Effects of Physical Activity on Appetite 🏃♂️
Modern lifestyles are profoundly sedentary. I probably don't have to tell you that. Surveys have suggested that Americans spend an average of 13 hours of their waking day sitting. For most of us, this isn't really a product of laziness, but rather due to the nature of our work.
Obviously, spending a lot of time sitting doesn't burn a lot of calories. You might assume that our energy intake would naturally go down to match this low energy expenditure, but research suggests that this might not be the case.
For example, researchers had six men live in a metabolic chamber for two 1-week periods, one during which they intentionally did very little, and another in which they were moderately active. The amount of food that they consumed did not significantly differ between the two conditions, despite the lower energy expenditure during the sedentary phase. This of course meant that energy intake outpaced energy output when they were sedentary, and those excess calories were stored as fat.
So, what about the other side of the equation -- does exercise cause appetite to be ramped up, as a way to compensate for the calories burned by the activity?
Overall, that doesn't appear to be so, at least not in the short term. Somewhat counterintuitively, a number of prior studies have shown that aerobic exercise, especially higher intensity movement like sprinting, actually lower appetite by altering levels of peptide hormones secreted from the gut. We've discussed that previously in this newsletter.
More recently, a couple of studies came out examining how other forms of activity (walking and resistance training) might affect the energy balance picture, so I thought we would take a look at them. I was particularly interested in the impact of strength training on appetite, since it hasn't been examined much.
This Week’s Research Highlights
🪑 Frequently breaking up bouts of sitting with walking breaks does not ramp up appetite or food intake.
A prior study showed that punctuating long bouts of sitting with walking breaks significantly increased postprandial levels of hormones associated with satiety (GLP-1 and peptide YY). However it did not look at actual energy intake. To examine how walking breaks affected appetite and subsequent food intake, researchers in the UK recruited 14 women and had them visit the lab on three occasions. During these sessions, they were exposed to three different conditions: 1) uninterrupted sitting for 7.5 hours; 2) sitting for 7.5 hours with 2-min walking breaks every 30 min; and 3) sitting for 7.5 hours with 10-min walking breaks every 180 min. So, the total duration of walking (as well as total energy expenditure) for the latter two conditions was the same, just distributed differently. At midday on each session, the women were provided a food buffet, and were instructed to eat as much as they liked, without being told the true purpose of this ad libitum feast. Sure enough, the lower energy expenditure during the sedentary condition did not result in a reduction in food intake. In fact, the relative energy intake for these women when they took walking breaks was a bit lower, although this was not statistically significant. Furthermore, the walking breaks did not shift appetite-regulating hormones in a way that would increase hunger. So, getting up and taking walking breaks may actually help keep you in energy balance – even if it doesn't necessarily burn a ton of calories or substantially dampen hunger.
A decent amount of prior research has shown that aerobic exercise can dampen appetite and even lower energy intake. But what about resistance training? Surprisingly, there's been very little investigation of this, until now. To examine how different training loads might affect appetite regulation, researchers in Taiwan recruited 11 healthy young men (aged 19-25), and had them come to the lab three times, during which they were put through three different exercise trials: 1) moderate-load resistance exercise, meaning 4 sets of 8 reps at 85% of the subject's 8 rep max; 2) low-load resistance exercise, meaning 4 sets of 15 reps at 45% of the subject's 8 rep max; and 3) control session, meaning no exercise at all. Rest intervals between sets were the same for both exercise trials, and the training volume was kept equal. Unfortunately, this study did not test actual food intake, like with the buffet in the study described above, but the men reported subjectively less hunger and lower predicted food consumption after both of the exercise trials compared to control. Furthermore, their blood work suggested a hunger-suppressing effect - concentrations of ghrelin were lower immediately after the resistance training, and levels of PYY were higher. Unsurprisingly, levels of lactate, a byproduct of intense physical activity, were also higher after the resistance exercise, and this is likely to be a key contributor to the appetite suppression. Prior research has shown that intravenous infusion of lactate (compared to saline) leads to significant reductions in food intake, and modes of exercise that elicit greater increases in lactate (such as sprinting) lead to substantial reductions in both ghrelin and energy intake.
Random Trivia & Weird News
Spam was introduced to South Korea by the US army during the Korean War. Around this period, food was scarce in South Korea due to the privations of the war, and especially meat, so the canned pork was embraced. Ironically, the product developed a reputation as a luxury treat, and its appeal has endured in South Korea to this day.
Podcasts We Loved This Week
- Dr. Paul Greenhaff: Inactivity – the unperceived muscle stressor. Via Inside Exercise.
- Merry Fitzpatrick: Jewish doctors in the Warsaw Ghetto secretly documented the effects of Nazi-imposed starvation, and the knowledge is helping researchers today. Via The Conversation Weekly.
Products We Are Enjoying
When you exercise hard, lactic acid is generated, which is subsequently broken down into lactate and hydrogen ions. This causes cellular pH to drop, producing the burning sensation that we all know and hate, and making it hard to maintain force production. Carnosine acts as a pH buffer, and higher levels of carnosine in muscle can help prevent accumulation of hydrogen ions during high-intensity exercise. This increases the power you can produce during a workout while still enabling you to reap the appetite-suppressing benefits of lactate-generating activity. To learn more about how it works, and how it could enhance your own training, check out our past interview with Brad Dieter, LactiGo’s lead scientist.
humanOS Catalog Feature of the Week
This week, we’d like to highlight one of the lessons from the Ideal Weight Program, developed by Stephan Guyenet. In this lesson, Stephan explains how exercise seems to prevent or slow weight gain, entirely apart from its impact on energy balance. Needless to say, findings like this should encourage most of us to try to maintain an active lifestyle.