Successfully maintaining weight loss for a long period of time with traditional caloric restriction is possible only for a small percentage of people. Is there an alternative to traditional dieting that actually works?
Fasting is nothing new. It has been a religious practice for centuries and Hippocrates advised medical fasting in 5th century B.C. Beginning in the early 1900s, animal studies demonstrated that fasting extended the life of rats. Animal research continued throughout the 20th century and suggested metabolic improvements as well.
How intermittent fasting works is not yet understood. Hypothesized mechanisms are based on rodent studies and involve the body’s circadian rhythm, or light-dark cycle. Proponents suggest that restricting food for a long period of time decreases insulin levels; they claim that when the gut is allowed to rest, gut permeability decreases, followed by decreased inflammation.
There are three kinds of intermittent fasting:
- Alternate-day fasting alternates days of zero calories with days of unrestricted calories.
- Modified fasting allows 20 percent to 25 percent of estimated caloric needs on fasting days and unrestricted intake on non-fasting days. For example, the 5:2 Diet is popular in the United Kingdom.
- Time-restricting feeding requires a prolonged nighttime fast.
In a 2013 article in the Canadian Medical Association Journal, intermittent fasting was predicted to be the next big weight-loss fad. This appears to be the case: The 2018 Food and Health Survey from the International Food Information Council Foundation found 10 percent of respondents said they follow intermittent fasting. Proponents claim it helps people lose weight, sleep better, improve diabetes and even live longer. But does human research support these claims?
A 2017 trial involved randomly assigning participants with a mean body mass index of 34 to one of three groups for one year: alternate-day fasting (25 percent of energy needs alternated with 125 percent), calorie restriction (75 percent of energy needs) and control (unrestricted calories). The alternate-day fasting group had a high dropout rate: 38 percent compared to 29 percent in the calorie restriction group. This group did not experience significantly different weight loss from the restriction group. Between the intervention groups, there also were no significant differences in blood pressure, heart rate, triglycerides, fasting glucose, fasting insulin, insulin resistance, C-reactive protein or homocysteine concentrations. LDL cholesterol rose significantly in the fasting group compared to the restriction group.
A 2018 study of adults with Type 2 diabetes and a mean BMI of 36 compared the 5:2 Diet to calorie restriction for one year and found both interventions improved HbA1c, fasting glucose and lipid levels but did not differ significantly.
While intermittent fasting appears to offer metabolic improvements for people with Type 2 diabetes, although not significantly different from those achieved by traditional caloric restriction, it may not be safe for all people with diabetes.
“Whether someone has Type 2 diabetes or Type 1 diabetes, what matters most is if they’re dependent on insulin and taking glucose-lowering medications,” says Hope Warshaw, MMSc, RD, CDE, BC-ADM, FAADE, author of Diabetes Meal Planning Made Easy, 5th Ed. (American Diabetes Association 2016). “Diabetes management is not the same for everyone. The safety of intermittent fasting for someone with diabetes depends on their medications and risk for hypoglycemia.”
The Academy’s 2016 position paper on adult weight management does not address any type of fasting and points out that research is very limited on the effect of timing of intake among people with obese BMIs. Additionally, a systematic review performed in 2013 and 2014 and published to the Evidence Analysis Library found that skipping breakfast was associated with higher body mass index and increased risk of obesity.
Recent systematic reviews and meta-analyses of intermittent fasting in humans have found most studies to be of short duration with few participants and mixed findings. Intermittent fasting does not appear to offer superior metabolic or short-term weight control advantages compared to traditional caloric restriction.
While there appear to be no health advantages, some people may find intermittent fasting easier to maintain. “With intermittent fasting, you are restricting food intake but only on certain days or times, and on other days or times, you have the freedom to eat and meet energy requirements,” says Abbey Sharp, RD, a Toronto-based dietitian. “In that sense, some people see this diet as a more flexible approach. Depending on which regime you choose, it also might not be too different from how you’re already eating if you eat an early dinner and late breakfast.”
Conversely, researchers found that some participants overate on fasting days and under-ate on non-fasting days, which may indicate intermittent fasting is difficult to maintain.
Side effects of intermittent fasting include persistent hunger, cold, irritability, low energy, distraction and lower work performance. Those at risk for disordered eating should not attempt any sort of fasting diet; fasting has been found to predict disordered eating and eating disorders. Meanwhile, multiple eating disorder organizations classify fasting as an unhealthy weight control behavior. According to Tiffany Haug, MS, RDN, EDOC, who specializes in behavioral health and eating disorders, “When restricting food, there is a higher dopamine release in the brain when you do eat, which increases the likelihood of binge.”
Another population of concern is athletes. While research is quite limited in this area, one small study found intermittent fasting decreased speed among male athletes.
Intermittent fasting could be risky for women of child-bearing age, since preconception is an important time for women to maximize their nutrient intake. “Going without food means going without energy as well as protein, vitamins, minerals and phytonutrients. In terms of nutrition, there is a lot to make up for on non-fasting days and it’s very difficult to catch up,” explains Elizabeth Ward, MS, RDN, author of Expect the Best, Your Guide to Health Eating Before, During, and After Pregnancy (Academy of Nutrition and Dietetics 2017). Pregnant and breastfeeding women also should not attempt fasting because they need consistent energy and nutrients to support the developing fetus and breast milk. Ward says she would not recommend intermittent fasting to a woman who is pregnant or breast-feeding.
“Premenopausal women have different metabolic expenditures depending on the time of the month. When you’re not meeting your needs due to fasting, you’re increasing your risk for bingeing and fixation on food,” Haug says.
Researchers agree there is not enough evidence to recommend intermittent fasting at this time and that studies have lacked long-term interventions and follow-up periods. There have been few studies investigating the mental, emotional and social consequences of fasting. Additionally, researchers suggest that future studies should investigate cognitive and immunological implications, dietary quality and impact on the microbiome.
