Aging happens to us all and brings about many changes in body composition, often with increased fat mass and decreased muscle mass. The change in body composition is thought to be because of a change in how our cells use energy (our metabolism). When one talks about metabolism or fat burning, it typically implies the basal or resting metabolic rate (BMR): how much energy the body uses or needs when in complete rest or immobilized state.Resting metabolism is thought to be high during childhood because of the energy cost of growth, while around the age of 20 it starts to decline. (1) The question is why.
Loss of Muscle Slows Metabolism
One reason might be that we lose muscle as we age. Muscle uses a lot of energy, both when used and when we rest. This means energy use during exercise decreases and therefore energy requirements also decrease. If a person then consumes more calories than their body needs, that person will gain weight. (2)
Organs Change Too!
Another reason for the lower resting metabolic rate of older adults may be due to slowed organ activity, and this may contribute to changes in fat-free mass that occur with aging (3). Some studies argue that the decline in resting metabolic rate can’t be explained solely by changes in our body composition (changes in muscle mass and fat mass). Genetics, tissue alteration, hormonal status and different organ mass may also influence resting metabolic rate (4).
Almost 8% Lower Metabolism in 65 Year-Olds
When comparing the resting metabolism of men and women, both young and old, the measured resting metabolic rate is lower in older individuals compared with younger individuals. (5-6). Comparing 27 year-olds to 92 year-olds, resting metabolic rate decreases by about 27% (6). Adjusting for changes in body composition, 65 year-old men have a 6-8% lower resting metabolic rate compared to 25 year-olds (5).
In practice, that means that if your resting metabolic rate is 1500 kcal today, it will probably decrease by 400 calories per day! However, if you keep lifting (keeping your body composition the same), it will only decrease by 100 calories per day. Another reason by you should keep lifting!
Take Home
In summary, it’s proven that older people have a lower resting metabolic rate compared to younger people. Most of the evidence seems to point to the consequence of a decline in muscle mass or fat-free mass. However, there are many more factors that can play a potential role in the decrease of resting metabolism.
Post provided by Maria Ekblom. Licensed physiotherapist and personal trainer.
Sources:
- Siöström C. Relationships Between Changes in Body Composition and Changes in Cardiovascular Risk Factors: The SOS Intervention Study. Obesity Research. 1997;5(6):519-530.
- Lührmann P. Longitudinal changes in energy expenditure in an elderly German population: a 12-year follow-up. European Journal of Clinical Nutrition. 2009;63(8):986-992.
- St-Onge M. Body composition changes with aging: The cause or the result of alterations in metabolic rate and macronutrient oxidation?. Nutrition. 2010;26(2):152-155.
- Lazzer S. Relationship Between Basal Metabolic Rate, Gender, Age, and Body Composition in 8,780 White Obese Subjects. Obesity. 2010;18(1):71-78.
- Krems C. Lower resting metabolic rate in the elderly may not be entirely due to changes in body composition. European Journal of Clinical Nutrition. 2004;59(2):255-262.
- Frisard M. Aging, Resting Metabolic Rate, and Oxidative Damage: Results From the Louisiana Healthy Aging Study. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences. 2007;62(7):752-759.
