As you might already know, especially if you’re someone who follows healthy eating and weight loss trends, intermittent fasting (IF) is a diet that calls for cycling between periods of eating and periods of fasting. The idea is that by restricting when you eat, you control the number of calories you’re consuming and, hopefully, benefit from the hormonal and cellular benefits fasting may bring about, like lowering your cholesterol, improving heart health, and ultimately helping you live longer. These claims are based on studies that have looked at the effects of IF on different markers, but they don’t tell the whole story. As an epidemiologist who studies the effects of fasting on longevity and health, I’ve spent much of my career trying to determine just how much IF can affect our health.
Let’s start at the beginning by looking at exactly what intermittent fasting is and why researchers are studying its effect on our health.
Fasting for religious, social, cultural, or political reasons is a practice that’s been observed for many thousands of years in a variety of societies and civilizations. Intermittent fasting, however, is a specific kind of caloric restriction that, at its core, really just means eating for a period and then not eating for another period of time. There’s no one way to practice IF, but typically it comes down to restricting food intake through one of two ways—cyclical day-long fasting or time-restricted eating.
Cyclical fasting includes protocols like the 5:2 diet, where you eat normally for five days and follow a fast of some kind for two days, and alternate-day fasting, where you fast every other day. Time-restricted fasting just means limiting the hours in the day when you eat to a specific window, like doing all your eating for the day between 10 A.M. and 6 P.M. and fasting the other 16 hours.
IF as a research topic grew mostly out of animal and laboratory studies of caloric restriction beginning in the early 2000s. In those animal studies, two primary biological mechanisms were found to be involved in creating health benefits from IF. One is that IF may induce ketosis (which you may be familiar with thanks to the ketogenic diet), in which the body draws on energy from stored fats instead of blood sugar (which is typically the body’s first go-to when it needs energy). The other mechanism, which, again has been studied in animal research, is that cells and tissues may enter a phase of rest, renewal, and rejuvenation. This could reduce chronic disease risk and increase longevity. More on these later, but the gist is that the science on IF is still very young.
So what does the science actually say?
In 2015, two cardiologist colleagues and I conducted a review of scientific publications on intermittent fasting. We found that clinical research studies of fasting with robust designs and high levels of clinical evidence were few and far between. With that review, we wanted to determine where the IF science was in terms of clinically good or excellent research that had been performed up to that point. When I say “good” research, I mean studies that have been designed and conducted in a rigorous enough manner that their results could be used as a basis for changing or guiding health practices. In particular, our aim was to find studies that were randomized clinical trials of fasting that used a particular kind of control group, or studies in which the research endpoint was a clinical outcome (like a diagnosis of diabetes). We found little that met the high standard of clinical trial research that could be used in developing guidelines around the use of IF for health improvement. (Since our review, just three more good–quality trials have been published.)
If there’s little solid science on the benefits of IF for health, why are there tons of enthusiastic claims all over the internet? One thing to keep in mind when looking at IF’s so-called proven benefits is the kind of studies that showed such benefits. The hype around IF, along with many of the claims associated with it, are primarily based on basic animal or laboratory research and on human pilot studies. The basic studies are often very good, but they only tell us what kinds of human studies we should conduct. Human—not animal—studies should be used to guide human health practices. The great majority of what we know about IF today is from animal and other laboratory research. Human pilot studies (which are preliminary studies in which the feasibility of a larger scale study is tested) have provided some valuable data showing that we should continue to do IF research, but, as the papers and reports themselves state, we need higher levels of evidence to provide actionable knowledge and modify nutrition guidelines. Let’s look at what we know so far.
IF and weight loss
IF has been demonstrated in a few good quality human studies to reduce weight to a similar extent as traditional caloric restriction-based dieting, but it has not been shown to be any more effective. One study—one of the good quality ones I mentioned above—found that “Alternate-day fasting did not produce superior adherence, weight loss, weight maintenance, or cardioprotection vs daily calorie restriction.” And the other (also one of the good-quality ones I mentioned) concluded that, “Three (fasting) cycles reduced body weight, trunk, and total body fat…” along with a myriad of other factors that need to be explored further in future clinical trials. IF regimens in those and other studies were fairly intense: fasting one full day every other day, fasting for five consecutive days once per month, and fasting two non-consecutive days per week. Participants found these regimens challenging to sustain.
Many intermittent fasting proponents claim that IF further triggers fat loss by altering hormone levels, but experts say that these hormone changes aren’t significant enough to cause weight loss in this way. “There are hormonal changes with intermittent fasting, but none are so profound to cause a clinical significance,” Deena Adimoolam, M.D., assistant professor of endocrinology at the Icahn School of Medicine at Mount Sinai, tells SELF. “For example, many intermittent fasting protocols speak of a rise in growth hormone levels triggering fat loss. Growth hormone can lead to an increase in lean body mass; however, significant amounts of growth hormone are necessary to achieve this, and these levels are not achieved with intermittent fasting.” Again, we need more research on this topic to draw any solid conclusions.
What does occur, however, during times of extended fasting, is that blood sugar and insulin levels drastically decrease, Caroline Apovian, M.D., director of the Center for Nutrition and Weight Management at Boston Medical Center, tells SELF. The body turns to glycogen—carbohydrates that are stored in the liver and muscles—for fuel. When no glycogen is available for energy, the body does go into ketosis, turning to fat for energy. However, with intermittent fasting, ketosis tends to be brief, if it even occurs at all because of the short time you fast (relative to how long it takes ketosis to begin) before you eat again. Although the actual timing of starting and ending ketosis will vary from person to person, for most people, most IF protocols are unlikely to result in ketosis.
IF and diabetes
IF has also been examined for effects on various body systems, including for metabolic, cognitive, and cardiovascular benefits. One good study among people with diabetes showed that IF was as effective as a conventional weight loss diet (but not better than) at controlling hemoglobin A1c, which is a marker of average blood sugar over the last few months.
The study of IF as a management technique for hemoglobin A1c among people with diagnosed diabetes showed that IF is an option that may work in place of other diet techniques used for blood sugar management. But it’s important to note that IF doesn’t replace medications prescribed to treat or manage diabetes. Using IF to manage a chronic disease like diabetes is a secondary prevention approach in which you are trying to avoid the worsening or progression of the disease and should be done under the care of a physician.
IF and heart health
Here’s where we delve into the research that I and my colleagues have been studying for years. First, some background: What first got me interested in studying IF was a series of studies about 40 years ago from the University of Utah, which showed that death rates in Utah from most cancers and from heart diseases were substantially lower than those in the rest of the U.S. This was ascribed to the low smoking rate in Utah. The idea was bolstered by a UCLA study around the same time that reported that religiously observant members of the Church of Jesus Christ of Latter-day Saints in California had greater longevity than other Californians, with a greater life expectancy by more than seven years. In 1998, I took a course on epidemiology taught by one of the Utah researchers. I couldn’t find any academic research on IF back then, but fasting (a regular and common practice in the Church of Jesus Christ of Latter-day Saints) stood out as a potential heart-protective behavior that had not yet been scientifically investigated.
In 2001, I began my Ph.D. program in genetic epidemiology and also continued to work at Intermountain Healthcare, where I had been since 1996. This position gave me the chance to do some medical research and in 2002 I proposed the idea of investigating whether some factor beyond smoking was impacting heart disease among the patients my cardiologist colleagues and I served. Using rich data available from Intermountain’s electronic health records (which had health data dating back to the 1960s), we indeed confirmed that another factor other than smoking was involved. This led to a prospective study in 2004 examining whether fasting (mostly for religious purposes) influenced coronary artery disease. In that study, we asked people: “Do you routinely abstain from food and drink (i.e., fast) for extended periods of time?”
The results of that 2008 study were profound. Among those who reported fasting routinely, the risk of being diagnosed with coronary disease was substantially lower than among those who did not fast, even when taking into account many other factors and behaviors. The risk of receiving a diagnosis of diabetes was also lower among fasters, although that had not been our primary hypothesis. To test whether the diabetes finding was valid, we conducted another study in 2012 that asked the same fasting question but investigated primarily if it were associated with diabetes. Diabetes risk was found to be considerably lower in those who fasted routinely. As we noted in the study, the lower risk of cardiovascular disease could have occurred either because of fasting or because of a behavior arising from fasting. For example, it’s possible that fasting improves a person’s self-control over appetite and desires, which could mean lower daily caloric intake. And this study did not look at caloric intake, so calories or some other dietary factor (vitamins, nutrients, etc.) could account for the finding, but adjustments in the study for many other factors did not affect the findings. In other words, more research is needed to learn more about the causal relationship between these findings and diet or dietary discipline.
The average fasting participant in my 2008 and 2012 studies had fasted about one day per month for 45 years (their age averaged 65 years). This indicated that some of the results we’re seeing from IF were because IF had been undertaken as as a long-term lifestyle, as opposed to a short-term solution for, say, weight loss. It had taken many decades for the coronary and diabetes benefits to become apparent. That wasn’t a bad thing. Since coronary disease, diabetes, dementia, and other chronic non-infectious diseases usually take decades to develop, having a small chronic protection via an IF lifestyle across that time could (and we anticipate it would) prevent those diseases from developing and silently progressing.
IF’s other possible benefits
Except for weight loss, research looking at the effect of IF on humans has been limited to scientific tests that look at effects secondary to weight loss or evaluating IF without a parallel control group that would allow researchers to control for and isolate any variables that could be bringing about a particular result. The possible benefits of IF from these limited pilot studies include improved blood pressure, cholesterol levels, markers of cognition/dementia, insulin, mood, and quality of life, and that IF may reduce depression and insulin resistance. Other benefits may also exist. No study has examined these outcomes, though, as a primary study question or hypothesis, so it’s unclear whether the non-weight loss effects in pilot studies are real and replicable, or are chance events.
So why aren’t there more studies on IF when its potential to affect our health seems promising?
Weight loss, although an important area of research, is not what I, as someone who studies intermittent fasting and cardiovascular disease, think is the most interesting or potentially impactful result of IF. Preventing chronic diseases, increasing longevity, and improving the quality of health at older ages seem to be potential results that IF may be able to deliver. IF may be able to provide benefits regardless of whether weight loss occurs. I don’t want the public or the scientific community to become so focused on the fad diet aspect of IF that we fail to explore whether IF is a dietary intervention that could preserve and regenerate human health. Unfortunately, research on non-weight-loss outcomes have not been examined as carefully as they should be for something that has created the social buzz that IF has. Weight loss is certainly easier to sell to grant funders and the public, partly because it can been seen and felt as it occurs. Further, research into nutritional influences on health are notoriously difficult to conduct because of all of the combinations of foods we can eat—or in the case of IF, don’t eat—and the challenge of measuring and accounting for all those combinations, so it is not surprising that the hype may not meet the reality of what IF can do.
But it’s often even more difficult to study whether the focus on using IF can change cardiovascular health outcomes and not just risk factors because the research would have to be carried out over the very long term and in a population that meets multiple cardiovascular health criteria (e.g., weight, blood pressure, and cholesterol). Even with the risk factors, nobody can feel when their cholesterol changes and most people cannot feel it when their blood pressure is high, for example. But cholesterol and blood pressure in and of themselves don’t matter the most as health outcomes. As healthcare providers and researchers, what we really care about is what tends to happen when people who are classified as overweight also have high cholesterol and/or high blood pressure. A higher proportion of people fitting this description develop coronary disease, diabetes, and other heart, lung, and related diseases compared to people who have normal levels of those risk factors (although it’s important to note that not everyone with a higher weight or a higher cholesterol or blood pressure will develop those diseases). Unfortunately, a study of disease outcomes among people starting IF today will take decades and be very expensive, so the epidemiologic evidence we have is likely to be the only outcomes data for some time.
No matter what, it’s important to remember that extremely restrictive diets have a notoriously high fail rate and usually aren’t very sustainable.
None of the weight or health effects of any diet, including IF, matter if you can’t stick with the eating protocol over the long-term. “There’s no one-size-fits-all diet that is best for everyone,” Donald K. Layman, professor emeritus of food science and human nutrition at the University of Illinois, tells SELF. “This is one strategy among many.”
If you want to try IF, the first thing to do would be to talk to your doctor and make sure it’s something that they feel would be healthy, both physically and psychologically, for you to try. You should also do it under the supervision of your doctor or a registered dietitian. I would recommend that anyone with a history of disordered eating not try IF. The duration and frequency of the IF protocol that you try, and the length of time that you use it (a few months vs. many years), may vary based on your current health status. For example, someone who is a younger adult and/or apparently healthy who wants to lose weight or establish a low lifetime risk of chronic disease is unlikely to need an intense IF regimen. It is unknown how IF affects the menstrual cycle, so tracking how it affects you is important. You should make any IF regimen into a sustainable lifestyle that you won’t need to stop abruptly (typically when a dietary practice seems difficult to stick with, it’s an indication that it’s too rigid or prescriptive).
Considering that IF is one of the more extreme ways to eat and live, if you’re curious about it, Adimoolam recommends talking with your doctor to make sure it’s a fit for you, physically and emotionally. If it’s weight loss you’re interested in, keep in mind when trying to decide how to do it that research shows that weight-loss diets do tend to fail. Instead, look for strategies that are not extreme, do not require deprivation of any kind, and that take your mental health into consideration.
The bottom line: IF is not a cure-all, and when it comes to weight loss, it’s not better than standard weight loss diets.
It’s not for everyone, and is one among many preventive methods that may improve your health and longevity. Other methods include not smoking, eating a healthy low-salt and low-cholesterol diet, and engaging in routine physical activity. These other methods of disease prevention have much more human scientific evidence behind them than IF does. That said, IF shows great promise as a health intervention and can move from alternative medicine into practical mainstream use if science and practice are conducted wisely. IF is making that transition, but we need additional research before we can create clinical statements that confidently inform nutrition guidelines and individual behavior.
In other words, scientists and researchers should be way more excited about the potential of IF than the average person who, as I mentioned above, can engage in more accessible (and more proven) healthy behaviors.
Additional reporting by K. Aleisha Fetters
Benjamin D. Horne, Ph.D., M.STAT., M.P.H., F.A.H.A., F.A.C.C., is the director of cardiovascular and genetic epidemiology at the Intermountain Heart Institute in Salt Lake City and holds an adjunct appointment in the Department of Biomedical Informatics at the University of Utah. Horne pursues research interests in population health and precision medicine through the unique resources and characteristics of Intermountain and its service area. These include developing and implementing clinical decision tools that maximize both scientific validity and clinical feasibility for personalizing medical care. They also include improving health across large populations by discovering the human health effects of intermittent fasting, evaluating genetic risk factors for associations with heart disease outcomes, and studying the influences of short-term air pollution elevations on health.