It is certain that nearly all adults lose muscle mass as they age. Muscle tissue requires more energy or calories to maintain than does fat, due to a higher requirement for blood and oxygen. Thus, the more muscle mass you have, the higher your metabolism will be, even at rest. According to a 2007 article published in "Clinical Nutrition," the average person loses muscle mass at a rate of 1 to 2 percent each year after age 50. The reduction in muscle tissue in your body lowers your metabolism, so even if you are eating the same amount as when you were younger, you will begin to deposit excess calories your body does not need as fat tissue.
Muscle mass aside, another component of your metabolic rate -- or the rate at which you burn calories -- is your physical activity level. One of the burdens of adulthood is the introduction of the desk job and lack of free time. Most young people have more time to engage in recreational activities and tend to be more active. Once you begin spending eight hours a day in an office chair and come home too tired to do anything but stare at the television, your metabolism begins to slow down since you are not challenging your body physically. This exacerbates muscle loss and again, even though you may not be eating more, the lower energy requirements of being sedentary cause your body to deposit excess calories as fat tissue.
With adulthood comes more responsibilities and worries, which often results in the development of stress. Although the relationship between stress and being overweight is still debated, recent studies suggest that stress influences the release of certain hormones that may change your metabolism. According to a 2000 study published in "Psychosomatic Medicine," the hormone cortisol is typically released during times of stress, which can cause your body to store more fat tissue primarily around your midsection. Abdominal fat is undesirable not only for aesthetic reasons, but also because it is associated with an increased risk of chronic disease. It is theorized that the cortisol stress-response relates to evolutionary survival mechanisms, since hundreds of years ago, times of stress typically occurred when food was scarce.
When you are young, your parents typically buy and prepare your food for you, greatly influencing your food choices. As you mature, you have to make your own dietary choices. When you are crunched for time, this might mean heading to the nearest vending machine or fast food joint, which typically does not result in low-calorie, nutrient-dense food choices. In addition, the stress that often accompanies adulthood can for some people result in compulsive eating habits. Excess calories from over-eating then become deposited as fat stores.
Although it might seem that evolution, genetics and society are all stacked against you, recent studies suggest there is hope for the battle against age-related weight gain. According to a 2003 viewpoint article by the Center for Human Nutrition, if you decrease your net calories by 100 per day, you may be able to stave off weight gain as you age. This may sound like a lot, especially since that is the number to maintain your weight -- not to lose weight as so many people in America are attempting to do. However, this could mean eating one less slice of bread, consuming one cookie instead of two, or taking an extra 10-minute brisk walk each day. If you combine eating a little less with a little more physical activity, you may be able to shave off 200 or even 300 calories per day -- that's 1,400 to 2,100 calories per week. In addition, engaging in strength training -- Will help you to maintain or even increase your muscle mass, thus increasing your resting metabolism.