After 40, your hormone levels (estrogen) drop. This causes your insulin (hormone that helps your body use sugar) rise. Your thyroid levels go down. This combination makes you hungrier. You end up eating more and burning fewer calories. Much of the weight gain occurs around your belly. Eat more foods with fiber (berries, whole grains, nuts) to fill you up and help you eat less. Aim for 25 grams of fiber each day after the age of 40. Other ways to increase your metabolism include:
Folate is most important for women of childbearing age. If you plan to have children some day, think of folate now. Folate is a B vitamin needed both before and during pregnancy and can help reduce risk of certain serious common neural tube birth defects (which affect the brain and spinal chord). Women ages 15-45 should include folate in their diet to reduce the risk for birth defects if one becomes pregnant, even if one is not planning a pregnancy.
It's a cliché, to be sure, but a balanced diet is the key to good nutrition and good health. Following that diet, however, isn't always that easy. One challenge is that women often feel too busy to eat healthfully, and it's often easier to pick up fast food than to prepare a healthy meal at home. But fast food is usually high in fat and calories and low in other nutrients, which can seriously affect your health. At the other extreme, a multimillion dollar industry is focused on telling women that being fit means being thin and that dieting is part of good nutrition.