Fasting Before Workout Boosts Metabolism in Obese, Overweight Men

By Anne Harding

November 01, 2019

NEW YORK (Reuters Health) - Obese and overweight people burn more fat and have greater improvements in insulin sensitivity when they exercise before rather than after eating, according to new findings.

"Exercise can be a powerful tool to improve health, but in order to maximise the efficacy of exercise we may need to consider the optimal time to perform exercise in relation to meals," Dr. Javier Gonzalez of the University of Bath, in the U.K., told Reuters Health by email. "Performing some exercise after an overnight fast, before breakfast seems to allow for more of the health benefits of exercise to be obtained."

He added, "This could be a strategy to consider as part of a wider preventative package for people without contraindications to fasting or to exercise."

Previous research in lean men has shown different responses to exercise training based on when nutrients are provided, Dr. Gonzalez and his team note in the Journal of Clinical Endocrinology and Metabolism, online October 19.

"In order to fully understand the potential for nutrient-exercise timings to alter metabolism, exercise-adaptations and metabolic health in individuals at increased risk of metabolic disease, there is a need to study the most relevant populations, such as individuals classified as overweight or obese," they add.

In the acute phase of the new study, the authors had 12 sedentary, overweight or obese men perform an hour of moderate-intensity cycling 90 minutes after eating a standard breakfast containing 65% carbohydrate, or before.

Plasma glucose area under the curve (AUC) and serum insulin AUC were both significantly higher with pre-breakfast exercise, and glycerol and non-esterified fatty acid (NEFA) concentrations were also higher.

In a second training phase, 30 overweight or obese sedentary men were randomized to no exercise, carbohydrate-only breakfast before exercise or exercise followed by a carb-only breakfast for six weeks.

The group who exercised before eating carbs had whole-body lipid utilization rates twice as high as those who exercised afterwards, while their whole-body carbohydrate utilization rates were lower.

"We were surprised at the magnitude of increase in some of the proteins in skeletal muscle. Based on prior work in athletes and people who are lean, we expected greater adaptations to exercise before breakfast, but were surprised by the size of some of the effects we observed," Dr. Gonzalez said.

"We need to understand if other populations also could benefit from fasting prior to exercise, as there are some differences in the capacity to burn fat between males and females, and also between people who are of South Asian origin compared to those of white European descent," he added. "We therefore are keen to understand if exercise before breakfast can have benefits for females, people of South Asian origin, and those with type 2 diabetes."

He added, "It is important to note that this strategy is probably only relevant to low- and moderate-intensity exercise. With high-intensity intervals, rates of fat burning are low regardless of prior fasting status. Therefore breakfast timing is likely to be less important for high-intensity interval training."


J Clin Endocrinol Metabol 2019.