TIME toxins

You Asked: Should I Dry Brush My Skin?

Illustration by Peter Oumanski for TIME

There may be benefits, but reducing cellulite isn’t one of them.

If you’re wondering what dry brushing is, the practice is exactly what it sounds like: Running a dry, soft-bristle brush over your bare skin. Methodologies vary, but most practitioners and beauty blogs recommend brushing your limbs and torso, always motioning toward your heart. Do this for a few minutes every day, they say, and you’ll increase blood flow and circulation, which will help your body and lymphatic system clear away toxins. Dry brushing is also thought to reduce cellulite and exfoliate, leaving your skin softer, more toned and better hydrated.

Unfortunately, there’s not much research to back up these health claims. “I know dry brushing is popular, but the actual benefits are unclear,” says Dr. Tina Alster, director of the Washington Institute of Dermatologic Laser Surgery and a clinical professor at Georgetown University.

Alster says that rubbing the skin—with a brush, your hand or anything else—will increase blood flow and circulation, giving your skin a flushed, youthful and “slightly swollen” appearance. (The same thing happens if you pinch your cheeks.) But your skin will return to normal very quickly after you’ve stopped brushing it, Alster says. There’s no evidence this temporary surge in blood flow will help your body remove waste or toxins, she adds.

Dry brushing will clear away dead skin cells. But exfoliating isn’t necessary for those in their teens and twenties. “When you’re young, your skin’s outermost layer will automatically turn over without any mechanical help,” Alster explains. Beginning in your thirties and increasing as you age, Alster says your skin’s cells can grow “stickier,” which can lead to accumulation and a dull appearance. “Exfoliation can help remove those stuck-together cells,” she says. “But you want to do it very gently and infrequently, or you may do more harm than good.”

Brushing too frequently or vigorously—or using a brush with rough bristles—could cause “micro-cuts” in your skin that may lead to infection, Alster says. Exfoliating more than once a week could also break down your skin’s protective barriers, leaving your hide less hydrated and prone to irritation, says Dr. Marc Glashofer, a New York-based dermatologist and member of the American Academy of Dermatology. For that reason, Glashofer says people with eczema or dry skin should avoid dry brushing altogether.

Glashofer mentions a common skin condition called keratosis pilaris (KP), which consists of many small rough bumps that tend to show up on the backs of arms and thighs. Dry brushing these areas could theoretically be beneficial, he says, but there’s no evidence yet.

And when it comes to reducing cellulite, both Glashofer and Alster say there’s nothing to back up such claims. “If brushing the skin twice per day would eliminate cellulite, you would have heard a lot more about it and there’d be some scientific proof,” Glashofer says.

Of course, not everything that benefits your body is easily captured by medical research. From meditation to massage, many practices once dismissed by clinicians have recently been linked to meaningful psychological and physical benefits. It’s possible dry brushing may one day fall into this category, but that day hasn’t arrived yet.

“If you like dry brushing and your skin looks good, that’s fine,” Alster says. “But would I encourage it as a dermatologist? Definitely not.”

TIME Mental Health/Psychology

You Asked: How Many Friends Do I Need?

You Asked: How Many Friends Do I Need?
Illustration by Peter Oumanski for TIME

Maybe just a handful, though quality trumps quantity.

Friends do your health so many favors. They protect your health as much as quitting smoking and a great deal more than exercising, according to a large 2010 review in the journal PLOS One. More research has shown that socially isolated people are more than twice as likely to die from heart disease as those with a solid social circle.

“Strong social relationships support mental health, and that ties into better immune function, reduced stress and less cardiovascular activation,” says Dr. Debra Umberson, a sociologist at the University of Texas, Austin. Umberson says emotional support is just one of a dozen ways friends may safeguard your health and extend your life.

MORE Here’s How Hugs Can Prevent the Flu

Unfortunately, though, many of us don’t have enough of them. According to data from the General Social Survey (GSS), the number of Americans who say they have no close friends has roughly tripled in recent decades. “Zero” is also the most common response when people are asked how many confidants they have, the GSS data show. And adult men seem to be especially bad at keeping and cultivating friendships.

That may seem strange in the era of Facebook, Twitter and boundless digital connectivity. But the “friends” orbiting at the farthest reaches of your digital galaxy aren’t the ones that matter when it comes to your health and happiness.

The vital friendships—the pals you hug and laugh and lament with—are the ones who have the greatest impact on your health and happiness. You need between three and five of them for optimal wellbeing, suggests research from Dr. Robin Dunbar, an evolutionary psychologist at the University of Oxford.

Dunbar’s name comes up a lot when you start digging into the subject of friendship. From his early work studying the brains and social circles of primates, he recognized that the size of a human’s social network might be limited by the size of a certain part of the human brain called the neocortex, a critical site for higher brain functions. After some complicated study, he came up with a figure now known as “Dunbar’s number.”

That number—usually cited as 150, but actually a range between 100 and 200—is the approximate size of a person’s social circle, or the perpetually changing group of friends and family members that you would invite to a large party. While you may have far fewer than 150 of these people in your life, your brain really can’t hold a close connection with more than 150, Dunbar’s research shows. Within that group, he says your closest 15 relationships—including family members or “kin”—seem to be most crucial when it comes to your mental and physical health.

But that’s not to say a brother or sister offers you the same benefits as a close friend, Dunbar says. While your kin are more likely to be there for you when you need help, your good friends tend to fire up your nervous system and trigger the release of feel-good neuropeptides called endorphins. Whether you’re laughing with your pal or feeling him or her touch your shoulder in sympathy, the resulting rush of endorphins seems to “tune” up your immune system, protecting you from disease, Dunbar explains.

So yes, for the sake of your health, you need friends—ideally the really close kind you see face-to-face on a regular basis. But even one very good friend can improve your life in profound ways, says Dr. Mark Vernon, a philosopher, psychotherapist and author of The Meaning of Friendship.

Despite their value in terms of your health and wellbeing, don’t think of them as your personal social doctors. Vernon warns against turning your friends into what he calls “service providers”—that’s not what friendship should be about, he says, even if your pals are good for you.

In the end, Vernon says Ralph Waldo Emerson may have offered the best advice when it comes to making and keeping close pals: “The only way to have a friend is to be one.”

Read next: 5 Types of Friends That Everyone Has

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TIME Exercise/Fitness

You Asked: Should I Do The Insanity Workout?

Illustration by Peter Oumanski for TIME

Expect results—though maybe not the ones you’d hoped for.

The name alone is a challenge—a dare. You’d have to be insane to attempt this workout. Of course, that’s a big part of the appeal. If the exercise is “extreme” or “crazy,” you assume the body benefits will be dramatic. And they may be—just not in the ways you’d expect from checking out the workout’s promotional materials, which emphasize weight loss.

While it’s continually changing, the latest iteration of “INSANITY” is a 60-day program composed of 30-minute bouts of very high-intensity aerobic and resistance exercise, to be completed six days a week. While research has linked physical fitness gains to Insanity-style interval workouts, the evidence that these programs lead to weight loss is anything but rock solid.

“We’ve shown that when it comes to cardiovascular fitness and function, greater intensity leads to greater adaptations,” says Todd Astorino, PhD, associate professor in the department of kinesiology at California State University, San Marcos.

Astorino has studied the health and fitness effects of very rigorous bouts of interval training, often referred to as high-intensity interval training (HIIT). He says there’s little doubt your athletic condition—the ability of your heart and lungs to handle physical activity—would improve if you completed something along the lines of an Insanity workout. He also says your blood sugar levels would likely drop, a change that could help protect you from metabolic diseases like type-2 diabetes. Throw in the body-resistance component of Insanity workouts, and you’ll certainly grow stronger too.

What about weight loss? “The research is very mixed, especially in the long term,” Astorino says. “I think it would help you avoid weight gain. But that’s not the same as losing weight.”

Hundreds of studies have looked into the effect of regular exercise on body weight. While physical activity is unquestionably good for your health, exercise alone doesn’t have a huge impact on the number you see on your bathroom scale, concludes one recent study appearing in the journal Obesity Reviews.

MORE You Asked: Why Are People Addicted To CrossFit?

Another study, this one from Stephen Boutcher, PhD, associate professor at Australia’s University of New South Wales, specifically examined the effect of Insanity-style HIIT training on body fat. Boutcher’s research found “significant” fat reduction—or a little more than 4 pounds of lost body fat after three months of training. That’s significant in science terms, but probably not what you’re expecting when starting a workout program that highlights happy customers who have lost 40, 50, or even 90 pounds.

Boutcher says there’s some evidence HIIT training may help suppress appetite in ways traditional aerobic exercise doesn’t. But to shed lots of weight—the kind of “total-body” transformation you see in product testimonials—you also have to eat a healthy diet, the research suggests. (Read the fine print on the INSANITY website, and you’ll find analogous disclaimers stating a “proper diet” is necessary to achieve and maintain weight loss.)

But for some people, even regular exercise and an improved diet won’t dramatically change the way their bodies look. “Many people are just naturally bigger,” Astorino says. “For them, trying to look physically lean would probably require some very dramatic diet restrictions.”

Of course, none of this touches on the psychological perks of challenging yourself with a butt-whipping exercise regimen. Talk to someone who has run a marathon or finished an Insanity program, and you can hear the sense of accomplishment and pride they feel when they talk about their achievement.

“If you can get through something like this, you’ll have confidence and a good understanding of what your body can tolerate,” Astorino says. Those benefits, along with improved endurance and metabolic health, are nothing to scoff at. But if you’re expecting to transform your body and drop several sizes with INSANITY, the results might not be as crazy as you’d hoped.

Read next: A Workout You Can Do Anywhere

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TIME Apple

How Apple’s New Health App Could Be Used — or Abused

Apple CEO Tim Cook speaks on stage during an Apple special event at the Yerba Buena Center for the Arts on March 9, 2015 in San Francisco.
Stephen Lam—Getty Images Apple CEO Tim Cook speaks on stage during an Apple special event at the Yerba Buena Center for the Arts on March 9, 2015 in San Francisco.

"There are some things you can't figure out by someone tapping into an iPhone”

The biggest news at Monday’s Apple event was the launch of the much anticipated Apple Watch. But the company also announced a new type of software — ResearchKit — that it says will help medical researchers collect health data directly from patients via their various iDevices.

ResearchKit is a software “framework” that hospitals and other health care organizations can use to create diagnostic applications, said Jeff Williams, Apple’s senior VP of operations, at the watch event. An example: Williams demonstrated one app called mPower, designed to measure hand and voice tremors related to Parkinson’s disease.

“With the use of this technology, we feel there’s an unprecedented opportunity to gain insight into what it’s like to live with Parkinson’s 24 hours a day, seven days a week,” says Dr. Todd Sherer, CEO of the Michael J. Fox Foundation for Parkinson’s Research. The foundation collaborated with Sage Bionetworks to help develop the Parkinson’s app featured at Apple’s event. Fox’s foundation has also announced a new data-collection initiative, Fox Insight, which Sherer says his organization hopes to soon pair with the mPower app.

Sherer said identifying qualified patients for clinical trials and other research efforts is another potential benefit of this new technology. “Getting people to participate in clinic-based trials is a challenge for many disease researchers, including those studying Parkinson’s,” he says.

Apple’s Williams mentioned other applications — now available — that can help spot symptoms of diabetes, heart disease and asthma. Williams said these apps were developed with the help of Massachusetts General Hospital, Stanford University, the Dana-Farber Cancer Institute, and other equally august health care organizations.

Apple’s Williams was adamant that any health information you share through ResearchKit will remain confidential. “You choose what studies you want to join, you are in control of what information you provide to which apps,” states information appearing on the Apple site.

While helping health care providers and researchers collect data and recruit qualified participants could lead to meaningful research advancements, some see reasons to be wary. “Two things concern me,” says Dr. David Ross Camidge, director of the Thoracic Oncology Clinical Program at the University of Colorado Cancer Center. For starters, big-name institutions may have more money and resources to develop these sorts of diagnostic apps, Camidge says. He worries patients may be directed away from the most-relevant or appropriate clinical trials to those that have the most funding behind them. “I certainly don’t know if that will be the case, but there seems like room for potential bias and commercialization,” he says.

Also, people tend to be lousy at self-reporting health-related information, Camidge says. A poorly designed app may generate misleading or contradictory patient data that could hinder instead of help the forward progress of some medical research. “I’m sure there are ways it will be very useful,” he adds. But he adds that some things are not so simplistic, cautioning that for conditions like cancer, Alzheimer’s and multiple sclerosis, he says, “There are some things you can’t figure out by someone tapping into an iPhone.”

TIME Diet/Nutrition

Why Full-Fat Dairy May Be Healthier Than Low-Fat

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Danny Kim for TIME

Dodging dairy fat may be bad for your waistline

Skim milk or whole? Non-fat yogurt or full-fat? For decades, public health officials have treated these decisions as no-brainers. Cut the dairy fat, they’ve maintained, and you’ll sidestep calories without missing out on good stuff like calcium and protein. Win-win. But they might have been wrong, a chorus of experts now say.

A recent review published in the European Journal of Nutrition of the existing research on dairy fat came to some surprising conclusions: People who eat full-fat dairy are no more likely to develop cardiovascular disease and type 2 diabetes than people who stick to low-fat dairy. When it comes to weight gain, full-fat dairy may actually be better for you, the review found.

“In terms of obesity, we found no support for the notion that low-fat dairy is healthier,” says Dr. Mario Kratz, first author of the review and a nutrition scientist at the Fred Hutchinson Cancer Research Center in Seattle. Of the 25 studies included in his team’s review, Kratz says 18 reported lower body weights, less weight gain, or a lower risk for obesity among full-fat dairy eaters. The other seven studies were inconclusive. “None of the research suggested low-fat dairy is better,” he says.

More research supports his team’s findings. A 2013 study in the Scandinavian Journal of Primary Health Care tracked the dairy intake and obesity rates of more than 1,500 middle-aged and older adults. Those who frequently ate full-fat butter, milk, and cream had lower obesity rates than those who eschewed dairy fat. “Based on my own research and on the research of others, I believe high-fat dairy is less likely to contribute to obesity that low-fat dairy,” says Dr. Sara Holmberg, first author of the study.

The belief that fat isn’t a health villain has been gaining traction the last few years, especially as data has piled up showing that low-fat diets don’t work. And while national health organizations seem to be softening their stance on fat, they still recommend reaching for low- or non-fat dairy at the supermarket.

Their justification: “Research has shown consistently that nutrient-rich foods—that is, foods that pack a lot of micronutrients into every calorie—are healthier,” says Isabel Maples, a registered dietitian and spokesperson for the Academy of Nutrition and Dietetics. Maples says reduced-fat dairy provides calcium, potassium, and other good things Americans need in their diet, and does so with fewer calories than full-fat dairy. She says reduced-fat dairy also contains less saturated fat.

Kratz doesn’t disagree with Maples’s comments. But he says they make assumptions about dairy that aren’t backed up by existing evidence. “Data should be weighed more heavily than assumptions,” he says. “And the data don’t support the notion that eating full-fat dairy is worse for your health than reduced-fat or non-fat dairy.”

How could something with more calories be better for your waistline? Some researchers argue that not all calories are equal—especially when it comes to weight gain. Also, focusing on calories-per-serving largely ignores a mammoth factor when it comes to obesity: fullness. Kratz says the fatty acids that are stripped out of reduced-fat dairy may help you feel full sooner and stay full longer—meaning you’ll eat less now and in the coming hours.

Dairy’s fatty acids may also play a role in gene expression and hormone regulation. In simple terms, these acids may crank up how much energy your body burns, or limit the amount of fat your body stores. “We don’t know any of these things for certain,” Kratz adds. “But they could help explain why our findings show full-fat dairy consumption is preferable to low-fat when it comes to a person’s risk for obesity.”

Holmberg, the author of the Scandinavian study, calls dairy “paradoxical,” and says it’s not possible to judge dairy’s health effects based only on its macronutrient content. “It is important to study the effect of real food and not just nutrients,” she adds.

Several more European studies have suggested similar links between full-fat dairy and lower rates of obesity, heart disease, and diabetes. And a just-published review from the journal BMJ concludes that, back in the 1970s—when health regulators established national diet guidelines that encouraged people to avoid fat—there wasn’t evidence to support those warnings. Basically, the foundation for all your “fat is evil” beliefs may have always been weak.

At the same time, none of this means you should gorge yourself on full-fat dairy. “We shouldn’t swing the pendulum too far in the other direction and say, ‘Put butter in everything and eat as much dairy as you want,’ ” Kratz warns. (Compared to many foods—especially vegetables and fruit—dairy contains no fiber, which is critical for digestion, for how the body manages sugar, and which plays an important role in maintaining a healthy weight.)

But if you’re deciding between skim milk and whole milk, the existing research argues you may be better off grabbing the full-fat stuff.

TIME Diet/Nutrition

You Asked: Why Can’t I Eat Raw Meat?

Illustration by Peter Oumanski for TIME

Well, you could. But you’d be rolling the dice.

Sushi restaurants are nearly as rampant as Starbucks stores. So why is raw fish okay to consume, while raw beef, pork and other land animals are typically not on the menu?

For one thing, the parasites and bacteria that set up shop in raw animal meat are different and more dangerous than the ones you’d find in raw fish, says Dr. Robert Tauxe, deputy director of the Division of Foodborne, Waterborne and Environmental Diseases within the Centers for Disease Control and Prevention (CDC).

From salmonella and parasitic E. coli to worms, flukes, and the virus hepatitis E, Tauxe says the creepy crawlies that may inhabit raw meat tend to be more harmful to humans than the microorganisms you’d find in raw fish. “Perhaps it’s because our bodies are more closely related to land animals than to those of fish,” he explains.

The way animals are slaughtered and packaged also has a lot to do with their health risks, says Dr. Eugene Muller, a microbiologist at Framingham State University in Massachusetts. “Parasites and bacteria tend to come from an animal’s gut, not its muscle,” he says. If your butcher nicks open an animal’s intestines, any harmful microorganisms released could contaminate all the meat the butcher is preparing.

Packaged ground beef is particularly likely to house sickness-causing bacteria or parasites, says Dr. Lee-Ann Jaykus, professor of food science at North Carolina State University. That’s because a single package of ground beef could contain meat from dozens of cows, Jaykus says. “One contaminated animal could corrupt dozens of batches,” she explains. For that reason, she advises never eating hamburger that’s red or rare in the center.

Both Muller and Jaykus say whole cuts of beef are less risky because they come from a single animal. “Anything harmful lives on the surface of the meat, not inside the muscle,” Muller says. “So if you like your steak very rare, just searing the outside will likely kill anything harmful.”

Jaykus agrees, but says you have to watch out for something called “mechanically tenderized meat,” which involves puncturing the beef with small needles or blades to make it more tender. She says many restaurants and grocery stores sell meat that’s undergone this process because it improves the texture of cheaper cuts like sirloin or round. “This process can force contaminants into the muscle tissue where searing the outside won’t kill them,” she says. “You don’t see this at high-end steakhouses, but it’s an issue with steaks purchased for home cooking and in some restaurants.”

Most of these concerns and caveats also apply to lamb, pigs, chickens and other land animals—though Muller says pigs and chickens tend to carry some harmful microorganisms you don’t find in cows or sheep. “But I don’t think many people really want to consume raw pig or raw chicken,” he adds.

Fish is a different story. Setting aside the differences between fish and mammals when it comes to the number, type, and frequency of potentially dangerous organisms they may harbor, fish tends not to be ground or mixed. That lowers the likelihood of a single disease-carrying salmon or tuna contaminating others, Jaykus says.

Also, any raw fish you consume at a sushi restaurant are caught in colder waters and frozen before you eat them. “This kills the encysted worms and other parasites,” Tauxe says. Unfortunately, freezing doesn’t kill parasitic E. coli and many of the harmful microorganisms you’d find in meat, Muller says.

With raw fish, oysters and other uncooked seafood, you’re taking a risk, Muller says—though not nearly as big a risk as eating that bloody tenderloin or tartare.

Read next: The 50 Healthiest Foods of All Time (With Recipes)

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TIME Research

You Asked: Why Is My Scalp So Itchy?

Illustration by Peter Oumanski for TIME

It’s probably dandruff. But everything you’ve heard about dandruff is wrong.

Itches are inscrutable. They arrive unannounced and recede at the rake of a fingernail. But the stubborn kind—the type that skittle across your scalp with terrible regularity—tend to have an easily identifiable cause: Dandruff.

“People think dandruff has to do with dry skin, but it’s actually a problem with how the skin cells on your scalp turn over or replace themselves,” says Dr. Adam Friedman, director of dermatologic research at the Albert Einstein College of Medicine in New York. Your skin is constantly shedding layers of cells while manufacturing new ones, and Friedman says this process can be touchy. “Producing too many cells too quickly can lead to a build-up of dead skin, and this build-up itches and flakes off,” he explains. “That’s dandruff.”

What causes this over-production of skin cells? Anything that puts stress on your immune system—from cold winter temperatures to a crazy week at the office—can switch on certain genetic proteins that speed up the production of skin cells, Friedman says. (Other skin conditions—acne, eczema—also flare up when you’re stressed.)

Yeast microorganisms living on your scalp can also mess with your skin’s cell reproduction, says Dr. Anthony Rossi, a dermatologist with Memorial Sloan Kettering Cancer Center. While usually harmless, these organisms—which live on everyone’s skin—can cause a reaction in some that leads to cell overabundance.

How do you stop the itching and flaking? Dandruff shampoo is a good start. Friedman says these shampoos work by killing scalp microorganisms and turning off the proteins that cause your skin cells to go nuts. That said, shampoos only help if you use them properly. “You’re trying to treat your scalp, so working these into your hair doesn’t do much good,” Friedman says. “You need to massage these products onto your scalp skin and leave them there for a couple minutes before rinsing.” (They aren’t usually very kind to your hair, though.)

He says dandruff shampoos typically include any one of a small number of chemicals that are all pretty much equally effective. While you could wash with them every day without over-drying your scalp, Friedman says this isn’t necessary. “Two or three times a week is plenty,” he says. “And if you don’t see improvement after a few weeks, switching to another product or using them more probably won’t do any good.”

There are many more explanations for an itchy dome. If your scalp is inflamed, red, and itchy, that may be seborrheic dermatitis—a more severe form of dandruff. “Scalp psoriasis is probably the next most common,” Friedman says. It can be hard to tell the difference between the two. But usually the flakes or “plates” of silvery gray plaques associated with scalp psoriasis are larger than dandruff flakes and tougher to brush from your clothing, he says. Scalp psoriasis could also cause some ear or face flaking.

Friedman mentions a few less-common issues: a skin disease called discoid lupus, or an allergic reaction. Rossi says an irritation to hair products like sprays or pomades is another possible itch-instigators. But trying to distinguish between those things and dandruff is really tough, Friedman says.

A good rule of thumb: If you have a red, itchy head and dandruff shampoos aren’t working after a month, see a doctor, he advises. He also cautions against waiting too long if dandruff shampoos don’t get the job done. “If you don’t treat inflammation of the scalp, there’s a chance of skin damage or hair loss,” Friedman says. “There’s often no coming back from that once it happens.”

TIME Diet/Nutrition

You Asked: Should I Take Probiotics?

Illustration by Peter Oumanski for TIME

From strengthening your brain to slimming your belly, you’ve likely heard the promises of probiotics. Headlines call them the future of preventive medicine and disease treatment. But for now, the key word is future.

“For those with bowel disorders or urinary tract infections, there’s little doubt probiotics can be helpful,” says Dr. Shekhar Challa, a gastroenterologist and author of Probiotics For Dummies. But for healthy adults? “Things are still open to interpretation,” Challa says.

Here’s what scientists know now: Your body houses hundreds of trillions of bacteria, many of which reside in your gastrointestinal tract. Of those gut bacteria, 90 percent appear to be system-supporting or “friendly,” Challa explains. Those are the probiotics. And the more types or “strains” of probiotic bacteria you have in your gut, the better.

Plenty of evidence shows that a diverse population of gut bacteria aids digestion and immune health, Challa says. Two bacteria strains in particular, Lactobacillus and Bifidobacteria, seem to be supportive, he says.

One recent U.K. study found Lactobacillus may combat the type of insulin resistance that contributes to obesity and the development of type 2 diabetes. If the tight junctions between the cells in the lining of your gut break down, the gap allows “inflammation-causing agents” to get into your bloodstream and cause trouble, says the study’s author, Dr. Carl Hulston of Loughborough University. But probiotics appear to seal those junctions and prevent this “leaky gut” situation.

“It’s a little too early to advise probiotic consumption as a prevention or cure for diabetes,” Hulston says. “But our initial findings are very positive.”

Read through the scientific literature on probiotics, and you’ll run into hundreds of similar statements. Probiotic potential seems limitless. The problem is figuring out how to get the right types of bacteria to set up shop in your body.

“You can eat probiotic foods like yogurt or kefir, which I think everyone should consume on a daily basis,” Challa says. “But just because you’re ingesting a probiotic doesn’t mean it’s being incorporated into your gut.” Harsh stomach acids and other aspects of digestion can kill the bacteria before they establish residency, Challa says.

Probiotic supplements—some of which contain coatings or ingredients designed to protect their contents from your belly’s acids—may be more effective, Challa says (though it should be noted that he’s part owner of a company that produces these types of supplements.) But he’s quick to add there are still a lot of roadblocks. Your age, your gut’s current micro-organic makeup, your diet and many other factors make it hard to offer broad recommendations, he explains.

It gets even murkier when you consider research from Dr. Daniel Bolnick and colleagues at the University of Texas. They’ve shown that—at least in some animals—eating a wider variety of foods actually reduces the number of probiotic bacteria in the intestinal tract. “We looked at the effects of diet mixing on gut microbes,” Bolnick says. “And we found the more you mix, the fewer you have.”

This doesn’t mean that consuming many different types of bacteria is bad. It just raises questions, Bolnick says. “Combinations of foods can do unexpected things,” he adds.

So what’s a wannabe bacteria-booster to do? Challa says that eating probiotic foods on a daily basis, like yogurt, kefir and kombucha may be beneficial, and certainly won’t harm you. “There’s no evidence that people can overdose on probiotics,” he says. And for those with gastrointestinal issues, he strongly suggests a probiotic supplement containing Lactobacillus and/or Bifidobacteria. (Just run it by your doctor first.)

When it comes to all the other benefits you’ve heard about, it’s too early for experts to offer firm advice. “There are just too many questions and not enough hard data,” Challa says. “The potential is there. The understanding is not.”

Read next: Is It Bad To Eat the Same Thing Every Day?

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TIME toxins

You Asked: Is Perfume Bad for Me?

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Illustration by Peter Oumanski for TIME

Many people suffer an allergic reaction to perfume—even when it's on someone else

For some it’s intoxicating. It may also be toxic.

From sneezing and wheezing to rash and headache, many people suffer an allergic reaction to perfume—even when it’s on someone else. “Anything that gives perfume an odor is very likely going to be an allergen,” says Dr. Heather Patisaul, a biologist at North Carolina State University who studies the way environmental exposures mess with human development. That means your cologne- or perfume-soaked colleague could be the cause of your runny nose.

“Look at your perfume bottle and read the ingredients,” she suggests. “It reads like a chemistry book.” You can get lost among all the benzyls and ethyls. But it’s the ingredient listed simply as “fragrance” or “parfum” that worries public health researchers like Patisaul. “Those terms are a catchall for 10,000 different ingredients,” she says.

Thanks to trade-secret laws protecting perfume and cosmetic makers from divulging the specific formulas for their scents, Patisaul says there’s usually no way to know exactly which of those 10,000 ingredients you’re dabbing onto your neck or wrist. But a good bet is that any scented cosmetic—from perfume and cologne to body lotion and deodorant—will contain a group of petroleum-based chemicals called phthalates.

“Phthalates are wonderful for cosmetics because they make things smear really well,” Patisaul explains. When it comes to perfume, phthalates keep all the liquid’s different elements suspended and evenly distributed, adds Caroline Cox, research director for the non-profit Center for Environmental Health—a watchdog group that monitors chemical-related public health concerns.

So what’s the problem? Some phthalates—namely one called diethyl phthalate (DEP)—are shown to disrupt our hormones, including testosterone. That’s a big concern for pregnant women, Patisaul says. “There’s evidence connecting phthalates to developmental disorders, especially among newborn boys,” she explains.

More research has linked DEP to poor lung function and myriad sperm issues, from lower counts to reduced motility, Cox says. Which means men—particularly adolescents who fumigate their still-developing bodies with aerosolized body sprays—could be at risk. “I mean, you’re purposely spraying these chemicals close to your body and rubbing them right on your skin,” Cox says. “There’s no question people are exposed.”

In women, research has linked phthalate exposure to problems such as polycystic ovarian syndrome, a cluster of symptoms that can include trouble conceiving, irregular periods, and hormonal imbalances that can lead to irregular hair growth and acne. Fetal exposure is better understood, and has some experts concerned.

Read more: Beauty Products May Trigger Early Menopause

What isn’t clear: Just how much phthalate exposure is too much. “It’s not like we can deliberately expose a bunch of pregnant women or boys to phthalates and see what happens,” Patisaul says. “So coming up with hard proof is difficult.” That same dilemma helped cigarette companies dodge health regulators for decades. And it’s bad news for American perfume and cosmetic users. “Our regulatory system assumes a chemical is safe until proven harmful,” Patisaul says. “In a lot of other countries, it works the opposite way. The burden is on the chemical producer.”

As a result, some perfume ingredients banned in Europe are still available in the U.S., Patisaul says. (France, the ne plus ultra of perfume-loving nations, is also one of the most strictly regulated when it comes to cosmetic components.) The list of banned ingredients includes natural animal musks and plant extracts that, for some, are potent allergens.

Read more: This Makeup Additive Is Linked To A Lower IQ

To protect yourself (and your coworkers), you could swap all your fragranced products in favor of unscented cosmetics, Patisaul says. Cox says you can check the safety of some ingredients and products that contain them using resources like Skin Deep, a product safety database run by the non-profit Environmental Working Group.

Pregnant women in particular should consider this sort of medicine-cabinet purge.

If you can’t bear to part with your favorite perfume, at the very least go easy when you apply it. And ditching scented lotions, deodorants, shampoos and aftershaves will still lower your exposure, Patisaul says. “You could argue endlessly about what is or isn’t a safe threshold,” she says. “The truth is, we’re only beginning to understand how these chemicals disrupt the body.”

Spritz at your own risk.

TIME

You Asked: Why Does My Eye Twitch?

Illustration by Peter Oumanski for TIME

Your eye spasms, decoded

Little lid spasms are common, but they can sometimes be a sign of trouble.

A slight tremor of the eyelid—the type that shows up without warning but scrams just as suddenly—is usually no cause for concern, explains Dr. Wayne Cornblath, a professor of ophthalmology at the University of Michigan’s Kellogg Eye Center. “I think everybody has these once in a while,” Cornblath says. “You rub it, and it eventually stops.”

He’s talking about the kind of whispery muscle spasm that happens in one eyelid (or just a portion of the lid, to be precise). It can be a nuisance, but it usually goes away on its own within a few days, if not a few minutes.

To get rid of eye twitches, you might want to cut back on the caffeine. Too much of it seems to be a trigger, says Cornblath. While the exact mechanisms are a bit of a mystery, research from York University in Canada has shown that caffeine prompts the release of excitatory neurotransmitters like serotonin and noradrenaline. “Caffeine is a stimulant, and it increases reactivity within the muscles and nerves,” Cornblath explains. That may go some way toward explaining how caffeine causes occasional bouts of eyelid quivering, he says.

Getting too little sleep also seems to have an effect, though the reasons why are less clear. “Research has shown a correlation, and we know that getting more sleep can help, but we don’t know why,” Cornblath says. The same can be said for muscle spasms in general, which are quite common but confound explanation. “You hear about low potassium or dehydration, but there doesn’t seem to be much hard evidence,” Cornblath says.

Stress may also play a role, says Dr. Rebecca Taylor, a clinical spokesperson for the American Academy of Ophthalmology. When you’re frazzled, you produce more epinephrine—a fight-of-flight molecule that primes your body for action. This heightened state of muscle arousal may manifest itself in small contractions or spasms, like the one in your eyelid, Taylor says.

In rare cases, when people address these sleep and stress issues and the twitch persists, Cornblath says a single treatment of Botox resolves the problem. Botox temporarily “shuts off” the connection between muscles and nerves, he explains.

Eyelid spasms are usually benign. But that’s not necessarily the case if the twitch spreads, Cornblath and Taylor both say. “If you’re experiencing spasms lower in your face or neck, that’s another story,” Cornblath says.

It’s not as common, but having a spasm in one side of your face—hemifacial spasm—is definitely something to talk to a doctor about, Taylor says. So is another condition, called a bletharospasm, where the whole eyelid closes or blinks involuntarily. There are a handful of potential explanations for both of them, and an eye doctor can help you figure out what’s going on, she recommends.

Back to that irksome little lid twitch: if it lasts for months, get it checked out. But in most cases, it’ll be gone in the blink of an eye.

Read next: Why Am I Cold All The Time?

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