Your Metabolism determines the speed of your internal engine. As we age, our metabolism slows down. We actually burn 100 calories per day less for each decade over age twenty. So, if you are in your forties, you are burning 200 calories less per day. If you do not adjust your diet and level of exercise accordingly, you are going to gain weight.
Many other factors have an impact on your metabolism including thyroid function, adrenal function, hormone balance, diet, neuro-chemical balance in your brain, nutritional deficiencies, quality of sleep, water intake, acid balance and your exposure to toxins.