Many people still think the changes that the body goes through when starting an exercise routine and losing weight is that fat turns into muscle. Not so. What actually happens is the muscle becomes more dense and shaped, or toned, and the fat cells reduce in size. Around the age of 16-18, we have established our own number of fat cells in our body. When we gain weight this means the cells have increased is size. When we lose weight this means the cells have decreased in size. So we have the same number of fat cells as we did as teenagers, they just may be a bit bigger.