Cutting Carbs Without Reducing Calories Can Trigger Fat-Burning

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What if you could get the metabolic benefits of fasting – without actually fasting? 

New research from the University of Surrey, published in the European Journal of Nutrition, suggests that simply cutting out carbohydrates – without reducing calories – can shift the body into a fat-burning state typically associated with fasting.

Rethinking the fasting-ketosis link

Over the past decade, intermittent fasting (IF) and ketogenic diets have dominated the nutritional landscape, each promising improved metabolic health by nudging the body into a fat-burning mode. Whether it’s through skipping meals or slashing carbs, the shared biological target is clear: deplete glucose stores, shift toward fat as fuel and trigger the production of ketones.

Traditionally, this shift – termed ketosis – has been linked to prolonged fasting or diets that contain fewer than 50 grams of carbohydrates per day. These conditions, while effective, are often extreme and difficult to maintain in the real world.

But what if there’s another way?

A growing question in nutritional science is whether it’s solely the carbohydrate restriction that results in the benefits associated with fasting and ketogenic eating – not the energy deficit. Past studies have not separated these variables cleanly. When people cut carbs, they often eat fewer calories naturally – making it difficult to know what’s really driving the shift toward fat-burning.

Fasting metabolism without the fast

To decouple these factors, the research team conducted a tightly controlled, randomized crossover trial, where 12 overweight or obese but otherwise healthy adults completed 3 1-day diet interventions, each separated by a 5-day washout. The three diet arms included:

• Normal energy-balanced diet: 55% carbohydrate, meeting full energy needs (control).
• Low-carbohydrate, energy-balanced diet (LCEB): 50g of carbs, but still meeting full energy needs.
• Low-carbohydrate, calorie-restricted diet (LC25): 50g of carbs, with a 75% energy reduction.

Each diet day ended with an overnight fast, followed by a standardized test meal the next morning: a 760 kcal liquid meal designed to mimic a full English breakfast, with fixed proportions of carbs (39%), fat (46%) and protein (15%). Researchers then tracked post-meal metabolic responses over six hours, measuring fat oxidation, ketone production, blood lipids, glucose, insulin and GLP-1 (a hormone linked to insulin secretion and appetite). Appetite scores and food intake were also recorded over the next 48 hours.

Both low-carb diets led to significantly lower post-meal triglyceride levels and signs of increased fat burning. The respiratory quotient was also significantly lower in both low-carb diets, and participants on these diets had higher levels of the ketone 3-hydroxybutyrate compared to the control – a clear signal that the body was shifting toward using fat as fuel. This effect appeared even when calories weren’t restricted.

 

 

“We found that by simply restricting carbohydrates, without enduring extreme calorie restriction, we can reap the metabolic effects associated with short-term fasting,” said corresponding author Dr. Adam Collins, an associate professor of nutrition at the University of Surrey. “This suggests that periodically reducing our carbohydrate intake could be a more accessible and sustainable way for people to manage and improve their metabolic health.”

Blood sugar levels were also slightly higher after the low-carb diets, although insulin levels didn’t change. This may be due to muscles relying more on fat for energy after a low-carb day, “which may impair muscle glycolysis and hence glucose uptake/utilization in muscle,” Collins told Technology Networks. He believes this reflects temporary glucose intolerance caused by the body’s increased use of fat, rather than a dysfunction in insulin production.

GLP-1 levels were lower after both low-carb diets compared to the control, although GLP-1 did still rise in all cases, likely due to the consistent protein intake.

“The GLP-1 response differences were a bit of a surprise given the test meal was identical on all days,” said Collins. “The slightly lower GLP-1 response following the low-carb legs does require further investigation.”

Participants reported feeling hungrier on low-carb days (up 60% for LCEB and 25% for LC25), but this did not lead to greater calorie intake during the following 2 days.

A smarter way to diet

The results suggest carbohydrate content alone may be a powerful, and often overlooked, switch for triggering metabolic benefits. Even a single day of moderate carb restriction led to changes in fat metabolism that continued into the next morning, despite participants returning to normal eating. That lingering effect could open the door to new dietary strategies.

Rather than prescribing full-time low-carb or fasting diets, this research supports a more flexible approach: intermittent low-carb days or meals. It’s a model that may be easier to maintain and more compatible with nutritional needs, especially when carbs are reintroduced strategically.

“I think having periodic low-carb could be useful, whether this be low-carb days or simply low-carb meals. This crosses over to the benefits of carbohydrate cycling. However, like anything, it is also about fuelling appropriately. If you are feeding carbs all the time and not using them, then this is going to be potentially problematic in the long term,” said Collins.

The fact that ketone levels rose even when participants ate a full day’s worth of calories also challenges the idea that ketosis requires fasting or extreme carb cutting. “People overemphasize the importance of being in ‘ketosis’,” said Collins. “Metabolic ketosis is more a reflection of the extent to which you switch to using fat as a fuel.”

One limitation of the study was that to maintain calorie balance, the LCEB diet was high in dietary fat, which means that much of the fat being burned was likely from food, not body stores.

“This means you may not see meaningful changes in body composition if this is your primary goal. Nevertheless, the metabolic benefits in terms of switching fuel utilization are comparable,” Collins added.

The findings also highlight the importance of how carbs are distributed throughout the day, not just how many grams are consumed overall.

“You could have a normal level of carbs in one meal, and other meals with very little. This may also be impactful,” Collins explained. “This is something we will continue to explore, in terms of carbohydrate meal patterning.”

“This research offers a promising new perspective on dietary interventions for metabolic health and could have an impact on managing conditions such as type 2 diabetes, heart disease and obesity,” Collins concluded.

 

Reference: Biyikoglu H, Robertson MD, Collins AL. Isolating the acute metabolic effects of carbohydrate restriction on postprandial metabolism with or without energy restriction: a crossover study. Eur J Nutr. 2025. doi: 10.1007/s00394-025-03646-5

 

About the interviewee:

Dr. Adam Collins is an associate professor in nutrition at the University of Surrey. Collins graduated with his undergraduate degree in 1996 before completing an MSc in nutrition and metabolism and a PhD focusing on energy expenditure and body composition. His current research includes exercise intensity and energy balance, intermittent fasting, meal timing and composition – specifically, the use of carbohydrate manipulation for metabolic health.