Given my 15-plus years in private practice I don’t believe there’s a one-size-fits-all approach to nutrition and weight loss. What works for one person may not work—or feel good—for another. That’s why I believe it’s so critical to listen to your body to determine what feels best and is sustainable for you, despite how popular an approach may be— from juice cleanses and detoxes to “caveman” versus vegan diets.
Regarding one strategy that’s currently trending, intermittent fasting, I’ve seen very mixed results. Many men, particularly those who struggle with excess weight and conditions like diabetes or metabolic syndrome, have reported positive results with this semi-fasting approach. But for many women I’ve counseled, any type of fasting—whether it be overnight for 16 hours every night, or capping calories at 500 two days a week—has seriously backfired. If you’re thinking of giving it a try, here are four potential unwanted effects to consider.
Limiting food intake to just eight hours each day or severely restricting calories a few days a week are two popular fasting approaches. I’ve seen both lead to intense cravings, preoccupation with food, and rebound binge eating, particularly for women. Some who attempted to cut off eating after 4pm (with the intention of eating again at 8am) have told me that after hours of lingering thoughts about food, or watching other family members eat, they just couldn’t take it anymore, and wound up raiding the kitchen and eating far more than they would have on a typical night. Others, who attempt to eat no more than 500 calories a day two non-consecutive days each week, often begin daydreaming on fasting days about what they can eat on nonfasting days, and end up eating decadent goodies more often, like baked goods, pizza, chips, and ice cream. The lesson: even if this tactic has worked miracles for a friend, co-worker, or family member, if it leaves you in a food frenzy, it’s not the best approach for you.
I’ve tried intermittent fasting myself, and like clients and others I’ve talked to, it interfered with my ability to fall asleep and stay asleep. This effect can not only wreak havoc with daytime energy, but a plethora of studies have shown that sleep length and quality are strongly associated with weight control. Too little sleep has been shown to increase hunger, up cravings for sweet and fatty foods, reduce the desire to eat healthy foods like veggies, and trigger excessive eating overall and weight gain. For these reasons, I don’t believe that fasting is an optimal strategy for many people. In fact, some clients have told me they got out of bed at 3am after waking up, and you guessed it, wound up either eating, drinking alcohol, or both, in order to fall asleep—not a good recipe for weight loss or wellness.
As a nutritionist, one of my biggest pet peeves with fasting is that I’ve seen it compromise overall nutrition by limiting the intake of veggies, fruit, even lean protein and healthy fats, which are strongly tied to keeping metabolism revved, boosting satiety, and reducing inflammation—all critical for weight control. I think this is especially the case when people become focused on calorie counts rather than food quality. If you do decide to try intermittent fasting, or even a modified version, make every morsel count by sticking with naturally nutrient rich whole and fresh foods rather than processed “diet” products.
Unfortunately, fasting doesn’t trigger your body to break down only your fat reserves. While that would make weight loss so much easier, metabolism is a bit more complex. Your body burns a combination of fat and carbohydrate and after about six hours or so, when carbohydrates aren’t being consumed and your body’s “back up” stores in your liver have been depleted, you begin to convert some lean tissue into carbohydrate. The ratio of how much fat to muscle you lose may vary depending on your body composition, protein intake, and activity level, but again, this is where I’ve seen women and men experience different results. Research shows that in postmenopausal women, a higher protein intake is needed in order to lose less muscle mass (not offset the effect completely), but many women tell me that when they fast they crave carbs, which may lead to a loss of muscle while maintaining body fat—the opposite of their intended goal. Bottom line: again, think through what feels good and in sync with your body’s needs, and remember, sustainability is key.
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Cynthia Sass, MPH, RD, sports nutrition consultant to the NHL's New York Rangers and the MLB's Tampa Bay Rays.