Intermittent Fasting: A Healthy Way to Manage Your Weight?

If you’ve been looking to lose weight or if you keep up with wellness trends, you’ve probably heard of intermittent fasting. This approach to eating—structuring your meals around certain times of day—has caught on with everyone from professional athletes to software engineers, but does it actually have any health benefits?

We sat down with Blandine Laferrère, MD, PhD, a professor of medicine in the Division of Endocrinology at the Vagelos College of Physicians and Surgeons and an expert in metabolism and weight management, to learn more.

Let’s start with the basics: what exactly is intermittent fasting?

Intermittent fasting involves structuring your day (or week) with consistent, predictable eating ‘windows’ and more prolonged fasting periods. Most diet plans ask you to carefully plan what you eat and how much you eat. With intermittent fasting, the idea is to think more about when you eat and hold yourself to that schedule daily, week after week.

What are the most common intermittent fasting schedules? What factors might someone consider when choosing an intermittent fasting schedule?

One type of intermittent fasting is time-restricted eating (TRE). An example of TRE is 16/8, which means you alternate a 16-hour fast with an 8-hour eating window. Another example is 14/10, which would alternate a 14-hour fast with a 10-hour eating window. In the second case, your 10-hour eating window might be 9 AM to 7 PM.

Your schedule can depend on your work schedule, your family’s needs, or personal preference. The main thing is to be consistent, day after day, week after week. And to be clear, when we say fast, we mean fast. Only non-caloric beverages such as water, tea, or coffee, but nothing more during the fasting periods.

Could you say more about the eating window and dietary choices? Does it matter what, or how much, someone eats during their eating window?

What you eat, and how much, still matters. You want to build your diet around whole grains, fresh fruits and vegetables, and lean proteins. The Mediterranean diet is a good model, and we can all learn from it.

As for how much to eat, I tell my patients to approach a meal as they usually would. Bring an appetite and enjoy the food.

How you schedule the eating window may also be important. An eating window that starts earlier in the day may be more successful for weight management than a later one, as over 50% of daily calories are eaten in the evening. So, in some cases, what starts as a fasting/eating schedule can become a way to reduce overall calorie intake.

In your clinical experience, is intermittent fasting an effective way to manage (or lose) weight?

Experience with patients is anecdotal, so I want to be careful not to overgeneralize. In addition, we usually don’t tell our patients to change just one aspect of their behavior but instead encourage them to improve diet composition, meal pattern, and timing, as well as physical activity. So, it would be hard in the clinical setting to isolate the effect of intermittent fasting/TRE.

That said, observational studies and controlled clinical trials of TRE have shown around a 3-5% reduction in weight, with improvement of blood pressure and blood glucose. It’s not a miracle cure for overweight or obesity, but it’s not trivial, either. Previous large studies have shown that even a slight weight loss can delay the progression of prediabetes to diabetes. And animal data of TRE show prolonged life span and other benefits, sometimes even in the absence of weight loss.

It is possible, therefore, that TRE may have potential in the management of type 2 diabetes, prediabetes, high cholesterol, and hypertension. My current research focuses on the effect of TRE on glucose metabolism in older adults with prediabetes.

There was a study announced at the 2024 American Heart Association meeting in Chicago that links a 16/8 TRE schedule to a 91% higher risk of cardiovascular death. Do you have any thoughts on that study?

I’m so glad you brought this up. There are serious concerns about this study's claims, including improperly adjusting for smoking, not accounting for other important risk factors, and using an assessment of meal timing with only two days of diet recall, which is very unlikely to represent long-term eating patterns.

In addition, reporting that someone has eaten over a period of less than eight hours for a single day is not the same as that person practicing TRE. Moreover, the group that reported an eating window of less than eight hours was small and represented only 2% of the entire sample! Unfortunately, the press jumped on this study's conclusions before it was peer-reviewed.

Over 100 published peer-reviewed clinical trials have reported that TRE has either small metabolic health benefits or no metabolic effect, but none have observed serious adverse health outcomes. And in contrast to the results presented in Chicago, rigorous testing of individuals in TRE programs has shown that shorter duration eating windows actually improve cardiovascular disease risk.

Are there some people who should definitely NOT try intermittent fasting?

Yes, and thank you for asking about this. Let me first say that anyone trying intermittent fasting should first consult with their doctor. Your doctor can help you choose a sustainable schedule, and if something doesn’t feel right, they can help you tweak your program. Your doctor can also offer support and motivation if you find yourself straying from the schedule.

As for particular groups of people for whom intermittent fasting is not appropriate, there are several: children and teens under 18; women who are pregnant or breastfeeding; people with type 1 diabetes; and individuals who have a history of eating disorders.