The Myth of Mitochondrial Density for Endurance Performance

Greg Marshall
6 hours ago
3 min read

A mountain landscape with yellow flowers at sunset

For years, endurance athletes have been told that the key to sustained aerobic performance is training that stimulates something called “mitochondrial density”; the number of mitochondria that are present in muscle cells. A new paper in Cell Metabolism puts the question to the test– is having more mitochondria always better for your health and performance?

We might remember mitochondria from high school biology or the internet of memes. If there’s only one cellular component to credit with generating the energy our cells use to live, mitochondria make a strong case to win the title. Essentially, mitochondria take in oxygen, carbohydrates and fats, and spit out the closest thing to raw energy that the body can use– a molecule called ATP.

If you’re seasoned in training for endurance performance, some faint memories are probably boiling to the surface of your coach telling you how all of those runs are stimulating your cells to develop a greater “mitochondrial density”; more mitochondria inside each muscle cell. More mitochondrial density, the dictum goes, means that our muscle cells are faster at producing large amounts of energy as they take in those carbs, fats, and oxygen from the air we breathe. In turn, more mitochondria should allow us to perform better as endurance athletes.

The implicit assumption is that all of those mitochondria are created equal. That’s where a new paper by Mikael Flockhart and colleagues at the Swedish School of Sports and Health Science in Stockholm picks up.

“The quality of the mitochondrial population and its effects on metabolic health is not merely regulated by the mitochondrial number and its respiratory capacity,” they write.

If not all mitochondria are the same, then what exactly do the bad ones look like, and how do we avoid them?

The Study

Eleven (six female and five male) athletes completed a four-week training protocol of ramping intensity volume on an indoor bike trainer. To keep the results of the study more clear-cut, there were only two workout formats used for all intensity days– a 5x8-minute ride or a 5x4-minute ride, both with 3-minute rest periods, with the aim of achieving the highest possible average power across the intervals that the athletes could. Effectively, every workout required an all-out effort.

Each week, the researchers had the athletes complete the workouts more often than in the previous week. The amount of time the volunteers spent riding hard on a weekly basis increased fast, and the total minutes of interval training prescribed in each week is shown in the histogram below:

A figure visualizing the exercise protocol of the study.

Throughout the study, the researchers collected muscle tissue samples from their quads to assess the mitochondrial density and function. They also ran several glucose tolerance tests to understand how the athlete’s bodies responded to sugar ingestion as training load shifted from light to heavy. They continued taking samples all the way until the end of the recovery period.

The results were that the muscles of the athletes indeed showed more mitochondrial density throughout each phase of training, but after the hard training condition, all of those mitochondria worked less well, negating any benefit of the increased density. Some of that function was restored after recovery, but not all of it. In the moderate training period, mitochondrial density was lower, but their function remained high. Instead of positive, performance-enhancing adaptation, the way muscle cells responded in the hard training period looked more like a short-term coping mechanism to keep producing the same amount of energy, as over-taxed mitochondria became less productive.

The dysfunction in mitochondria has wider implications for our health too. If mitochondria are the machinery that use glucose coming to them from the blood, and suddenly those mitochondria aren’t working as well, what does that mean for blood sugar control? Glucose patterns of the athletes after the hard training period looked as bad as a Type II diabetic, and only partly recovered by the end of the recovery week that followed the hard training period.

Bottom Line

Training hard remains a surefire way to build better health and fitness quickly. But like anything, there’s such a thing as too much of a good thing. What Flockhart’s team seems to show is that there’s more to lose than to gain from doing all-out intervals every other day. Most athletes aren’t doing training like that anyway, thankfully, but at least there’s more clarity as to what overdoing it could cost us. Mitochondrial density is great, but only if we’re following practices that are building them strong and sturdy for the long haul.