The modern lifestyle is profoundly sedentary. Contemporary post-industrial humans are uniquely inactive, compared both to our ancient ancestors and to other non-human mammals. Surveys suggest that Americans are spending an average of 13 hours of their waking day sitting.
The difference in physical activity becomes particularly obvious when you look at hunter-gatherers, or people living traditional agricultural lifestyles. When researchers gave pedometers to Old Order Amish, they found that the men took on average more than 18,000 steps per day. The average American, in contrast, barely hits 5000 per day. And let’s be honest, it can be a whole lot worse than that. If you just spend the whole day at home, without going anywhere, you can wind up with about 600 steps or fewer – basically the same as someone who is seriously ill or bedridden. It’s no wonder that so many people are struggling with their weight.
It’s easy to dismiss this as laziness, but for many of us, our livelihoods are tied directly to spending a lot of time sitting. Most Americans make their living not through physical labor, but from mental work, and that does not burn a lot of calories.
But beyond the sheer reduction in energy expenditure, there is another, perhaps less obvious drawback associated with mental work.
Mental Work and Overeating
Cognitively demanding work seems to stimulate overeating. I think most of us have probably either experienced or observed this phenomenon.
And research bears this out.
Here’s a good example. In one study, students at Laval University in Quebec were assigned to either a 45-minute mental work session or a 45-minute rest session. Afterwards, researchers presented the subjects with an all-you-can-eat buffet. Following the mental work session, participants consumed an additional 229 calories, compared to the rest group. Furthermore, participants did not seem to compensate by decreasing their food intake later in the day, suggesting that this did put them into an energy surplus.
Why does mental work have this effect? It is thought that it might be due to the energy demands of the brain, which is a very glucose-dependent organ. Studies have shown that prolonged cognitive work causes rapid fluctuations in blood sugar and insulin levels, as much as 7-8 fold greater variability than at rest. It is thought that this might stoke desire to eat, at least transiently, and often for stuff that isn’t especially good for you.
But unfortunately, thinking really hard doesn’t burn a whole lot of energy. The researchers for the Laval study used indirect calorimetry to assess and compare energy expenditure during mental work versus just sitting. They found that the mental work session only burned a measly 3 calories more than resting.
So, mental work is kind of a double whammy as far as energy balance. It doesn’t expend many calories, and it seems to compel people to take in more food. From a public health standpoint, this kind of sucks, because many of us spend a tremendous portion of our day engrossed in exactly the sort of cognitively demanding work that this research is examining.
Alas, quitting our jobs and working on an Amish farm just isn’t a realistic option for most of us. So what can we do?
Luckily, a team of researchers may have found an elegant solution to this dilemma. Let’s take a look.
Researchers recruited 38 participants from the undergraduate population at the University of Alabama at Birmingham.
Participants were initially summoned to the lab for a baseline visit, during which they spent 35 minutes just hanging out. This visit was effectively a control condition, to compare with the experimental protocol later on. At the end of their time there, they were presented with pizza – complete with their favorite toppings, and as much as they wanted (sounds like my kind of study).
One week after the baseline visit, participants were randomly assigned to one of two different conditions:
- Mental Work + Rest
- Mental Work + Exercise
At this point, both groups engaged in 20 minutes of mental work, in the form of a series of graduate entrance exam level problems.
After they were finished with the problem sets, the first group just spent 15 minutes resting, not doing anything cognitively or physically stimulating. The second group, however, spent that 15 minute window doing interval exercises on a treadmill. Activity during the work intervals was designed to be at about 80-85% of their estimated VO2 max.
For both groups, the lab visit concluded with another all-you-can-eat pizza buffet (again, kind of jealous here – I participated in research as a student, and not one of those studies involved endless pizza).
So what did the data reveal?
- First, the researchers looked at how much pizza the subjects ate during that first baseline visit when they just chilled for 35 minutes and then chowed down on pizza. They then compared that intake to how much the participants consumed after the mental work assignment. They found that participants in the Mental Work + Rest group increased pizza intake by approximately 100 calories following the mental work, compared to the baseline rest visit. Subjects in the Mental Group + Exercise group, in contrast, decreased their consumption by 25 calories on average.
- Next, the researchers calculated the individual energy expenditure during the exercise, based on VO2 submax and heart rate values which had been obtained during a prior screening exam. The average estimated energy expenditure of the exercise was about 90 calories. That doesn’t sound like a whole lot, but certainly compares favorably to the ~16 calories expended during the rest period.
- Putting it all together, the participants in the Mental Work + Rest group found themselves in positive energy balance by around 100 calories on average. In contrast, subjects in the Mental Work + Exercise group were in negative energy balance by almost 100 calories – partly because they ate a little bit less after exercising, but also due to the additional caloric burn from the intervals.
So we see that the exercise group in this study achieved a 100 calorie deficit. Now, that does not sound like much, but you can imagine how that might add up over time if it became part of a long-term change.
In fact, researchers have estimated that shifting energy balance by 100 calories per day – either by reducing energy intake or increasing physical activity – could prevent weight gain in a significant percentage of the population. I can’t help but think that adding fifteen minutes of interval training after work would make a surprisingly big difference in overall energy balance for most people, along with the other obvious health benefits associated with regular exercise.
One thing that is worth noting here is that intense physical activity seems to have a unique impact on appetite, which is probably one reason the researchers chose to test that form of training. Why this is the case is not entirely clear. Previous research suggests that intense aerobic activity modifies appetite-regulating hormones, particularly ghrelin and neuropeptide YY. Blood flow may also play a role. When you are exercising hard – like the high-intensity intervals in the study described above – blood is directed to skeletal muscle and away from the digestive system. For most of us, this makes eating and sprinting mutually exclusive from a physiological standpoint, and that effect might linger for a bit.
It’s certainly not a magic bullet, but if you struggle with overeating when you’re studying or working, doing some intervals seems like a relatively easy way to rein in the munchies.
If you’re not sure where to start, I would encourage you to try Daily HIIT with Krista Stryker. Workouts are about thirteen minutes long (similar to the duration of the interval training in the study), and demand minimal to no equipment, so you can do them wherever you are – no gym required.