Diabetes and Exercise

There is a complex relationship between insulin, glucose and muscles which makes prolonged exercise a challenge for people with type 1 diabetes.

Without insulin the muscle cells can't use the glucose circulating around the body. The insulin acts like a key that allows glucose to enter into the muscle cells. The glucose can either be used as an immediate source of fuel for the muscles to work or be stored for use later. Glucose is converted into a polymer called glycogen which acts as a food store. This is the energy source our muscles use if we need to make a quick getaway. It also is available to use if our muscles are used for a long time. When people exercise to exhaustion the muscles have used up all their glucose stores. Muscles can also use other fuels such as small fat molecules and amino acids, but they don't work as well as when they use glucose.

This is one of the reasons why people with diabetes feel so tired and lethargic if they don't have enough insulin around. So if somebody is exercising they need to have insulin around, even if it is in small amounts.

The other action of insulin is to control the speed at which our livers make glucose from the fat and protein stores in our bodies. The more insulin around the less sugar able to be made by the liver. For somebody able to make their own insulin as soon as the blood sugar levels fall below a certain amount (about 4.3 mmols/l) insulin production is turned off. This allows the liver to start making glucose and prevent us from running out of sugar or going hypoglycaemic. As sugar levels rise insulin is produced again. It is only when extreme exercise is being undertaken such as running a fast marathon that more glucose can be used by the brain and muscles than can be produced by the liver.

This can lead to hypoglycaemia and is why some people collapse whilst running long distances. They stagger about at first as the brains energy levels fall. Even if they don't become hypoglycaemic the lack of available sugar renders the muscles less able to work and the running slows right down.

For somebody with diabetes they must have insulin around for the muscles to use sugar, but not too much else they will have a hypo. It is a very difficult balance and for elite athletes requires a lot of dedication, testing and injections. But they can do it. Just look at the achievements of Sir Steve Redgrave.