When it comes to diet, you can totally have too much of a good thing. Even the healthiest foods, in big quantities, may have side effects. And today, with our increased zeal for superfoods, the risk of overdosing on certain power eats has multiplied. Here are my top culprits—plus how to stay balanced.
For nutrients and antioxidants per calorie, few foods compare to kale. But our current obsession with this leafy green may be overkill: Kale’s appearance on U.S. restaurant menus jumped nearly 400 percent from 2009 to 2013. And the number of new kale products introduced globally more than tripled between 2007 and 2012, per Innova Market Insights.
Women I know are whipping up a green juice (many of which contain far more kale than they could eat in one sitting concentrated into 16 ounces) in the morning, having kale salad for lunch and snacking on kale chips at night. The potential side effect? Kidney stones. Kale contains oxalate, which can bind with calcium to form stones. While other foods (like spinach) are higher in oxalate, megadoses of kale could make it a problem for those susceptible to stones.
The best balance: Choose a kale-rich green juice or a big kale salad per day. On days you have whole kale, you can still do green juice; just make one with a low-oxalate ingredient like cucumber and a variety of other vegetables.
Read more: 13 Healthy Kale Recipes
One of the simplest (and yummiest) ways to get the recommended twice-weekly servings of ocean fare is to hit the local sushi joint. But plenty of busy women overrely on it.
While sushi does offer lean protein and heart- and brain-protective omega-3s, mercury in fish (from pollution) is a real concern. And it can be easy to OD on it via sushi, per a recent study. Researchers at Rutgers University interviewed 1,289 men and women about their sushi intake and tested fish samples. Among the tuna, eel, salmon and crab, tuna had the highest levels of mercury. Researchers estimated that mercury exposure for people who ate seven sushi meals consisting mostly of tuna per month exceeded the EPA’s recommendations. The scary part: Symptoms of mercury overexposure (vision issues, tingling fingers and muscle weakness) may not show up for months, or even at all.
The best balance: A good sushi option is a brown rice California roll; it’s made with crabmeat, so it’s fine twice a week. It’s best to limit bigger fish like certain types of mackerel and tuna, which tend to have more mercury—particularly if you’re pregnant, planning to get pregnant or nursing.
Read more: 10 Fish You Should Avoid (and Why)
If you shop for cereals, energy bars, orange juice and bottled water, you’ve probably seen labels bragging about added nutrients. Check those labels carefully, however. Some cereals boast 100 percent of the daily value (DV) for many vitamins and minerals. While fortification helps ensure that you’re not lacking in nutrients like vitamin D, ingesting a total day’s worth of zinc, iron, B vitamins and more from one product ups your likelihood of getting a surplus. This is especially true if you take supplements or have more than one fortified food a day. If you want a boost, buy brands fortified with no more than 50 percent of the DV for any nutrient.
Consistently going far above the daily allowance can push you toward your tolerable upper intake level (UL), the point at which good nutrients can become dangerous. One European study suggested that people who ate large amounts of dairy could exceed the UL for calcium when adding things like calcium-fortified orange juice and oatmeal. Passing the daily 2,500-milligram limit regularly can cause constipation and kidney problems. Too much zinc may reduce immune function and HDL (good) cholesterol levels.
The best balance: Skip products that pack 100 percent of your DV for any one nutrient. Consume plenty of whole foods instead.
Read more: The 20 Best Foods to Eat for Breakfast
Once only health food staples, chips, spice concoctions and salad mixes made of sea plants like nori and kombu are now in the grocery. You can also find this power green—which has been linked to heart health—in sushi rolls and seaweed salads. The problem? Seaweed is often super rich in iodine. Too much can lead to thyroid problems, which can cause weight fluctuations. In one study, a 39-year-old woman was diagnosed with hyperthyroidism after downing tea that contained kelp for four weeks.
The best balance: I generally recommend that clients get their kelp fix safely by stopping at one fresh seaweed salad (in addition to sushi) once a week. Steer clear of the teas, unless prescribed by a doc, and keep seaweed snacks to one serving a day. If you notice fatigue or weight changes, though, cut them out completely.
Read more: 19 Signs Your Thyroid Isn’t Working Right
Cynthia Sass, MPH, RD, is Health’s contributing nutrition editor, and privately counsels clients in New York, Los Angeles, and long distance. Cynthia is currently the sports nutrition consultant to the New York Rangers NHL team and the Tampa Bay Rays MLB team, and is board certified as a specialist in sports dietetics.