Can you really lose weight by cutting gluten from your diet, as Matt Damon claims?

When Matt Damon recently credited his weight loss to going gluten-free, it reignited a familiar debate about this divisive dietary approach.

But while "The Odyssey" star’s claims have sparked discussion, the science behind weight loss tells a far more nuanced story than simply cutting out a single protein.

Gluten is a naturally occurring protein found in grains such as wheat, barley and rye, which means it’s commonly consumed in everyday foods like bread, pasta and cereal. For most people, gluten doesn’t cause any health problems.

But for those with coeliac disease – which affects about 1% of people – avoiding it is essential. This autoimmune condition triggers an immune response to gluten, damaging the small intestine’s lining and impairing nutrient absorption.

Then there’s gluten intolerance, or non-coeliac gluten sensitivity, a condition associated with symptoms like bloating and reflux. People with this condition also commonly experience problems beyond the digestive system, including headaches and skin rashes.

Despite growing numbers of people reporting such symptoms, gluten intolerance remains hotly debated in terms of its causes and management. Currently, the only recommended approach is to adopt a gluten-free diet.

For everyone else – those without coeliac disease or gluten intolerance – avoiding gluten-rich foods may be unnecessary and potentially problematic.

Foods high in gluten, such as bread, pasta and cereal, don’t just provide carbohydrates, they’re also excellent sources of fibre and B vitamins.

Removing these foods may inadvertently contribute towards nutrient deficiencies. Yet the market for gluten-free products continues to surge, with projections suggesting it will reach US$13.7 billion (£10.2 billion) by 2030.

Given that Damon didn’t disclose any medical condition when discussing his weight loss goals, the likely explanation for his results lies in his overall diet and behaviour rather than gluten itself.

Research published in Nutrients found no significant differences between gluten-free and gluten-rich diets in body fat or body weight among healthy adults.

Mechanics, not magic

The weight loss many people experience on gluten-free diets often comes down to mechanics rather than magic. Because gluten is in many energy-dense, carbohydrate-based foods, people eliminating it typically cut out items like pizza, fast food and pasta.

This carbohydrate restriction leads to a reduction in glycogen – the stored form of carbohydrate in the human body. When glycogen is stored, water is stored alongside it.

So when glycogen levels drop, water weight follows, creating the illusion of rapid fat loss. This phenomenon explains why people often see dramatic results in the first week or two of any new diet or exercise programme.

Beyond reduced carbohydrate intake, people following gluten-free diets often shift towards consuming more naturally gluten-free whole foods. This dietary restructuring often results in fewer calories being consumed overall.

A small preliminary study, published in Frontiers of Sports and Active Living, found that adhering to a gluten-free diet for six weeks led to significant reductions in body weight compared to a control diet.

But these changes were probably the result of a calorie deficit and fluid loss, rather than any metabolic advantage from removing gluten.

There’s another factor at play. Wheat-based carbohydrates contain fermentable sugars called fructans, which are broken down by bacteria in the large intestine. This fermentation produces gas that can cause bloating, pain and changes in bowel movements.

When these foods are removed, symptoms subside, and the stomach can appear flatter – an aesthetic change that people may mistake for fat loss.

Gluten may have health benefits

Adopting a gluten-free diet that isn’t medically necessary could actually increase health risks. A large study published in the BMJ found an association between higher gluten intake and reduced heart disease risk.

Similarly, research has revealed a link between low gluten intake and increased type 2 diabetes risk.

The culprit behind these concerning links may well be the gluten-free products lining supermarket shelves. When gluten is removed from a product, it changes the texture and palatability of the food. To compensate, manufacturers add other ingredients to improve taste and consistency.

The result? Gluten-free products have been shown to contain significantly less protein, higher saturated fat, lower fibre and higher sugar than their conventional counterparts. Over time, this nutritional profile may lead to poor diets and hence poor health.

So while people may believe that going gluten-free causes weight loss, the reality is usually different. Subtle changes in diet structure and composition, alongside behavioural modifications, are typically the real reason.

∎ Writer Guy Guppy is a lecturer in performance nutrition and exercise physiology at Kingston University.