Exercise on an empty stomach to burn more fat

It is now well established that exercise in all its forms improves overall health. In addition to increasing cardiorespiratory capacity, regular exercise improves insulin sensitivity and reduces insulin secretion after meals. However, each individual’s response to similar exercises is very variable: some people become fitter or lose more weight or stabilize their blood sugar more than others. One of the factors that could be important is the timing of meals and exercise sessions. Muscles use energy in the form of sugars and fats, which can come from the last meal or from reserves in the body when fasting. The accumulation of too much fat in the muscles is problematic for health, because the fat-engorged muscles do not respond well to insulin, a hormone that stimulates the absorption of glucose by muscle, adipose and liver cells. Therefore, excess fat in the muscles can contribute to insulin resistance, hyperglycemia and increased risks of type 2 diabetes and other metabolic imbalances.

In a randomized controlled study, an international team tested the effect of the timing of meals on the metabolic benefits associated with exercise. Thirty sedentary and overweight or obese men were divided into three groups (see figure below): a control group that continued to live normally and two other groups that did supervised exercise in the morning (treadmill running), three times a week for six weeks, without breakfast; the second group ingested a vanilla-flavoured drink containing 20% sugar two hours before each exercise session, whereas the third group ingested a vanilla-flavoured placebo beverage containing water and no calories. After each morning exercise session, participants in both groups drank the beverage they had not received prior to the session. This means that all the runners ingested the same number of calories and did the same amount of exercise, only the timing of calorie consumption differed, i.e. before or after the exercise session.

Figure. Protocol schematic for the training study.  Adapted from Edinburgh et al., 2019.

The study was randomized and controlled, so participants did not know what type of drink they ingested before and after exercising. 83% of participants reported that they could not detect differences between the sugary and placebo drinks or were unable to identify which beverage contained sugar. It should be noted that the sugar used here, maltodextrin (partially hydrolyzed starch), has a very low sweetening power and is therefore difficult to detect.

Blood samples and biopsies of a muscle located in the thigh (vastus lateralis) were taken before and after the intervention in order to measure different metabolites and proteins of interest. Glucose, glycerol, triglycerides, HDL and LDL cholesterol, insulin, C-peptide, and fatty acids were measured in the blood, and phospholipid composition, protein content involved in glucose transport, insulin signalling and lipid metabolism were measured in the vastus lateralis muscle samples.

Not surprisingly, the control group did not improve their physical fitness or insulin sensitivity during these six weeks. On the contrary, the other two groups who exercised saw their fitness improved and their waistline decreased, although only a few of the participants lost weight.

The most striking finding of the study was that participants who exercised on an empty stomach “burned” twice as much fat as those who consumed a sugary drink before the exercise session. Yet participants in both groups who exercised expended the same number of calories.

Participants who ran on an empty stomach also saw their insulin sensitivity improve further and their muscles synthesized greater amounts of certain proteins (AMP-activated protein kinase, an energy sensor, and the glucose transporter GLUT4) involved in the response of muscle cells to insulin and the use of sugars.

Studies on exercise and metabolic health will need to consider the timing of meals in the future. Since it is not possible for everyone to exercise in the morning after the night fasting period, it will be interesting to check if it is possible to obtain the same metabolic benefits after a shorter daytime fasting period, for example, when exercising in the early evening after skipping lunch. However, it should not be forgotten that any physical activity (walking, housework, etc.), performed at any time of the day, is beneficial for health.