So yesterday I completed a sprint duathlon. Just for fun and on my own because we are still in the midst of social distancing due to the Covid-19 pandemic. In the morning I got up and had some Greek yogurt and fruit and then I went and did a 5 km run, 20 km bike, 2.5 km run. After I finished I did have a glass of chocolate milk because I deserved it but I actually had to convince myself to eat about an hour and a half later.
I’ve noticed this before too. If I go to the gym at lunch and do strength training I’m starving afterwards and can’t wait for lunch. But, if I go for a run, I won’t eat for quite a while afterwards. Which of course makes me wonder, what does the research say?
Most of the papers I found had pretty small sample sizes. And they either studied healthy adult males (read male university students) or they looked at specific obese populations. However, the results were pretty consistent across the research.
Aerobic exercise temporarily decreases appetite. Strength exercise does not appear to have the same effect (e.g. Laan et al, 2010). Kawano et al (2013) compared two different aerobic workouts: stationary cycling and jumping rope. They found that jumping rope resulted in higher perceived appetite suppression but the levels of a related circulating hormone were the same between the two. Grhelin is a hormone that circulates in your blood at higher levels when you’re hungry; aerobic exercise decreases the amount of this hormone that is circulating. When present, it increases food intake. So the fact that it is suppressed by exercise contributes to reduced appetite post aerobic workouts.
King et al (1996) studied a group of women and found that they did not exhibit the same appetite suppression post aerobic workout that was commonly found in male subjects at the time but Pomerleau et al (2004) found that low intensity workouts did result in reduced appetite in women and both high intensity and low intensity workouts resulted in lower relative energy intake. Which basically means that even though the high intensity workout triggered increased food intake it was less than what would be expected given the amount of energy the women used up during the workout.
Schubert et al (2013) conducted a meta-analysis (analyzing other studies) and found that the fitness level of the participants played a role with low to moderately fit people showing bigger effect sizes (higher appetite suppression). However, they also comment on the extreme variability between studies in terms of exercise type and intensity, the pre workout state – fasted or not, and the duration of both the exercise period and the period of waiting before eating post workout. Oh, and the food the participants are offered after the workout also changes, which to me is super significant. And significantly more than half of all the participants across all the studies in their analysis were male.
So, what does all that mean? Well, as I said at the start, I notice that I am less hungry immediately after aerobic workouts and the research backs this up that it is likely, at least in part, due to reduced amounts of the hunger hormone being active. But it also means that we need some more systematic research if we want this research to be able to contribute to addressing the current obesity epidemic.