Why am I the short fat one?--The Secret Life of Twins

Long term changes
 
Most people are used to thinking of our DNA as a fairly fixed code, a bit like a blueprint for a building.

We know that some things can change the code itself - exposure to radiation can cause mutations that lead to an increased risk of cancer for example - but usually the code remains the same.

 
Finnish twins Miia (left) and Noora (right) are 24-years-old
      Noora weighs 17kg more than Miia due to their different lifestyles
           But Miia is also likely to put on weight if she becomes less physically active

However, production of the molecules for which our genes are responsible - things like digestive enzymes and muscle protein - is constantly getting switched on and off.

And it seems that some environmental influences can have much more lasting effects - permanently activating or inactivating certain genes.

It is becoming apparent that this aspect of genetic control - a process dubbed epigenetics - is very important in human health.

In 1944 there was a severe famine in Holland. The children born during this period are more susceptible to diabetes, obesity and heart disease compared to siblings who were older during the famine.

It seems that the period of starvation prompted their bodies to switch certain genes on or off to cope with the lack of food.

These changes may, in subsequent times of plenty, have made them gain weight and get diabetes.