Here's What Dietitians Really Think of Intermittent Fasting
There's always a new trending diet that's gotten everyone's attention. One diet or eating plan, in particular, that has been around for a while but has become popularized in recent years is intermittent fasting (IF). We wanted to get a breakdown of what intermittent fasting exactly is and if it's really good for you from the experts. Here's what we discovered.
THE BENEFITS OF INTERMITTENT FASTING
It's important to note that the research on intermittent fasting is not 100% conclusive and definitive. That's because many of the studies have been conducted on animals, mainly mice. "Some of the studies also use a slightly different form of fasting called continuous calorie restriction, or CR, when studying the effects of fasting on longevity," Harbstreet says. "Research conducted in animal models doesn't always translate into the same benefits for people, so I encourage readers to use caution and consider the limitations of the available evidence of IF."
With that in mind, there have been quite a few promising studies that show the positive effects of intermittent fasting. Some are outlined below.
IT COULD INCREASE INSULIN SENSITIVITY
"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 Metavolism," 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.
IT STIMULATES METABOLIC SWITCHING
"Metabolic switching occurs when a fasted body runs out of sugar, or glucose, to use as its main source of fuel," Samuels explains. "As a result, the body switches to start breaking down stored fat. Broken-down fats are turned into ketone bodies by the liver and then used by the body for energy. According to research published in The New England Journal of Medicine, ketone body production may start within eight to 12 hours of fasting, hence why popular IF methods recommend fasting for at least 14 hours to promote weight loss."
IT MAY BOOST MEMORY AND THINKING
According to John Hopkins Medicine, studies have shown that intermittent fasting can boost verbal memory in adult humans (and working memory in animals).
IT COULD HELP REDUCE INFLAMMATION
Samuels 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."
IT COULD SUPPORT HEART HEALTH
John Hopkins Medicine also points to research that showed that IF improved blood pressure and resting heart rate in study subjects.
IT COULD SUPPORT WEIGHT LOSS
"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.
THE CONS OF INTERMITTENT FASTING
YOU'LL WANT TO AVOID IF YOU HAVE A CERTAIN CONDITION
You 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.
IT COULD LEAD TO DISORDERED EATING
"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.
FLUCTUATIONS IN HUNGER
Harbstreet 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."
IT COULD LEAD TO BINGE EATING
Harbstreet 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."
IT COULD INCREASE STRESS LEVELS
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."
IT MIGHT NOT FIT CERTAIN LIFESTYLES
Intermittent 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."