Peter K. Dearden
Last week, on Facebook, an interesting article from the New York Times was being shared. The headline read “How Exercise Changes Our DNA”, and, despite the provocative title, the article underneath was pretty good. The article explains how a recent published experiment showed that exercise changes some aspects of our DNA. To uncover what they mean we need to delve into the murky waters of epigenetics.
We all have a genome, the sequence of DNA that encodes all our genes and is present in almost every one of our cells. Our genome contains information that is encoded in the order of bases (different chemicals) in the DNA. We inherit this sequence of bases from our parents, but, in the main, it doesn’t change.
As an aside, the ‘in the main’ bit is a touch strange. In some cells, especially ones related to the immune system, a small region of our DNA is modified through bits of DNA being cut and joined together. Changes might also occur in our DNA through errors when it is being copied, UV light hitting it, or environmental chemicals that damage DNA. These are minor, although they can have important consequences. Our DNA sequence doesn’t really change much in our lifetimes.
So what is meant by ‘How exercise changes our DNA?’ At the level of DNA sequence it doesn’t, but what it does seem to do is change the way we use it.
We don’t use all our genes all the time. In each cell, some genes are turned on, and some turned off. Our genes respond to changes in diet, the environment, health, infection etc, by turning on or off genes. This process, called gene regulation, is very complex, but is associated with changes in the way DNA in a cell is structured, and how it is modified chemically.
Perhaps the easiest way to think about this is to realise that in each of our cells there is an approximately two-meter length of DNA. Now cells are, as you know, very VERY small. While the biggest cell in the human body, the ovum, is just about visible to the naked eye, the rest are tiny. This two-meter length of DNA is twisted, crushed, folded and packed in the nucleus of each cell, in a controlled and changeable way. Some regions of the genome are going to be very tightly packed, some not so much. This packing, named chromatin conformation (because we like alliterative and incomprehensible names) is associated with gene regulation. Genes that are turned on tend to be in less densely packed regions of DNA; so when you turn a gene on you change the way the DNA is packed. Gene regulation is also associated with a chemical change to DNA – DNA methylation. DNA methylation involves the placing of a tiny chemical onto DNA, not changing the sequence but marking particular points. It too is associated with packing of DNA and the turning on and off of genes.
If this all sounds terrifyingly complex, it is, and it must be said that we still do not have a full understanding of how these process work and what they do. These modifications of DNA, which we call epigenetics, have been shown to be important in health and disease.
So, when the article states that exercise changes our DNA, this is what they mean. Long-term endurance training causes changes in the way DNA is packed and genes are regulated.
The experiment goes like this. The scientists took healthy people and biopsied the muscle from each of their legs. They then exercised one of these legs with regular endurance training for 3 months and biopsied them again. They took their biopsy samples and looked at the DNA. They didn’t look at the DNA sequence, just changes in DNA methylation and which genes were turned on or off. And they found changes! Lots of genes are turned on or off, and lots of DNA methylation sites differ between the exercised and non-exercised leg. They don’t show that exercise changes our DNA, but they do show that it changes the way we use our DNA.
There is a problem here however. Epigenetic changes are also involved in making our cells different. Liver cells turn on different genes from kidney cells, and epigenetics is involved in this. If you took the volunteers, biopsied their liver and kidneys, and compared them, you would find many gene expression changes and many epigenetic changes. The way we use our DNA depends on what cell that DNA is in.
In the exercise experiment, if the composition of cells in the biopsy was different from before and after exercise, then the epigenetic measurements will show those differences. I would be very surprised if the cell composition was the same before and after exercise, as we all know that exercise causes changes in muscle tissue, including making more blood vessels. That difference in cell composition must confound the experiment. While the authors of the experiment do mention this, they don’t do anything to get around this problem.
So in the end, I am sure that exercise does change the way we use the DNA in our muscles, but it also changes the composition of cells in our muscles, and measures of health and fitness very clearly show that exercise is good for you. To do that it doesn’t change the sequence of our DNA. While gene regulation is dynamic and responds to lots of things, DNA sequences are static and don’t change.
Perhaps a better headline, though a less interesting one, is that ‘Exercise changes your muscles.’
Oh, and in Sweden, where the experiment was carried out, there are quite a few people who have one huge muscly leg, and one tiny one!