What science says about intermittent fasting

What you need to know before trying intermittent fasting. Photo / 123RF

Many of my friends have been trying intermittent fasting for weight loss. Does it work, and is it better than other diets?

Intermittent fasting has been shown to lead to some weight loss in several studies. But it might not work better than other dieting practices.

When scientists compared intermittent fasting to simply eating fewer calories throughout the day, they found that intermittent fasting wasn’t more effective in helping people shed pounds.

Conventional dieting focuses on what you eat, but intermittent fasting focuses on the time you eat it – within an eight-hour window per day, for example. Because intermittent fasting doesn’t typically require monitoring calories or changing what you eat, it can feel easier to stick with.

Interest in the practice took off in the last decade or so, after experiments in animals showed that eating restricted to certain times had a profound impact on metabolic health and the microbiome. But in people, the weight-loss benefits linked to intermittent fasting may be because they took in less food in general – up to 550 fewer calories per day.

Both intermittent fasting and overall calorie restriction approaches can result in weight loss for a few months, but they are very hard to sustain; studies have shown that some of that weight comes back after a year, regardless of the approach.

Given all these factors, intermittent fasting is not my go-to recommendation for healthy eating. Instead, I suggest the Mediterranean diet, which has been better studied for improving your heart health and lowering the risk of Type 2 diabetes. Because of the diet’s higher fibre and healthier fat intake, you may lose a bit of weight – even if you aren’t thinking about cutting calories.

How does intermittent fasting work?

Intermittent fasting can take on many forms, each with potentially different impacts on our health. There are two common variations:

The 5:2 approach

In this method, people each week severely restrict calories by at least 75% for two usually non-consecutive days, and then eat without restrictions for five days.

A striking 2024 study published in JAMA Network Open looked at 5:2 intermittent fasting for overweight or obese adults with newly diagnosed Type 2 diabetes. Researchers found that intermittent fasting led to more weight loss (on average 21 pounds after 16 weeks) compared to eating as usual and taking prescription drugs for diabetes. It also led to greater improvements in HbA1c, a long-term measure of blood sugar levels.

But it’s important to know that in this study, the intermittent fasting group consumed one prepackaged low-energy meal replacement per day that the control group on diabetes medication did not. That means the control group was likely consuming more calories.

Time-restricted eating

This approach involves fasting for a part of the day – say, 16 hours of a 24-hour cycle – and then eating only within the remaining window.

One large study from last year found that time-restricted eating was just as effective as calorie-restricted diets for weight loss – about 10-12 pounds in both groups after 12 months. This study was notable because it included participants from a diverse American population.

But there are also potential risks: a widely publicised 2024 abstract presented at an American Heart Association meeting indicated that an eight-hour time-restricted eating pattern was linked to a 91% higher risk of cardiovascular death over a median of eight years.

Most scientists would agree that this study raises questions but is not conclusive. To date, a full study has not been published in a peer-reviewed journal.

That said, a lifelong habit of skipping breakfast, which often occurs with time-restricted eating, has been linked in multiple studies to a higher risk of death from cardiovascular disease. These studies can’t fully exclude the possibility that people who skip breakfast engage in other less healthy activities. But other benefits of eating breakfast, such as improved work performance and an optimised circadian rhythm in the gut, are worth considering.

So my advice to intermittent-fasters – and everyone else – would be to eat a healthy breakfast every day and avoid late-night meals. The evidence shows that late-night snacking and eating impacts how we burn calories and store fat, putting us at greater risk of obesity.

Are there downsides to intermittent fasting?

Many of my patients who try intermittent fasting suffer from hanger – much like with any diet – and can also experience headaches or fatigue, at least in the beginning.

It’s also important to consider how this approach can impact your social life. Meals are often what brings us together, and intermittent fasting can affect that. The risks and benefits are worth considering carefully.

What I want my patients to know

Intermittent fasting has become popular as a lifestyle choice even among non-dieters: limited studies in people have found that, for instance, intermittent fasting can improve insulin sensitivity and blood pressure regardless of weight changes.

But you should always run any big changes in your diet by your physician. For example, I wouldn’t advise intermittent fasting for people with certain medical conditions – such as diabetes, which makes people more prone to fluctuations in blood sugar when skipping meals.