Fasting before exercise may be a trend, but science is running the other way

In recent years, trainers and celebrities have bought into the trend for “fasted” gym sessions and skipping breakfast when hitting the treadmill before work, particularly when the primary exercise goal is weight loss.

The rationale behind exercising on an empty stomach is that your blood sugar and glycogen levels fall when you fast. In theory, exercising in this state should force your body into a ketogenic state where, having exhausted carbohydrate reserves, it starts to burn fat.

However, the evidence is mixed and a new study by a consortium of scientists from UK universities suggests that eating breakfast before an hour’s cycling could be more beneficial for shedding calories than not eating. It found that a light carbohydrate breakfast of porridge not only primed the body to burn carbohydrate far faster than without food, but also that it stimulated the digestive system, increasing the rate at which meals were metabolised for the rest of the day.

While this is not the end of the argument, as the study was conducted on only 12 volunteers, it adds weight to research that showed increased calorie consumption during a workout, and in the 12 to 24 hours following exercise, among gymgoers who ate a light meal beforehand.

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Diet experts have always advised people – especially those who do not exercise regularly and the over-50s – to have a small snack consisting of protein and carbohydrate before a workout. They point out that studies promoting the benefits of fasted cardio are typically conducted on young, regular gymgoers with no underlying health issues who have a regulated diet that ensures they are not deficient in any key nutrients.

Those who attempt fasted workouts without proper dietary planning risk unhealthy changes in body composition. For older adults, protein recommendations in particular are higher, as this group is at more risk of muscle wastage.

There are concerns that over-50s who regularly exercise in the morning before eating anything could prime their bodies towards a metabolic state where they lose muscle.

– Guardian