According to conventional wisdom, your resting metabolic rate tends to drop as you age because you lose a small amount of muscle each year.
However, many people don’t realize that muscle mass is NOT the only thing that affects your resting metabolic rate.
Studies show that your metabolic rate declines with age independently of muscle loss. In other words, young physically active men tend to have a higher metabolic rate than their older counterparts, even if they have the same amount of muscle.
In fact, researchers from the University of Colorado have found that as well as maintaining your muscle mass, there are two other “controllable factors” that affect your metabolic rate as you age .
They compared a group of young (age 19-36) and older (age 52-75) inactive men with physically active men of a similar age. In total, there were 32 young and 34 older sedentary men, 39 young and 32 older physically active men.
The young and older physically active men were local runners and triathletes who placed in the top 25% of their age divisions in either 10-kilometer running events or triathlon competitions.
All of the physically active men were endurance athletes whose training consisted of running and one or more types of cross training (biking, swimming, hiking, or skiing). The sedentary subjects performed no regular physical activity.
At first glance, the results show what you’d expect them to show — older people eat less, move less and have a lower metabolic rate than their younger counterparts.
The older physically active men had more body fat and less muscle compared with their younger counterparts. The younger men did more exercise (9.9 hours per week) than the older men (5.7 hours per week). They also ate more (3564 calories per day) than the older men (2468 calories per day).
Even when differences in muscle mass between the young and older men were taken into account, resting metabolic rate was lower with age in both the inactive and physically active subjects.
The older men burned around 64-68 calories per hour, compared to 72-77 calories per hour in the younger subjects.
At this point, you might be wondering why the research team even bothered to do this study.
After all, the fact that older people eat less, move less and have a lower metabolic rate than younger people is stating the bleeding obvious.
Here’s where things get a bit more interesting. The researchers went a step further and compared a smaller group of older and younger subjects doing the SAME amount of exercise and/or eating the SAME number of calories.
In contrast to the main group, there was very little difference in adjusted resting metabolic rate in the subgroup of older and younger men matched for calorie intake (71.8 versus 73.8 calories per hour) and exercise volume (73.2 versus 70.4 calories per hour).
In summary, the age-related decline in metabolic rate, even when muscle mass is taken into account, is because of two reasons.
Firstly, there’s a strong link between exercise volume and your metabolic rate. In other words, the more exercise you do, the higher your metabolic rate. The fact that people tend to exercise less as they age is partly responsible for the drop in metabolic rate.
Second, metabolic rate is also linked to total calorie intake. This means that the more you eat, the higher your metabolic rate. A reduced metabolic rate in older physically active men is also down to the fact they eat less than their younger counterparts.
In short, a drop in your metabolic rate is NOT an inevitable consequence of aging and has a lot more to do with the way you live your life — the food you eat and the amount of exercise you do.
There are two main “take home” messages here.
Firstly, chances are you know already about the importance of staying active as you age. So I won’t harp on about it. But something I do want to emphasize is the importance of strength training, as the amount of muscle you have is one of the primary determinants of the age-related decline in resting metabolic rate.
Second, don’t rely on diet alone to control your weight. This is a perfect example of a “low energy flux” state, where a low level of energy expenditure is matched by an equally low calorie intake.
It’s a lot easier to get the nutrients your body needs to stay healthy if you’re eating 2500 calories per day and burning them off with a high level of physical activity than if you’re eating only 1500 calories per day and sitting around doing nothing.
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ABOUT THE AUTHORChristian Finn is a former "trainer to the trainers" and fitness writer based in Northamptonshire, England. He holds a masters degree in exercise science, and has been featured in or contributed to major media on two continents, including the BBC and Sunday Times in the U.K. and Men’s Health and Men’s Fitness in the U.S.
1. van Pelt, R.E., Dinneno, F.A., Seals, D.R., & Jones, P.P. (2001). Age-related decline in RMR in physically active men: relation to exercise volume and energy intake. American Journal of Physiology, E281, 633-639
2. van Pelt, R.E., Jones, P.P., Davy, K.P., Desouza, C.A., Tanaka, H., Davy, B.M., & Seals, D.R. (1997). Regular exercise and the age-related decline in resting metabolic rate in women. Journal of Clinical Endocrinology and Metabolism, 82, 3208-3212