If you are a woman, then as you age you start losing muscle.
Worse, it is usually replaced by fat.
And for hundreds of years that has just been the way life was.
As women aged they became plump and matronly.
But increasingly researchers are finding that if you can retain muscle as you reach middle age -- or, better still, develop more muscle mass -- you can stave off fat.
Because, the more muscle your body has, the better you are at burning fat.
There are many other benefits from building up your muscle mass in middle age.
For a start, you gain extra strength, and that is without doubt a good thing.
It also means that you become less clumsy and better able to avoid accidents.
You will also find that, as a woman, muscle development tends to help you maintain -- or even develop -- curves.
This is in contrast to men, who will bulk up from their muscle work.
It can even help you develop and lift the chest, which for many women -- and their male admirers -- is also a good thing.
A further benefit is that regular exercise, and particularly weight training, is thought to stimulate the production of human growth hormone, which carries anti-ageing properties.
In addition, when you build up your muscles you are also strengthening your bones, boosting blood sugar levels and lowering your blood pressure and your cholesterol.
Studies have also shown that muscle work can help lift your mood.
So what are you waiting for? You have nothing to lose but your fat.
And lots -- especially muscle -- to gain.