Birds eye view of the Charles Perkins Centre staircase at the University of Sydney

Intermittent fasting leads to weight loss, not improved health

10 February 2023
Health claims not as significant as once thought.
New research published in Aging Biology finds weight loss experienced by people on intermittent fasting diets, such as the popular 5:2 diet, does not bring the same health benefits as other weight loss programs.
A round plate with a salad sits on a light green background. The salad is arranged in a wedge to take up a third of the plate, borded by carrots and cucumbers to mimic the hands of a clock. Sprigs of rosemary make up the hour markers around the edge of the rest of the plate to signify intermittent fasting and time restrictive diets.

Intermittent fasting can lead to weight loss, but the foods you do eat should be nutritious to maximise the health benefits of the diet, says Professor Fontana. Image credit: Adobe Stock.

Intermittent fasting, such as the 5:2 diet has surged in popularity in recent years as a way to lose weight and improve health.

However, new research published in the prestigious journal Aging Biology has found while these diets involving intermittent fasting could potentially help people lose weight , they lack the beneficial health effects of other forms of weight loss such as improved insulin resistance and reduced inflammation.

“There have been a lot of claims that these diets are associated with other benefits alongside weight loss, including increased health and longevity,” says Professor Fontana, the Leonard P. Ullman Chair in Translational Metabolic Health at the University of Sydney's Charles Perkins Centre and Faculty of Medicine and Health.

“Our randomised clinical trial was designed to test these claims by measuring the effects of intermittent fasting-induced weight loss on inflammatory, metabolic, and molecular pathways of healthy aging.”

The clinical trial involved 50 participants aged 30 to 65 placed into an intermittent fasting group and a control group who consumed their usual diet. After six months, blood samples were taken to measure and compare markers of ageing and inflammation before and after weight loss.

If you want to experience the full benefits of weight loss, it’s not just about the quantities of calories you ingest or when, but what nutrients they come with.
Professor Luigi Fontana

The study found participants who underwent intermittent fasting did not have the same beneficial anti-inflammatory response seen in other forms of weight loss.

The study also showed there was some level of improved insulin sensitivity, but it was not a ‘clinically significant’ level.

Professor Fontana says health benefits claims around intermittent fasting were based on studies in animal models

“Early studies on this diet were done in animal models which showed the same pro-longevity benefits we expect of other calorie restrictive diets,” says Professor Fontana, who is also the Scientific Director of the Charles Perkins Centre Royal Prince Alfred Clinic.

Previously, studies on intermittent fasting in animal models found lowered risk of cardiovascular disease, diabetes, inflammation and other illnesses.

“Our research shows this isn’t the same in humans, who do not have the same response to limiting calories on alternate days when associated with usual diets.”

Three multi-coloured kettle bells on a turquoise background

Image credit: Pixabay

Intermittent fasting benefits lower than expected

Intermittent fasting restricts calorie intake by limiting the times in which you eat. There are several diets that use the principles of intermittent fasting.

The popular 5:2 diet, for example, involves eating as normal for five days of the week and only limiting calorie intake on two days.

The results of clinical trials show that many of the health benefits claimed by intermittent fasting either lack data to back them or were significantly lower than expected.

“This study shows that, unlike with chronic daily calorie restriction with optimal nutrition, a similar eight percent weight loss induced by intermittent fasting does not reduce markers of inflammation, such as C-reactive protein or any other circulating inflammatory cytokine. We also found that intermittent fasting caused a statistically significant but very small improvement in some insulin sensitivity indexes.

“This is consistent with the results of recent randomised trials of alternate-day fasting showing no improvements in either markers of inflammation or insulin sensitivity.”

Not all calories are the same

This study adds to an emerging concept that, from a metabolic point of view, not all calories are the same or have the same effect on the body, says Professor Fontana.

“In contrast to intermittent fasting, calorie restriction with optimal nutrition and exercise training have a massive beneficial effect on insulin sensitivity and glucose tolerance.”

This means that people looking to experience the full benefits of weight loss, such as a lowered risk of cardiovascular disease, diabetes or inflammation, should consider the nutrient balance of their diet and not just the calorie content.

“While intermittent fasting clearly is a powerful instrument to lose weight, the quality of diet during non-fasting days and the amount of exercise people get are crucial factors in maintaining or improving health and wellbeing,” says Professor Fontana.

“If you want to experience the full benefits of weight loss, it’s not just about the quantities of calories you ingest or when, but what nutrients they come with.”

Declaration: Professor Fontana has no financial conflicts of interest to disclose. Co-authors RDH and RAB may receive royalty income based on the CompBIo method developed by RDH and RAB and licensed by Washington University to PercayAI. All the other authors have no financial conflicts of interest to disclose.

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