Men, Women, and Workout Intensity — Surprising New Findings on Exercise and Appetite
This Week’s Research Highlight
The Hunger Hormone Connection
With the holiday season soon upon us, many folks may be thinking about ramping up their exercise routine to make up for the extra calories. But the association between exercise and appetite is far more complex — and sometimes counterintuitive — than you might think.
One of the most compelling insights into exercise and appetite control comes from a pioneering study conducted by Mayer and colleagues in the 1950s. The researchers were studying male industrial workers in West Bengal. These men’s jobs demanded dramatically different levels of physical activity, ranging from sedentary desk work to intense physical labor.
They found that workers who performed moderate to high levels of physical labor showed an incredibly precise matching of their caloric intake to their energy needs. As their work became more physically demanding, they naturally ate more to compensate. Kind of makes sense, right?
However, when the researchers looked at the sedentary workers, they found that this unconscious balancing act went awry. Instead of eating less to match their lower energy expenditure, these men actually consumed more calories than those performing light to medium physical work! This seemed to suggest that at very low levels of physical activity, the tight coupling between energy intake and energy expenditure becomes disrupted.
Needless to say, this finding seems uncomfortably relevant to the modern sedentary lifestyles that most of us lead now.
This representation of the “Mayer curve” illustrates the relationship between energy expended through physical work and calories consumed per day. As you can see, being in the sedentary range, or the “non-regulated zone,” leads to an inability to accurately regulate appetite despite much lower energy expenditure. From Gibbons & Blundell, 2015.
Seven decades of subsequent research have continued to expand upon these findings. Some very cleverly designed experiments have shown that regular exercisers are indeed able to better adjust their appetite in response to their energy intake — and they do this without even thinking about it.
When researchers give physically active individuals a test meal with hidden differences in calorie content (called a "preload"), they're better able to adjust their later food intake to compensate. In contrast, sedentary individuals often show a deficient homeostatic feedback control of hunger and satiety, unable to distinguish between high and low-calorie preloads and eating similar amounts at subsequent meals.
The benefits of exercise on appetite regulation appear to develop relatively quickly. A 6-week moderate-intensity exercise program in previously sedentary individuals led to improved appetite control and more sensitive eating behavior in response to previous energy intake. This suggests our appetite control system remains plastic, and can be "retrained" even in adulthood.
But why exactly? We think that improved appetite control in active individuals may be linked to how exercise affects appetite-regulating hormones.
And that brings us to ghrelin. Released primarily by the stomach, ghrelin levels typically rise before meals and fall after eating, so it basically acts as your body's internal meal scheduler. Thus, ghrelin is often referred to as the "hunger hormone." But it's actually a little more complicated than that.
You see, ghrelin exists in the bloodstream in two distinct forms. The first form, acylated ghrelin (AG), is what triggers appetite. The second form, deacylated ghrelin (DAG), was initially thought to be inactive. Scientists now know that DAG can either have no effect on appetite or might even suppress it, essentially acting as an appetite controller.
The aforementioned link between exercise and appetite control has led scientists to look more closely at how exercise affects ghrelin. However, this relationship isn't straightforward. The impact of exercise on ghrelin appears to hinge on multiple factors, including the intensity of the workout and even the gender of the exerciser. Women typically have naturally higher baseline levels of ghrelin than men, and emerging evidence suggests they might respond differently to exercise in terms of both ghrelin production and appetite suppression.
A new study from the University of Virginia has begun to unravel these complex relationships, with findings that could change how we think about exercise timing around meals. The results offer some insights into why some people feel less hungry after working out, while others don't experience the same benefit.
Inside the Study
The research team designed a comprehensive crossover study to investigate how exercise intensity affects appetite and hunger hormones. (This means each participant served as their own control by completing all experimental conditions, helping to reduce the impact of individual differences on the results.)
They recruited 14 participants (eight men and six women), all of normal weight and not regular exercisers.
Before beginning the main experiment, researchers conducted thorough baseline assessments, including fitness testing to determine each participant's peak oxygen consumption and lactate threshold on a cycling ergometer. This way, “moderate intensity” and “high intensity” could be carefully calibrated to each individual individual subject.
From there, each participant completed three different testing sessions in random order:
- A control session with no exercise
- A moderate-intensity cycling session (about 55 minutes) at their lactate threshold.
- A high-intensity cycling session (about 30 minutes) at 75% of the way between lactate threshold and maximum capacity.
The researchers inserted a small catheter into participants' arms to collect blood samples throughout the three-hour testing period — frequently during the first hour and then every 30 minutes afterward. These samples were analyzed for different forms of ghrelin, as well as blood lactate levels. Throughout each session, participants also completed detailed appetite questionnaires every 30 minutes.
Importantly, the researchers matched the caloric expenditure between the two exercise sessions. This meant that while the high-intensity sessions were shorter, they burned the same number of calories as the longer moderate-intensity sessions.
What They Found
Role of intensity
The key finding here was that high-intensity exercise was most effective at reducing all forms of ghrelin in the bloodstream.
Surprisingly, moderate-intensity exercise sometimes led to increased ghrelin levels, rather than decreased!
And remember that the two exercise sessions were matched for caloric expenditure, so it's not like the body was compensating for a greater energy deficit here.
This suggests that the intensity of exercise, rather than just the amount of energy expended, plays a crucial role in how our bodies regulate hunger hormones and appetite.
It would also explain why some previous research on exercise and appetite control have shown mixed results.
Role of gender
One of the most intriguing findings was how differently men and women responded to exercise.
Overall, women started with naturally higher baseline levels of total ghrelin and deacylated ghrelin compared to men. That’s pretty normal.
And both men and women showed reductions in deacylated ghrelin during high-intensity exercise. However, only women experienced significant decreases in acylated ghrelin (the form responsible for stimulating appetite) during high-intensity exercise.
Effect on hunger
The hormone differences observed in the blood were reflected in how hungry participants actually felt: men reported more hunger during moderate exercise compared to high intensity, while women experienced more hunger during moderate exercise compared to rest.
These subjective ratings aligned with their ghrelin responses, indicating that the hormonal changes weren't just numbers in a lab test — they translated to real differences in appetite sensation.
Differences in lactate
Lactate is a molecule produced during intense physical activity. And as you might expect, there was a clear effect of exercise intensity on lactate levels.
High intensity exercise elicited a significant increase in lactate (2.59 ± 0.1 mM/L), compared to moderate exercise (0.85 ± 0.1 mM/L) as well as control. Moderate exercise was also significantly greater than control, but not nearly as much as high intensity exercise. They also found a clear time pattern, wherein high intensity exercise elicited elevations in lactate lasting up to 90 minutes, compared to just 60 minutes for moderate exercise.
Lactate has historically been characterized as a mere waste product of exercise. But in fact, lactate might just be the key to the appetite control effects of intense physical activity.
Mechanism: The Lactate Link
A pile of prior research has shown persuasively that lactate dampens appetite and reduces energy intake.
In a study of overweight men, high-intensity exercise that raised blood lactate led to both lower ghrelin levels (-30%) and reduced food intake, not just immediately after exercise but for up to 38 hours afterward. And research that tested different exercise intensities found that sprint interval training, which produced the highest lactate levels, also resulted in the greatest reductions in ghrelin (-37%). After the sprint exercise, participants consumed 300 fewer calories, compared to no exercise.
Finally, when Swiss researchers infused lactate directly into men's bloodstreams (raising levels to 3.5 mmol/L, pretty close to what we saw in today’s featured study), participants were less hungry and consumed 17% fewer calories at a buffet later, compared to when they received a saline solution.
Remarkably, the stomach's ghrelin-producing cells are richly populated with lactate receptors, suggesting our bodies evolved to use lactate as a direct signal to suppress appetite during intense physical activity. And if you stop and think about it, that makes good sense from an evolutionary standpoint. In a natural environment, intense physical activity that generates high amounts of lactate would probably have occurred when dealing with an immediate physical threat (like a predator). In such a scenario, trying to find food is decidedly a lower priority.
Furthermore, digestion requires significant blood flow to the gut. But during intense activity, this blood has to be directed to active muscles. Suppressing appetite helps resolve these competing circulatory demands. If you’ve ever tried to sprint on a full stomach, you probably already know that eating and hard exercise generally don't mix well.
Making It Work For You
Our ancestors would have experienced high lactate levels during genuine survival situations. Now, in a modern environment where food availability is endless, we can strategically use high-intensity exercise to activate these ancient appetite control pathways for our own purposes.
Consider HIIT: High-intensity exercise appears more effective at controlling appetite than moderate-intensity workouts. If you're looking to manage hunger, incorporating some high-intensity intervals might be more beneficial than steady-state cardio. That having been said, classic steady-state cardio (sometimes referred to as zone 2) has other benefits, so you don’t want to skip it entirely.
Gender-specific approaches: Women seem to experience stronger appetite-suppressing benefits from high-intensity exercise, making it a particularly effective strategy for female exercisers looking to rein in their appetite.
Listen to your body: While moderate exercise increased hunger in some participants, responses did vary between individuals. Pay attention to how different exercise intensities affect your own appetite patterns. Experiment with different modes of activity and durations, and make note of how they affect you.
Random Trivia & Weird News
The TV dinner was supposedly invented because Swanson needed to repurpose a temporary surplus of Thanksgiving turkey.
Back in 1953, Swanson had a problem: They severely overestimated how many turkeys they would need for Thanksgiving, and as a result were sitting on 260 tons of frozen meat (that's about 520,000 pounds), and had no clue what to do with it.
As they say, necessity is the mother of invention.
Gerry Thomas, a Swanson salesman, was inspired by the aluminum trays used for airline food. He came up with the idea to create pre-packaged turkey dinners to use up the extra meat. He ordered a few thousand aluminum trays and designed a three-compartment meal of turkey, corn bread dressing and gravy, peas and carrots, and sweet potatoes.
The company wasn't super optimistic about this scheme. They expected to sell about 5,000 units. But to everyone's surprise, they sold more than 10 million TV dinners in the first year.
(Incidentally, this was before microwaves were widely available, so these early TV dinners had to be heated in a regular oven for about 25 minutes at 425°F. So compared to today's frozen meals, you'd need to exercise some patience!)
Finally, it is probably worth noting that doubt has been cast on this explanation, with some sources saying that the initial turkey crisis may have been exaggerated, or even outright fabricated, as a marketing story.
Either way, it launched a revolution in how Americans eat - all because of (perhaps) too many Thanksgiving turkeys.
Photo credit: Advertising Archive / Everett Collection
Podcasts We Loved This Week
- Nick Petrić Howe & Elizabeth Gibney: Bone marrow in the skull plays a surprisingly important role in ageing. Via Nature Podcast.
- Steve Jones, Staffan Müller-Wille, Stella Sandford: Linnaeus. Via In Our Time.
Products We Like
Sportlegs
Following up on our discussion of lactate's role in appetite and exercise, let's examine a supplement that purports to work with your body's lactate system: SportLegs. The formulation is remarkably simple: it's 87% lactate, in the form of calcium and magnesium lactate.
Sportlegs takes an interesting approach. Instead of trying to block lactate production (which would actually be counterproductive), it provides a small dose of lactate compounds before exercise. This early elevation of blood lactate ostensibly signals muscles to moderate their lactate production during exercise, limiting fatigue.
To be totally honest, I'm not 100% sold on this mechanism, but can't argue with the results. Our team has found SportLegs useful for high-intensity training sessions and exhausting high rep exercise. The effect is subtle but noticeable - it doesn't eliminate the burn entirely, but it seems to push back the threshold where that burning sensation begins. Worth a try!
humanOS Catalog Feature of the Week
Physical Activity and Weight Control
This week, we’d like to highlight one of the courses from the Ideal Weight Program, developed by Stephan Guyenet. There is abundant evidence that physical activity can help attenuate fat gain — although not necessarily in the way that most people tend to assume.
Moderate amounts of exercise do not usually burn a ton of calories (certainly not enough to balance out hardcore holiday feasting). However, physical activity can indirectly rein in fat gain in other ways — for instance, exercise seems to help with appetite regulation, and may even reduce expression of genes that are associated with obesity.
In particular, exercise seems to be crucial in weight maintenance, as we alluded to earlier in today’s Research Highlight. The US National Weight Control Registry tracks over 10,000 individuals who have successfully maintained a weight loss of at least 13.6 kg (30 pounds) for over a year. One consistent finding is that these successful weight loss maintainers are extremely physically active. On average, registry members report expending about 2,800 calories per week through physical activity - equivalent to walking about 28 miles.
In this course, Stephan describes how physical activity patterns have changed in recent history, and how the relative absence of physical activity in our lives undermines our health and body composition goals. He then explains what you can do to naturally build more movement into your day.
Wishing you the best,
