Intermittent fasting (IF) is a diet that calls for cycling between periods of eating and periods of fasting, as you may already be aware, especially if you follow a healthy eating and weight loss trends.
The idea is that by restricting when you eat, you can regulate the number of calories you consume and, presumably, benefit from the hormonal and cellular benefits of fasting, such as lower cholesterol, improved heart health, and ultimately helping you live longer. These assertions are based on research that examined the impact of IF on various indicators, but they do not convey the entire story. As an epidemiologist researching the impacts of fasting on longevity and health, I’ve spent a large portion of my career attempting to determine how much IF can alter our health.
Let’s start by defining intermittent fasting and why researchers are interested in its impact on human health.
Fasting for religious, social, cultural, or political purposes has been practiced for thousands of years in many nations and civilizations. On the other hand, intermittent fasting is a type of calorie restriction that, at its heart, implies eating for some time and then not eating for another period. There is no one way to practice IF, although it often involves restricting food intake in one of two ways: cyclical day-long fasting or time-restricted eating.
Cyclical fasting regimens include the 5:2 diet, in which you normally eat for five days and then fast for two days, and alternate-day fasting, in which you fast every other day. Time-restricted fasting means limiting the hours in the day when you eat to a specified window, such as eating all of your meals between 10 a.m. and 6 p.m. and fasting the remaining 16 hours.
In the early 2000s, animal and laboratory investigations of caloric restriction sparked interest in IF as a study issue. Two key biological processes were discovered to be involved in creating health advantages by IF in those animal experiments. One is that IF may cause ketosis (which you may be familiar with owing to the ketogenic diet), a state in which the body pulls energy from stored fats rather than blood sugar (which is generally the body’s first go-to source of energy when it needs it). Another process examined in the animal study is that cells and tissues can enter a state of rest, renewal, and rejuvenation. This could minimize the risk of chronic disease and prolong longevity. More on this later, but the main point is that the science of IF is still in its infancy.
So, what exactly does science say?
In 2015, two cardiologist colleagues and I did a literature review on intermittent fasting. We discovered that clinical research studies on fasting with strong designs and high levels of clinical evidence were scarce. With that review, we hoped to assess where the IF science stood in terms of clinically good or exceptional research completed to that date. When I say “good” research, I mean studies conceived and carried out so that their findings can be used to change or guide health practices. Our specific goal was to discover studies that were randomized clinical trials of fasting with a specific type of control group or studies where the study endpoint was a clinical result (like a diagnosis of diabetes). Unfortunately, we discovered nothing that reached the high quality of clinical trial research and could be utilized to generate guidelines for IF for health improvement.
Why are there so many enthusiastic claims about the benefits of IF for health if there is so little reliable science? When considering IF’s so-called established benefits, consider the type of studies that demonstrated such benefits. The hoopla around IF, as well as many of the claims associated with it, are based mostly on basic animal or laboratory research, as well as human pilot studies. Basic studies are frequently excellent, but they merely inform us what types of human studies we should perform. Human research, rather than animal studies, should inform human health practices. Animal and other laboratory studies have provided the vast majority of what we know about IF today. In addition, human pilot studies (which test the feasibility of a larger-scale study) have provided valuable data indicating that we should continue doing IF research. Still, as the papers and reports themselves state, we need stronger evidence to provide actionable knowledge and modify nutrition guidelines. Let’s take a look at what we’ve learned thus far.