After an hour in the gym you’re feeling pretty good about yourself. Less so two hours later, when you’ve demolished half the fridge. But the relationship between exercise and weight loss is complicated: not all exercise stimulates appetite to the same extent. And individuals vary in how much weight they lose from exercise.
This latest research, a small study of 16 healthy young men, measured the effects of different types of exercise on levels of acylated ghrelin, a hormone in the blood that is thought to increase appetite. The researchers provided standardised meals after the exercise and asked the men to rate their hunger levels. The study showed that the more intense and long the exercise, the more the levels of acylated ghrelin were suppressed. Those who ran for 90 minutes still had lower levels an hour after exercise. They also felt less hungry for longer. Shorter, more intense workouts reduced hormone levels more than easy jogging but the men still felt peckish a bit earlier than those who ran for longer.
Prof. David Stensel of the School of Sport, Exercise and Health Sciences at Loughborough University, one of the authors of the study, says that their previous work does not show that people overcompensate in how much they eat after exercise. It’s a difficult area to research as, unless you directly watch what people do, you have to rely on them self-reporting the length and intensity of their exercise and how much they ate – not always accurately.
But if you want to lose weight, you should exercise as well as diet. To be effective you need to exercise at a level above 75% of your maximum heart rate, according to David Broom, the lead author of the study and professor at the Academy of Sport and Physical Activity at Sheffield Hallam University. To get round the small size of many of the trials, his team have pooled data from studies. Broom says this shows that exercise needs to be high intensity. “Even short bursts of exercise – as little as 30 second sprints – has been shown to suppress appetite and acylated ghrelin,” he says. The suppression of appetite due to the lowering of acylated ghrelin lasts up to roughly two hours, but there is variation between individuals.
Broom is clear that the evidence shows you will not feel hungrier or eat more at a subsequent meal. “You are more likely to put on weight if you are inactive,” he says. You should exercise regardless of whether you want to lose weight, of course. It lowers blood pressure and makes you happier.