Aging and Metabolism
Weight gain is a common occurrence as people get older because the body's metabolism slows as we age. Thankfully, exercise can help mitigate this weight gain.
After age 45, the average individual loses around 10% of their muscle mass per decade.1 This equates to losing about one-third to one-half a pound of muscle each year and also gaining that much in body fat. Because muscle mass burns a lot of calories compared to fat, the total number of calories needed goes down.
How Aging Impacts Metabolism
Many studies have been done to understand why people gain weight as they age and the answer is clear – the change in body composition
accounts for the vast majority of the decline in metabolism.
There are also a growing number of studies, however, that suggest that body composition does not account for all of the weight gain associated with the aging process. Decreases in the calories used by the body's organs, such as the heart and liver, also seem to occur as the body ages.2,3,4
Physical activity plays a role in both body composition and metabolism during the aging process. Research shows that most individuals gradually reduce their level of physical activity as they age, which further reduces their number of calories needed to maintain weight. Less activity also means less use of the body's muscles, which contributes to the general decline in muscle mass and subsequent changes in body composition.
Overall, these age-related changes means that the average 50 year-old woman needs around 300-500 fewer calories per day than she did in her twenties to maintain the same body weight. So for those who gain weight while aging, the reason is not necessarily eating more; but rather eating the same, while needing fewer calories.
Aging and Exercise
Is there anything that can help to slow this part of the aging process? Fortunately yes. Exercise appears to help a lot. Several studies have shown that resistance (i.e. weight) training alone can boost metabolism and offset the decline seen with aging.
Other Science Library topics:
The Physics of Weight Loss
1 Janssen I, Ross R. Linking age-related changes in skeletal muscle mass and composition with metabolism and disease. J Nutr Health Aging. 2005 Nov-Dec;9(6):408-19.
2 Alfonzo-Gonzalez G, Doucet E, Bouchard C, Tremblay A.
Greater than predicted decrease in resting energy expenditure with age: cross-sectional and longitudinal evidence. Eur J Clin Nutr. 2005 Sep 7.
3 Wang Z, Heshka S, Heymsfield SB, Shen W, Gallagher D. A cellular-level approach to predicting resting energy expenditure across the adult years. Am J Clin Nutr. 2005 Apr;81(4):799-806.
4 He Q, Heshka S, Albu J, Boxt L, Krasnow N, Elia M, Gallagher D.Smaller organ mass with greater age, except for heart. J Appl Physiol. 2009 Jun;106(6):1780-4.