How exercise changes your body for the better

Regular exercise causes profound changes in the body and mind. But what exactly is going on in there?

The gyms are full to capacity and there are joggers everywhere this time of year as people struggle to get fit. But what does it mean to be fit and what changes take place in the body when you get there?

A quick tour of human physiology shows that profound changes take place and these changes also bring huge health benefits. Risks of cancer, heart disease, diabetes and most other diseases drop when you take regular exercise.

"We are talking about a reduced risk of heart disease and stroke by between 20 and 35 per cent," says Maureen Mulvihill, head of health promotion at the Irish Heart Foundation. "Type 2 diabetes risk drops by 33-50 per cent, and being active can help to control our weight and prevent and lower high blood pressure."

Many people try to lose weight through exercise, but health benefits will come regardless of your weight. "You can lose weight, but actually exercise is independently cardio-protective, " says Prof Alan Donnelly, professor of exercise physiology in the department of physical education and sport sciences at the University of Limerick. "If you are heavy it is better to be heavy and exercising."



Exercise has an immediate effect on the heart. "The heart is a muscle, and if you exercise and train it will improve its effectiveness and efficiency for pumping blood," says Dr Giles Warrington in the school of health and human performance at Dublin City University. He is also a sports science consultant to the Olympic Council of Ireland.

“The heart has to work less hard if you are fit,” he says. “It becomes stronger and more efficient and so it can deliver more oxygen, and the removal of waste products is better as well.”

The heart expands during exercise, says Prof Aidan Bradford, professor of physiology at the Royal College of Surgeons in Ireland. "It stretches, and when that happens it contracts more [and] the amount of blood pumped out goes up." With regular exercise, the heart gets larger and its wall gets thicker and stronger. "When you are not training, you might have 70 beats a minute, but it can be 35 beats a minute for highly trained endurance athletes," he says.

It is beyond doubt that exercise is good for your heart, but it remains a puzzle why this is the case.


The biggest change in the body caused by regular exercise is to muscle tissue. “Once you take up exercise, you get early changes in gene expression, within hours, and this continues,” says Prof Donnelly. “The genes are for improved aerobic performance and muscle growth. Several hundred genes are up-regulated, and if you stop they are down-regulated.”

Active muscles are excellent for dealing with food energy, both fat and sugar. If you are sedentary, the muscle becomes less able to absorb glucose, so there is a higher risk of Type 2 diabetes. It is also less able to help burn off fats, so this accumulates elsewhere, including the arteries, and so there is an increased risk of stroke and heart disease.

At the cellular level, exercise triggers the release of substances that dilate the blood vessels in the muscle, letting in more oxygen, says Prof Bradford. These are short-term effects, but deeper changes come with regular exercise. “The ability of the blood vessels to open up is dramatically improved. This has a huge effect on the muscle. The tissues themselves change, with a rise in the number of mitochondria in each cell. These are the powerhouses of the cell.”

Exercise also stimulates the body to produce more antioxidants to mop up the free radicals produced through the burning of energy. This helps during exercise, but they also operate against other free radicals that might be produced.

Our muscles have evolved to perform in this way, which suggests that humans were meant to be active and on their feet.


“Why is exercise so beneficial? We don’t know,” says Prof Bradford. “But it is indisputable, and some of the numbers are quite extraordinary. Long-term aerobic training increases your longevity by decreasing incidence of almost every disease you can think of – arthritis, osteoporosis, cardiovascular disease, diabetes, even things like depression.”

There is a payback for all the hard work, and it relates to how exercise affects the brain. “Aside from the health benefits, there are the mental benefits as well,” says Dr Warrington. “Exercise makes your body produce endorphins, which give you a positive mental outlook. You also benefit from the sense of wellbeing.”

The value of this is being recognised more and more, along with the possibility of using exercise as a medicine, he says. “It is a great preventative tool but also a treatment.”

Prof Donnelly agrees. “Exercise is the best medicine. There is no drug company involved. Exercise isn’t a cure-all but it is one of the key factors in determining long-term health.”

“It is as important as looking after cholesterol or blood pressure but, because it is free, we don’t value it as much as we should,” says Mulvihill.


Those who train regularly are soon aware that their breathing becomes more effective, but only minor changes are taking place in these tissues, says Prof Bradford. “The lungs don’t change very much with training. During exercise, the depth of breathing and the rate goes up, and gas exchange increases, but the training effects aren’t significant. The muscles used for breathing grow stronger.”

“Your lungs don’t grow; you are stuck with the ones you were born with,” says Prof Donnelly. But the “second wind” that sports people talk about is a real phenomenon. “Your lung lining secretes a surfactant, effectively a detergent that breaks the surface tension of water.” At rest you make less surfactant. When you begin to exercise, it is released but at too low a level. The second wind comes when the amount of surfactant catches up with demand.

How much exercise?

Most studies suggest a minimum of 150 minutes of exercise a week, broken up, say, into five days of 30 minutes each of moderate exercise. “Moderate is the key phrase,” says Prof Donnelly. “During moderate exercise, you could speak but you could not sing. One of the big problems is people don’t work hard enough.” One way to gauge this is walking pace. Walking at 100 steps a minute equals moderate. There are benefits if you push that little bit harder, says Dr Warrington. “If you move to a higher intensity, you get greater benefit for your heart and respiratory system,” he says.

“The evidence suggests you need to do some vigorous exercise, but if you have not exercised for a long period there is no advantage. Do things gradually and progressively. Try cycling, swimming or walking first before running.”

This will cut down on the risk of injury and avoid putting the system under too much pressure.

It may seem difficult to meet recommendations for exercise, but don’t let this put you off, says Mulvihill. “Stick with it and do something that you enjoy. It is not only about pumping iron; you can cycle and brisk walk.”

Dick Ahlstrom

Dick Ahlstrom

Dick Ahlstrom, a contributor to The Irish Times, is the newspaper's former Science Editor.