As you get older your muscle mass decreases and your metabolism slows down; thus it reduces the number of calories you need to maintain your current weight. Your metabolism is your body’s ability to break down nutrients and convert them into energy or store them as fat if you eat too much. Usually, a person’s metabolism naturally starts to slow down around the age of 40. After that, if you continue to eat the same number of calories every day without changing your level of physical activity, you’ll gradually gain some fat.

A pound or two a year may not seem like much at first, but over the years the weight can add up. You can’t turn back the clock and become young again, but gaining unwanted weight as you age doesn’t need to be inevitable if you put forth the effort to lose weight or maintain a healthy weight. Here’s what you can do about it:

Stay active (or get active). Increased physical activity can help you maintain your weight. Resistance exercises like weight lifting can increase your muscle mass, which may increase your metabolism and number of calories burned. It also increases your strength and overall fitness. Aerobic activities such as running or walking burn calories while you’re moving. They’re also good for your heart health.

Count your calories. Calculate your daily calorie need with the Harris-Benedict formula. Remember to go back and recalculate your calorie needs every so often as you get older and as your weight changes. Keep track of the calories you consume by joining Calorie Count or by keeping a food diary. You may also want to invest in a kitchen scale until you become comfortable with estimating the serving sizes of the foods you eat.

Watch what you eat. Although you may need fewer calories, you still need to get your daily vitamin and mineral needs met. Choose nutrient-dense foods such as fruits and green and colorful vegetables that are low in calories and rich in vitamins and minerals. Lean protein sources such as fish and seafood are low in calories and contain omega-3 fatty acids that many diets are lacking.

Make sure you get plenty of high fiber foods (non-starchy vegetables, whole fruits, legumes, and 100-percent whole grain products).

Watch your alcohol intake. Alcohol has more calories per ounce than carbohydrates or proteins but has no other nutritional value. Plus it’s often combined with sweetened mixers that add even more calories.

http://nutrition.about.com/od/nutritionaging/f/Do-We-Need-Fewer-Calories-Per-Day-As-We-Age.htm