On paper, the benefits of one meal a day, or a 20-hour fast, sound perfect. You don’t have to worry about what you’re going to eat during daylight hours, which could save you time, money, and kitchen clean up. Plus, studies of time-restricted eating suggest it can improve insulin sensitivity, fight disease, and boost weight loss. You can thank ketosis for the latter; without a constant supply of carbs and sugar in your system, your body burns fat as a default.
But, of course, there are some major caveats. An empty stomach for prolonged periods of time can also lead to fatigue, headaches, irritability,brain fog, and that hangry feeling—something women are more prone to (experts suspect this is because women have evolved to seek out food more regularly than men to ensure the health of their future offspring). In fact, it’s not clear that one-meal-a-day or other varieties of intermittent fasting would benefit women much at all, given the fact that nearly all the research has been done on men.
Fasting can also lead to rebound binging on the wrong foods. “When you have cravings and you’re cranky, it’s easy to over-consume foods high in carbs, fat, and calories,” warns Serena Marie, RD, a Brooklyn-based registered dietitian. And then there’s the social side: “While not eating during the day gives you an easy out for skipping office birthdays and other unhealthy food opportunities, it’s also rather anti-social,” saysLauren Slayton, MS, RD, author of The Little Book of Thin.
Needless to say, this type of intermittent fasting isn’t for everyone. There are the obvious groups who should avoid it, namely children and anyone pregnant or trying to get pregnant, but what about the average person? While Marie says it can work for runners who want to become more efficient at using fat as fuel before a race, fasting for any period of time is not something she recommends for the general population, and certainly not for the long term. “You’re probably not eating enough fruits and vegetables or getting your recommended daily 25 grams of fiber in one meal, which is the optimal number for health and fighting chronic disease,” warns Marie. (Find out what happens when you don’t get enough fiber.)
For Slayton, it’s a lot more reasonable to fast 16 hours (aka: skip breakfast) than to go food-free all the way to dinner, not that she endorses doing either on the regular. “However, I do think there’s something to be said for paying attention to when you eat in addition to what you eat,” says Slayton, whose clients use a 12-hour window rule for typical days—so if they start eating at 7 AM they finish by 7 PM, effectively eliminating unnecessary late-night snacking.
For optimal health, weight, and mental focus, it’s more important that you’re eating the right kinds of foods at your meals so your blood sugarisn’t erratic and crazy, says Marie. The solution doesn’t have to be fasting