Here's What Dietitians Really Think of Intermittent Fasting
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Arx0nt/Getty Images"Because insulin levels drop when we haven't eaten in a while, IF may be a powerful tool for increasing insulin sensitivity, per a 2018 study published in Cell Metabolism," Samuels says. "It's important to note, however, that a ton of the research on IF has been done on overweight individuals with insulin resistance, meaning results may not apply to the average healthy person."
Harbstreet adds that there is some evidence that IF can support blood glucose management through regulating specific hormones, which may decrease insulin resistance or prevent/delay the onset of type 2 diabetes.
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Brook Pifer/Getty ImagesAccording to Johns Hopkins Medicine, studies have shown that intermittent fasting can boost verbal memory in adult humans (and working memory in animals).
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Duet Postscriptum/StocksySamuels cites evidence that IF may bolster the body's stress response because it forces cells to be more adaptive. "This phenomenon harkens back to when our ancestors regularly experienced periods of starvation. As a result, cells became more adaptive, revving up their antioxidant activity and DNA repair and decreasing inflammation," she explains. "The thinking goes that, if these same adaptations occur in people who fast, IF may be protective against conditions like cardiovascular disease, type 2 diabetes, cancer, and Alzheimer's disease, but research in this area is still limited."
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Bymuratdeniz/Getty ImagesJohns Hopkins Medicine also points to research that showed that IF improved blood pressure and resting heart rate in study subjects.
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Nadine Greeff/Stocksy"Finally, IF may be beneficial for weight loss. However, it might just be due to the fact that you’re eating fewer calories overall and not necessarily from the act of fasting itself," Samuels says. She adds that other healthy diets like the Mediterranean diet may be just as effective.
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Eva Blanco/EyeEm/Getty ImagesYou might want to steer clear of intermittent fasting if you fall into one of these groups: people with advanced type 2 diabetes, pregnant or breastfeeding women, and those with a history of disordered eating. "People with insulin-dependent diabetes should eat at regular intervals to maintain stable blood sugar levels," Samuels says. "Pregnant and breastfeeding mamas need to consume adequate energy to support both themselves and their babies. When it comes to individuals with a history of disordered eating, IF can exacerbate unhealthy habits like overeating, binge eating, and restrictive dieting." She also adds that you might want to avoid intermittent fasting if you're prone to stress and anxiety.
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Foxys Forest Manufacture"At the end of the day, IF is a diet. It involves the planned and continued restriction of food, with or without a caloric deficit, and we know that the majority of diets are not sustainable beyond a two to five–year period for most people," Harbstreet says "Due to the potentially harmful impact on one's relationship with food, lifestyle limitations, and mental/emotional health, I do not recommend this approach for most people, especially those with a history of weight cycling, disordered eating, or eating disorders."
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Julien McRoberts/Getty ImagesHarbstreet says this is a short-term drawback. "This may go without saying, but when switching to a fasting regimen from a normal or typical schedule of eating, there will likely be periods of hunger (potentially extreme)," she explains. "For me, the issue here lies in how this promotes a disconnection from our body's normal hunger cues. Many dieters have long histories of numbing their sensation of hunger or otherwise disrupting or delaying their body's signals for hunger."
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Westend61/Getty ImagesHarbstreet says that some clients she's worked with who have tried intermittent fasting started becoming fixated on the clock. "They described feeling obsessed with watching the time tick down to their eating window, and several experienced binge-eating patterns that were also absent when they weren't following a fasting regimen," she says. "When we eat very quickly, as we tend to do when overly hunger, we can lose the ability to eat mindfully, thus reducing the overall pleasure and satiety we derive from meals."
Fasting can increase levels of cortisol, which is the stress hormone. "This makes sense. Not eating for an extended period of time is bound to stress out our body's systems," Samuels says. "If you're someone who already struggles with anxiety, elevated cortisol levels related to IF may further exacerbate physiological symptoms of stress, like rapid heart rate and breathing, high blood pressure, and high blood sugar."
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Juri Pozzi/StocksyIntermittent fasting also might not be right for your particular lifestyle. Sometimes, your fasting times might not work with your regular activities, like family or social gatherings, which can be stressful. "For instance, if you're someone who lives for breakfast, there's no reason to subject yourself to time-restricted feeding," Samuels says. "Similarly, if you know you get light-headed when you haven't eaten in a few hours, fasting may pose a serious threat to your health."
When it's time to eat, make sure you're eating well-balanced meals. Samuels says that includes healthy proteins (eggs, animal protein, dairy products); healthy fats (nuts, seeds, olive oil); complex starchy carbohydrates (legumes, 100% whole grains); and nonstarchy carbohydrates like leafy greens and colorful fruits and vegetables. "When we consider portion size of each food, I recommend making half of your plate nonstarchy vegetables, 1/4 of your plate 100% whole starchy carbohydrates, 1/4 of your plate healthy proteins, with one to two servings of healthy fats," she suggests.
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Aleksandar Nakic/Getty ImagesHarbstreet says that working out while in a fasted state is not necessarily a problem, but it depends on the length and intensity of exercise. You also might need to adjust your exercise habits, depending on the type of IF method you are doing. "I would not recommend exercise on days when people are doing 500-calorie fasts, 24-hour fasts, or OMAD fasts. Exercising without appropriate fuel can impair metabolism, disrupt stress hormones, and negatively impact physical and mental performance," Samuels adds.
Across the board, it's always important to stay hydrated and get quality sleep. "Depending on what type of IF you are doing, these habits may be especially important because fasting can alter stress hormones, which can make sleep worse," Samuels says. "Furthermore, being adequately hydrated can help to minimize hunger, which may be more of an issue with people who are doing IF."
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