Intermittent fasting has fast become a diet trend worth noting. For the past two years, the IFIC Foundation’s Food and Health Survey has found that intermittent fasting is one of the most widely cited diets that people have followed. But what is intermittent fasting? Its growing popularity justifies a deeper dive into the specific parameters, research and potential considerations for trying this diet.
What is intermittent fasting and why is it so popular?
Intermittent fasting is a diet that cycles between times of fasting and eating. Unlike other eating philosophies, intermittent fasting does not prescribe what to eat; rather, it’s focus is on when to eat. For most who adopt this eating plan, the goal is to reduce the number of calories eaten to lose weight.
There are a few variations to the eating and fasting periods of this diet, including a daily 16:8 ratio of fasting hours to eating hours; the 5:2 method (five days of normal eating and two days of eating 500–600 calories per day); and the “Eat-Stop-Eat” method (which involves fasting for 24 hours one to two times per week). Regardless of fasting stage length, no food is allowed during this time; but coffee, tea and water are permitted.
What does research on intermittent fasting show?
One of the top reasons people adopt a new diet is to lose weight, and some believe intermittent fasting may help with this. A systematic review and meta-analysis in 2018 examined whether different forms of intermittent fasting results in weight loss. The research included six studies ranging in duration from three to 12 months. Four of these studies included continuous energy restriction as a comparator intervention, meaning that study participants ate fewer calories throughout the day than they usually would. Two studies included a no-treatment control intervention, meaning that the control group didn’t change their eating habits at all. In total, the effects of intermittent fasting on body weight were assessed on 400 overweight and obese individuals. The researchers concluded that intermittent energy restriction was more effective than no treatment for weight loss. There was no difference in weight loss between intermittent energy restriction and continuous energy restriction. These results are based on a small number of studies, and more studies are needed to confirm the effect of intermittent fasting on weight loss.
A more recent study examined the impact of alternate day fasting (i.e. eating as much as desired every other day and nothing on the days in between) on calorie intake among 60 healthy-weight and overweight individuals. After four weeks, researchers found that while fasting on alternate days, subjects ate 37% fewer calories than they reported eating before the study and lost almost eight pounds of body weight on average. The study’s control group also ate fewer calories and lost body weight during the study by following an “ad lib diet” (i.e. they could eat as much as they wanted each day). This resulted in them eating eight percent fewer calories than they reported eating prior to the study and losing almost a half-pound of body weight on average. While these results may sound promising, the promise of fasting comes with some important advice. Researchers noted that “even healthy adults should not perform alternate day fasting without consultation by clinicians to rule out adverse effects due to critical medical conditions. Importantly, although not directly assessed in this study, a wholesome and balanced diet is likely crucial to foster the beneficial effects caused by alternate day fasting.”
Beyond body weight, overall health has been discussed as a potential benefit to intermittent fasting. In this review, different types of intermittent fasting regimens and their potential health benefits and associated physiological mechanisms were examined. The health outcomes of interest were changes in weight and in metabolic parameters (e.g. blood glucose levels and total cholesterol) associated with type 2 diabetes, cardiovascular disease, and cancer. Notably, 16 human studies were included in the review, and the majority enrolled fewer than 50 participants for short periods of time, which means these results should be interpreted with caution. The authors of this review found that intermittent fasting may be an effective way to improve metabolic health for people who can safely tolerate not eating for extended stretches of time. However, they conclude by saying, “an important clinical and scientific question is whether adopting a regular, intermittent fasting regimen is a feasible and sustainable population-based strategy for promoting metabolic health.”
Our body composition (i.e. ratio of body fat mass to lean mass) has implications for our health. For example, excessive fat mass contributes to poor health and lean mass like skeletal muscle supports good health. This review examined studies done on intermittent fasting programs to determine their effectiveness at improving body composition and biomarkers associated with disease. It assessed three types of fasting regimens: alternate-day fasting, whole-day fasting and time-restricted eating. Alternate-day fasting trials of three to 12 weeks in duration appear to be effective at reducing body weight, body fat, total cholesterol and triglycerides in normal-weight, overweight, and obese humans. Whole-day fasting trials lasting 12 to 24 weeks also reduced body weight and body fat and improved blood lipids (total cholesterol and triglycerides). Importantly, research on time-restricted feeding was found to be limited, and clear conclusions could not be made. These studies were limited in duration and size. Longer, more robust studies are needed to determine if intermittent fasting is an effective way to improve body composition and biomarkers associated with health.
What else is there to consider?
Intermittent fasting may not work for everyone’s daily schedule, especially those who have abnormal or long working hours or extremely active lifestyles that require eating more frequently. Moreover, research suggesting that restrictive dieting (of any kind) leads to long-term, successful weight loss is limited and inconsistent. Additionally, adhering to a strict diet (like intermittent fasting) may increase one’s risk for developing an eating disorder.
What’s the bottom line?
Despite intermittent fasting’s popularity, it lacks enough long-term studies to thoroughly demonstrate its health benefits. This time-restricted eating style may work for some, but it may result in an unhealthy relationship with food for others. Plus, since specific foods and nutrients are not encouraged or limited by this diet, the healthfulness of an intermittent faster’s diets could vary widely. Eating styles like the Mediterranean, Nordic, MIND and DASH diets that incorporate a wide variety of healthy foods, and do not require strict monitoring or eliminating of certain foods, are more likely to be beneficial to health and easier to stick with over the long term.
This article contains contributions from Ali Webster, PhD, RD and Kris Sollid, RD.