Hormones, Metabolism and the Benefits of Exercise
Bruce Spiegelman (ed.), 2017
Why does the same training programme give one person a lot and another person very little? Scientists from Boston, Stockholm, Melbourne and Basel gathered in Paris and showed that a working muscle does not only move the body but also sends signals to fat, the liver and the brain. A book about the fuel gauge in the cell, the muscle repair crew, brown fat, irisin, and how training can protect neurons.
Who it is for
For people who train or are about to start and want to understand what exercise really does to muscles, fat and the brain, without advertising promises.
Endurance training is one of the very few ways an adult can set off the formation of new neurons.
What a muscle tells the rest of the body
Exercise researchers gathered in Paris, and the authors of this collection's chapters work in Boston, Stockholm, Melbourne, Basel, Paris and Beijing. They were interested in a question that seems long settled: what exactly does physical exercise do to the body? People have known for centuries that exercise is good for them, and in obesity and type 2 diabetes it is prescribed first, along with diet. Less is known about something else: training improves the work of the liver, of the muscles themselves and, especially important, of the brain.
The editor of the collection, the biologist Bruce Spiegelman, came to this topic through his own discovery. His lab found the PGC-1α protein in brown fat, and then saw that during endurance training there is more of this protein in muscles, and that it starts their remodelling. Scientists became interested in how changes in a muscle help organs that the muscle does not reach directly. That is how irisin was found, a small protein that a muscle releases into the blood. The substances a muscle uses to pass news to other organs are called myokines.
This leads to the main idea of the book. A muscle does not only move the body. While it works, it sends signals to fat tissue, the liver and the brain, and food, cold and the exercise itself change what it reports. And exercise is not one thing: running and weightlifting switch on the cell differently and bring different benefits.
Here you will learn why one programme gives people different results, how a half-empty fuel supply becomes a signal, who repairs a muscle after exercise, how a cool bedroom wakes up brown fat, and how trained muscles protect the brain.
Twenty weeks on an exercise bike
In the HERITAGE study, the main condition for taking part was one thing: a person had to lead a sedentary life. The adult participants were put on exercise bikes three times a week for twenty weeks. At first they pedalled for half an hour at a heart rate matching 55% of their pre-measured limit. Every two weeks the sessions became longer and harder, until they reached fifty minutes at 75%.
The benefit was measured through maximal oxygen uptake: how much oxygen the body can take in and put to use at peak effort. This is the main measure of the endurance of the heart and lungs, and it predicts health and life expectancy well. At the start, men averaged about 36 millilitres per kilogram of body weight per minute, and women about 28. For people around thirty-five, that is low.
Then something familiar happened. Even with the same programme and honest effort, the results differed enormously. The group led by geneticist Claude Bouchard tested a simple explanation: maybe hormones decide everything? In 488 participants, equal numbers of men and women, they measured fourteen steroid hormones and the protein that carries sex hormones in the blood, twice before the programme and twice after it.
The answer was modest. In men, testosterone and a few other hormones were weakly linked to starting endurance: testosterone explained about 3% of the differences between people. When endurance was recalculated per kilogram of lean body mass, the link with testosterone disappeared. It seems it ran through the amount of muscle, not through the ability to deliver oxygen. In women, only the free androgen index gave a noticeable link, also about 3%.
And here is the main point. No hormone predicted how much endurance would grow over twenty weeks. Even the thirteen men with the lowest testosterone responded to training no worse than the others. If your progress is slower than a friend's, these data give no reason to blame your hormone level: many other factors create the difference in response.
About the author
Bruce Spiegelman, a biologist at the Dana-Farber Cancer Institute and Harvard Medical School, in whose lab the PGC-1α protein and the hormone irisin were discovered. The original was published by Springer as open access under the CC BY 4.0 licence.
All books by this author: Bruce Spiegelman (ed.)
Terms from the book
- Cortisol
- BDNF
- Testosterone
- Hippocampus
- glycogen
- AMPK
- mitochondria
- insulin
- Hansen
- capillaries
- serotonin
- melatonin
- PGC-1α: A switch protein inside cells that turns on the genes of mitochondria. Spiegelman's lab discovered it in brown fat, and muscles make more of it after endurance training.
- irisin: A small protein that muscle releases into the blood during exercise. It is cut off from the FNDC5 protein; in experiments on mice it helped white fat turn beige.
- myokines: Substances that muscle releases to send a signal to other organs or neighbouring cells.
- brown fat: Fat tissue that does not store energy but burns it and turns it into heat. It is found in babies, small animals and many adults.
- HERITAGE: A large American family study in which people with a sedentary lifestyle exercised on stationary bikes for 20 weeks under a single program.
- maximal oxygen uptake: How much oxygen the body can take in and use per minute at peak load. It is the main measure of the endurance of the heart and lungs, written VO2max.
- Bouchard: Claude Bouchard, a geneticist at the Pennington Biomedical Research Center (USA), one of the leaders of the HERITAGE study and author of the chapter on hormones.
- steroid hormones: Hormones the body makes from cholesterol, mostly in the sex glands and adrenal glands: testosterone, estradiol, cortisol and others.
- free androgen index: A calculated figure: the ratio of testosterone to the protein that binds it in the blood. It shows how much testosterone is free and able to act.
- ATP: Adenosine triphosphate, the molecule that carries energy inside a cell. Almost every process uses it, from a muscle contracting to a neuron working.
- Viollet: Benoît Viollet, a researcher at the Cochin Institute in Paris, author of the chapter on the AMPK sensor.
- Hawley: John Hawley, a physiologist at the Australian Catholic University in Melbourne, author of the chapter on how the fuel supply affects training.
- Yeo: A researcher who in 2008 tested training with a reduced glycogen supply in experienced athletes.
- Furrer: Regula Furrer, a researcher at the University of Basel (Switzerland), co-author of the chapter on muscle repair.
- Handschin: Christoph Handschin, a researcher at the University of Basel, co-author of the chapter on muscle repair.
- macrophages: Immune cells that engulf debris and foreign matter. They come in inflammatory (type one) and inflammation-calming (type two) forms.
- satellite cells: Stem cells of muscle that lie dormant under its sheath and, when the muscle is damaged or loaded, divide and join the fibres.
- sarcopenia: Loss of muscle mass and strength with age.
- white fat: Ordinary fat tissue that stores energy in reserve.
- New England Journal of Medicine: One of the most authoritative medical journals in the world, published in the USA.
- Celi: Francesco Celi, an endocrinologist at Virginia Commonwealth University (USA), author of the chapter on brown fat and cold.
- metabolic chamber: A sealed room in which the composition of the air is used to measure exactly how much energy a person spends.
- beige cells: Cells inside white fat that, under cold or signals such as irisin, start to turn energy into heat, as brown fat does.
- Ruas: Jorge Ruas, a researcher at the Karolinska Institute, co-author of the chapter on kynurenine and mental health.
- kynurenine: The substance the body turns most of its tryptophan into. It crosses into the brain easily, and its buildup there is linked to inflammation and depression caused by stress.
- tryptophan: An amino acid the body cannot make and gets only from food. It is used to make proteins, serotonin, melatonin and kynurenine.
- blood-brain barrier: The protective wall between the blood and the brain, which lets some substances into the brain and holds back others.
- KAT: Kynurenine aminotransferases, the enzymes that turn kynurenine into kynurenic acid. The human genome has four such genes.
- kynurenic acid: The product of kynurenine processing. It does not pass from the blood into the brain. In the brain it has properties that protect neurons, although an excess there is also linked to disorders.
- albumin: The main protein of blood plasma. Many substances, including tryptophan, are carried by the blood bound to it.
- Val66Met: A common variant of the BDNF gene, in which the cell releases less of this protein.
- Wrann: Christiane Wrann, a researcher at Massachusetts General Hospital and Harvard Medical School, author of the chapter on irisin and the brain.
- FNDC5: A protein on the surface of muscle and brain cells. Part of it, when cut off, becomes irisin.
- mass spectrometry: A method that identifies molecules by their mass and does not depend on the quality of antibodies.
- cathepsin: Cathepsin B, an enzyme that muscles also release during exercise; it is linked to the benefits of exercise for the brain.
All terms
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