Physiology Friday #131: How Does Heat Therapy Compare to Exercise for Improving Mitochondrial Function?
Elevated muscle temperature is a potent stimulus for skeletal muscle adaptations.
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Quick finding of the week
In case you needed another reason to drink coffee…
A recent analysis of over 449,000 people found that drinking 2-3 cups per day of ground, instant, or decaffeinated coffee was associated with a significantly reduced risk for cardiovascular disease and all-cause mortality compared to never drinking coffee.1
Using heat to mimic the mitochondrial benefits of exercise
One reason why exercise is so good for us is that it increases the amount and function of our mitochondria — cells that are responsible for a majority of our body’s energy production.
Increasing our mitochondrial capacity is one of the best things we can do for long-term health and performance.
One of the more interesting areas of research is the exploration of strategies to improve the health of mitochondria in the absence of exercise. Of course, all of us should engage in regular exercise, but there are certain instances where non-exercise therapies to improve mitochondrial function could be beneficial — recovery from injury, bed rest, or immobilization due to injury or illness, for example.
There’s also the fact that any strategy that’s proven to increase mitochondrial function could be stacked onto an exercise routine to augment training adaptations. In this regard, heat is…well…hot.
Sauna use is increasing in popularity, in part because of convincing data on the cardiovascular benefits of regular sauna use. One study also found that combining exercise and sauna increased VO2 max and exercise performance more than exercise alone.
Heat can be applied in many ways — sauna bathing, hot-water immersion, or local heating using a heating pad or other type of device. It’s practical, widely available, and may have mitochondrial benefits.
A newly-published study investigated the effects of local heat therapy on mitochondrial content and function, and also compared these effects to exercise.2 Could heat induce comparable mitochondrial adaptations to muscle-strengthening exercise?
35 healthy adults (~21 years old) were randomized to 1 of 3 groups: a control group, a heat therapy (HT) group, and an exercise training (ET) group.
The heat therapy group received 2 hours of passive local heat therapy 3 times per week for 6 weeks. The exercise training group engaged in 40 minutes of knee-extension exercise 3 times per week, also for 6 weeks. The heat and exercise were both designed to target the quadriceps muscle, which is also the site from which a muscle biopsy was taken from each participant.
Mitochondrial content, mitochondrial respiration, and mitochondrial enzyme activity were measured at baseline, 3 weeks, and 6 weeks.
After 6 weeks of training, there were no changes in mitochondrial content in either group. However, the activity of an enzyme known as citrate synthase — which is correlated to mitochondrial content — did increase in the exercise training group.
The mitochondrial respiratory capacity — an indicator of mitochondrial function — increased in the heat therapy group by 25% and in the exercise training group by 28% after 6 weeks. The increase was similar between groups, suggesting that heat was equally effective as exercise.
Fat-burning capacity increased after exercise training, while heat therapy did not improve mitochondrial fat oxidation.
It might seem surprising that something as “simple” as heat could produce similar benefits to exercise for improving mitochondrial function. But it’s not so unbelievable when we break down the many shared pathways that heat and exercise activate in the body. Both of these “stresses” cause an increase in reactive oxygen species, which signal beneficial adaptations in the mitochondria. Exercise and heat also activate stress-response molecules called heat shock proteins (HSPs), which play a role in skeletal muscle mitochondrial health.
How hot is “hot”? In this study, the local heating increased muscle temperature to around 40 degrees Celsius or 104 degrees Fahrenheit. Exercise increased muscle temperature a bit less — to 37.5 degrees Celsius or 99 degrees Fahrenheit.
What’s not surprising is that there were some outcomes for which heat therapy was inferior to exercise. In particular, heat therapy did not improve mitochondrial fat oxidation, whereas exercise did. Since passive heating does not increase the muscle’s utilization of fatty acids, or any fuel source for that matter, we shouldn’t expect to see this as an adaptation. On the other hand, fat utilization does increase during exercise, and training allows the muscle to become more efficient at using fat as a fuel source.
So, while it’s neat to see the proof-of-concept that heat may be a viable strategy for stimulating some adaptive pathways in muscle, this study also highlights the shortcomings of any intervention compared to good old exercise. Heat might be the closest thing to an “exercise mimetic” that we have, but it still can’t recapitulate all of the adaptive aspects of exercise.
You probably won’t improve your VO2 max or increase muscle size or strength using heat alone. I think the synergy of exercise + heat is more interesting. On that note, you can apply these findings in several ways.
End your regular exercise training session with 15-20 minutes in the sauna or hot tub. This will prolong your body’s temperature (and perhaps heart rate) elevation and may stimulate further skeletal muscle/mitochondrial adaptations like those seen in this study.
On a rest or recovery day, make a point to visit the sauna or a hot tub. You might be able to gain the benefits of recovery while also stimulating your mitochondria and your cardiovascular system to some degree.
If you are injured or recovering from an injury, use heating to prevent some of the loss in strength or mitochondrial function that come from short-term disuse. In fact, the authors of today's study also published a paper where daily heat therapy mitigated muscle atrophy that occurred during 10 days of immobilization.3
The body thrives from variation — in temperature, stress, and routine. Heat therapy is just one example that, in order to get stronger, we’ve got to get a bit uncomfortable from time to time.
Thanks for reading. See you next Friday.