Early research suggests that intermittent fasting may help some people lose weight. However, scientists are still studying how intermittent fasting affects people with diabetes. Krista Varady, PhD, professor of nutrition at the University of Illinois, Chicago, discusses some of the early findings on intermittent fasting and how health care professionals can advise their patients who are considering whether to try an intermittent fasting eating plan.

Q: What is intermittent fasting?

A: Intermittent fasting is defined as a period of eating followed by a period of not eating. It’s an umbrella term for two major types of diets.

The most researched type of intermittent fasting is time-restricted eating, which is when people eat within a shortened window of time each day. We know from observational studies that people typically eat within a 12- to 14-hour window each day. Time-restricted eating is shaving off hours of eating. Most people who are doing time-restricted eating eat within a 6- to 8-hour window instead of that longer 14-hour window. During the eating window, you don’t have to count calories; you just pick the time when your eating window starts and stops.

Most people pick 12:00 p.m. to 8:00 p.m. because they like to include the dinnertime meal. It’s hard for people to miss dinner because that’s such a social eating time. In this case, you would start eating at 12:00 p.m., eat what you want without tracking, stop eating at 8:00 p.m., and then from 8:00 p.m. until noon the next day, you fast with water or calorie-free beverages, like black coffee or tea.

The second major type is alternate day fasting. Alternate day fasting typically involves a fast day, where people can either have only water or 500 to 600 calories in a day. The fast day alternates with a feast day, where people can eat what they want, without tracking calories, carbs, or anything.

Q: What is the state of research on intermittent fasting? How extensively has it been studied in people with type 2 diabetes or prediabetes?

A: I started studying intermittent fasting about 20 years ago, and at that time, maybe one or two studies were published per year. Around 2018, time-restricted eating really took off. That’s when all the research started coming out. There’s probably a new clinical trial published once or twice a month at this point. Most of the data now is in time-restricted eating, but they’re short studies—usually 2 to 3 months—and generally, just in people with obesity.

My research group published a long-term 12-month study last summer in Annals of Internal Medicine, and I think that’s the longest study of time-restricted eating. We compared time-restricted eating to daily calorie restriction, just to see if the intermittent fasting regimen was easier for people to stick with and if they would lose more weight. Both diets resulted in about 5% weight loss over 12 months. So, I don’t think intermittent fasting is better than anything else; I think it’s just another option for people who are tired of tracking calories.

A few intermittent fasting studies have included people with prediabetes. I think the study of time-restricted eating in people with type 2 diabetes that my research group published is probably the most comprehensive one on type 2 diabetes. Two other controlled trials have been done, but both were very short. Overall, the data are promising on intermittent fasting in type 2 diabetes. We just need much longer studies with larger sample sizes. My research group also just started working with endocrinologists to study intermittent fasting in type 1 diabetes. Hopefully that study will be done in about a year or so.

Q: What are some of the benefits of intermittent fasting for people with type 2 diabetes or prediabetes?

A: In general, when people eat within a 6- to 8-hour window, they naturally reduce their energy intake. I think that’s the coolest thing about time-restricted eating. Even though you get to eat what you want in that 8-hour window, you naturally cut out about 500 calories because you’re eating in a shortened timeframe. This is what people strive for when they’re counting calories every day. I don’t think it’s the fasting. I think they’re just eating less, so they’re losing weight. You don’t really get significant increases in ketones after fasting for 16 hours.

Weight loss has a downstream effect. In some people, it can lower blood pressure. We also sometimes see improvements in LDL cholesterol, but rarely. I think the main benefit is improved blood glucose regulation. With most studies, we see reductions in fasting insulin and improvements in insulin resistance in people with obesity and prediabetes.

In people with type 2 diabetes, we’re seeing reductions in the major marker of glucose regulation, A1C. Some studies have shown that people with type 2 diabetes can experience up to a full-point reduction in A1C within 3 to 6 months. Some studies in which people wear continuous glucose monitors have found that people spend more time in their target glucose range.

I think the issue is that so many of the studies are small sample sizes. We need longer studies with bigger sample sizes to really get to the bottom of things.

Q: What are some of the risks associated with intermittent fasting in people with diabetes?

A: I must admit, I was kind of scared to run a study in type 2 diabetes. I was worried that in these extended fasting periods, people would become hypoglycemic. But we didn’t find that in our study. In the few other studies out there, they’re also not finding an increased risk of hypoglycemia when they compare the intermittent fasting group to the control group. So that’s been interesting.

I think people are also worried that eating disorders may develop. That’s a big question I get about intermittent fasting in general. First, almost all studies screen out people who have a history of eating disorders. I don’t think people with a history of eating disorders should do fasting regimens. We start with people who don’t have a history of eating disorders, and we give them questionnaires throughout the study. We don’t find that their risk of eating disorders or their likelihood of developing eating disorders goes up.

At the same time, certain people probably shouldn’t do intermittent fasting, like pregnant women since no studies have been conducted in this group just yet. Also, people do lose a bit of lean mass; that’s just a natural result of weight loss. So, I‘d be a little worried about applying it in older populations because of the risk of excessive muscle mass loss. We need more studies in older adults too.

Q: Would intermittent fasting on its own be enough to affect health for an extended time, or are other weight-loss measures needed for people who have diabetes and obesity?

A: We’ve shown that people still maintain a reduced calorie intake when they follow intermittent fasting for up to 12 months. Our 12-month study was 6 months of weight loss and 6 months of weight maintenance. We wanted to see if time-restricted eating can be used for keeping the weight off. We found that participants could maintain their weight loss when they stuck to an 8- to 10-hour eating window. But, I think we need 2- to 3-year studies to figure out if time-restricted eating can affect health for an extended period of time.

I don’t think you need to combine it with calorie counting. That’s the beauty of it. You don’t need a lot of nutrition literacy, and it’s easy to incorporate into your lifestyle. It’s also completely free, and it doesn’t require people to buy expensive food products. I think that’s why people like it and why people stick with it.

Our studies show that people are able to stick with time-restricted eating as well as, or slightly better than, people are able to stick with calorie counting. On average, people doing time-restricted eating stick to eating within a 6- to 8-hour window on 6 out of 7 days per week. For people who count calories, about 60% of them stick to their calorie goals during the study. Comparing adherence to the two diets can be difficult, because adherence is measured differently for each diet (i.e., counting time versus counting calories).

Q: How can health care professionals advise their patients with type 2 diabetes or prediabetes who are interested in trying intermittent fasting for weight loss? Is intermittent fasting more appropriate for some patients than others?

A: The literature is relatively new, but it is very promising. Three or four different studies all show similar weight loss and similar improvements in A1C. I think it does work.

You’d have to ask the person if they’re interested, because some people really like calorie counting, and other people like carbohydrate counting. See if it’s something that could fit into the patient’s lifestyle. And if they’re interested, you could tell them that it is simple and free—people really like that.

But the key thing with doing intermittent fasting in people with type 2 diabetes is that they must work closely with their doctors to adjust medications, specifically sulfonylureas and insulin. Medications must be monitored and adjusted because the patient is eating in a new pattern, which now includes fasting windows. We have all the specifics of how to adjust medications based on the patient’s A1C level at baseline in our article on time-restricted eating in people with type 2 diabetes.

How do you advise your patients about intermittent fasting? Share in the comments below.