If you are interested in health, wellness, and nutrition, chances are you have heard of intermittent fasting or IF. It is also known as time-restricted eating. Proponents of IF argue that it can do wonders for one’s health- from helping to lose weight to reversing diabetes, heart disease, and slowing down the aging process. All these sound amazing, but my scientific (ie. skeptical) mind wanted evidence. I’ve gathered the evidence, and I am excited to share it with you.

So, what is intermittent fasting and why and when did it become popular?

Fasting, has been around for millennia as a way to observe a religious order, due to scarcity of food, or as a means to lose weight. Intermittent fasting is simply alternating periods of eating with periods of fasting. Intermittent fasting was propelled into the mainstream by a British doctor and journalist Dr. Michael Mosley, who wrote a book and produced a documentary on the subject. The first version of what is now known as IF was the 5:2 eating plan, with 5 days of eating normally, then severely restricting calories to 500 for the other 2 days. In the past decade many different version have come out and many diet books have been written on the subject. Fasting is a type of calorie restriction (CR). Calorie restriction has traditionally been used as a way to lose weight, and weight loss in itself (even moderate weight loss of 5% of body weight) can contribute to a variety of positive health effects.

What Do We Know About Fasting From Animal Interventions?

Scientists have been very interested in calorie restriction since the 1930’s when Dr. Clive McCay published astounding results with lab rodents subjected to caloric restriction. Calorie restriction here refers to about 30-50% reduction in calories compared with the control group, while maintaining adequate protein and micronutrients to avoid deficiencies. Calorie-restriction in lab animals has been shown to increase life span and delay aging. Many lab rodents died at a very old age without any evidence of disease, and in several studies CR has been described to be associated with significant delay in age-related degeneration and onset of chronic diseases such as diabetes, heart disease, and cancer. Much like the earlier versions of calorie restriction tested on animals, intermittent fasting has been shown to have positive cardiometabolic effects on lab animals as well, among them improved insulin sensitivity and reduced rates of stroke and diabetes. This is great news! However, we need studies on humans to determine if CR or intermittent caloric restriction is safe and beneficial.

What is The Evidence From Human Studies on Calorie Restriction and Intermittent Fasting?

Human studies that are available are limited, but growing in number. To summarize what’s known, similar to animal models, CR can markedly reduce risk factors for heart disease, such as cholesterol levels and blood pressure, as well as hormonal markers that are associated with longevity. Reduction in inflammation is also observed, which is great news as inflammation is said to be the root cause of many modern diseases. Reduction in insulin-like growth factor 1 or IFG-1 is well understood to contribute to longevity and delay in age-related disease onset, particularly cancer, is observed as well. A buzz word used in relation to intermittent fasting is autophagy. Autophagy is a process by which the body breaks down old cells and gets rid of cellular waste, while also using the products of breakdown to create new structures or in a sense renew itself. Calorie restriction or fasting delays aging and disease by increasing autophagy.

How can Intermittent Fasting Be Practiced?

Intermittent energy restriction can be practiced in different ways: alternate day fasting, fasting 2 consecutive days per week, fasting 2 separate days per week, intermittent (time-restricted eating), or fasting-mimicking diet. In a large study, 100 obese subjects were either calorie restricting every day (75% of requirement) or every other day (25% or calorie requirement on fast days and 125% every other day) or were in the control (non-intervention) group. Weight loss and cardiometabolic risk factors were similar for both intervention groups at the end of the 1-year study. People in the alternate-day fasting group found it very difficult to adhere to the 75% restriction in calories on the fasting day (easting 25% of required calories) and reported eating more than prescribed. Based on this information, this type of intermittent fasting appears to be difficult to adhere to and therefore to sustain.

Intermittent fasting or time-restricted eating, has been popularized on social media. Proponents of this method advocate for eating all the meals for the day in a shorter time window, say 10am-6pm, 12pm-6pm, or even 8am-3pm (you can see the common theme here of eating last meals of the day as early as possible). In one study of men with pre-diabetes, intervention group limited their eating window to about 6 hours early in the day, from 8am-3pm for 5 weeks, while controls ate the same number of calories over a longer period. The groups did not lose any weight which was by design of the study. But, early time-restricted groups participants, however, had improved insulin sensitivity, beta cell function (beta cells produce insulin), oxidative stress markers, and reduced blood pressure.

In summary, intermittent fasting is a promising intervention for metabolic health. A large review showed it to be an effective way to lose weight. However, there were no marked difference between the amount of weight lost on alternated-day fast, 2-days-per-week fast or even fasting 4 days per week. It appears that intermittent fasting has beneficial effects independent of the benefits of weight loss. Time-restricting eating to the early-to-middle part of the days is showing promising results. However, consuming meals in a shortened window later in the day, say 12pm-6pm is beneficial as well. Autophagy appears to be the primary beneficial mechanism. When our bodies are not digesting food, tissues (including brain) can “clean house”.

More research is coming out on intermittent fasting, and it is being shown to have positive metabolic effects on the body and can be used in conjunction with an overall healthy diet.