Age Related Metabolic Decline

So, you may have heard off and on that people have the tendency to believe that their metabolism declines with age, and that decline is due to the body succumbing to its natural state of slow degradation. The held belief is the body simply “slows down” in some capacity. Well, in this article we will delve into the science and find out what metabolic decline is, who suffers from it, what causes it, and what can be done to prevent it (or even reverse it?) – so, that said, let us begin!

What is metabolic decline?

First off, what is metabolism? Metabolism is a multifaceted definition, but is, in layman terms, the use of energy to maintain bodily function; that bodily function being anything related to breathing, pumping blood, contracting muscles, digesting nutrients, nervous system function, etc. [1]. Then, if we combine the two words “metabolic decline”, we are talking about a decrease in energy expenditure from the body performing all of these functions.

Does metabolism decline with age?

Yes, basal and total energy expenditure decrease with age [2][3].

How much of a decline?

There is a 1-2% decline in basal metabolic rate per decade of life [2].

What is metabolic decline?
Basal metabolic rate (BMR) is the energy required for an individual to survive after fasting for 12 hours (typically inclusive of sleep) and at complete physical and mental rest [2].

What is total energy expenditure?
Total energy expenditure (TEE or TDEE – total daily energy expenditure), is basal metabolic rate plus activity level from walking around, working, exercise, as well as energy used for digestion of food [5].

What causes metabolic decline?

Well, this is a bit of a difficult question, because we know a lot about metabolic decline, but we do not know everything. While most articles would boil this down to the first factor discussed here, we do need to delve into other factors that impact basal metabolic rate, as well as total energy expenditure, to get a complete picture of the subject.

Muscle & Fat

As we age, our capacity to retain muscle diminishes due to a variety of factors that include, but are not limited to, decreased movement, decreased anabolic hormones, lower protein synthesis, etc. [6][7]. So, as muscle (and other fat free tissue) is the preferential metabolically active tissue, as it uses more calories than fat tissue, its decline leads to a decrease in basal metabolic rate. This, then, is usually replaced with fat mass as metabolic rate reduces and intake remains the same, leading to increase in weight from less metabolically active tissue – fat mass. Even if this is controlled for, the loss of metabolically active tissue in fat free mass leads to a decrease in bodily metabolic rate through a decrease in basal metabolism [2].

This is where debate often occurs, with some research indicating, with variables controlled for, that the loss in fat free mass (muscle, tendons, bones, etc.) is the sole reason for metabolic decline [2]. Meanwhile, other studies still see a discrepancy once fat free mass changes have been accounted for in calculation [2][3]. So, this subject is not clear and more extensive research needs to be done to understand the amount of impact fat free mass has on basal metabolic rate; is it fully explained by this loss? Majority explained? Partially explained? We know it is a main reason, but the extent is unclear.

Quality, Physiological Changes in Tissue

Another aspect that many do not consider are the quality differences between younger and older individuals; meaning, if a younger and older individual were to retain the same amount of muscle mass, would it be possible for the older individual to still see a reduction in basal metabolic rate?

It seems that this may be the case based on some preliminary research on the subject. Apart from the amount of fat free tissue, the quality of that tissue seems to also diminish. Quality of the muscle cells, for example, is reduced in that enzymatic activity decreases, mitochondrial ATP production decreases, sarcomere organization is skewed, and mitochondria genesis is also lowered [4]. In all these ways, there is some evidence that basal metabolic rate would then decrease; however, again, what is not known is by how much this impacts metabolic rate.

 

Brain metabolism

Although this is highly unlikely to make a significant difference, it still falls under the parameters of this discussion and should be mentioned. Apparently, as age increases, the brain metabolizes slightly less glucose (5 grams less, or, in caloric terms, 20 calories), which could lead to slight decline in basal metabolic rate [2]. This slight decline metabolic rate has been determined to be less than 2% [2]. Still, interesting enough to mention, although unlikely to be a large factor on basal metabolic rate.
 

Changes in Eating Behavior

While this does relate more to a main effect on fat free mass and fat mass as mentioned above, it is a large enough, and interesting enough, subject for us to discuss it. In terms of how much, and what, we eat, there seems to be some evidence that strongly implies our feedback mechanisms that inform of us of satiety seem to be altered in a way that could have an effect on basal metabolic rate. It seems that these altered feedback mechanisms stem from hormonal changes, changes in taste and smell, and possible slight blood glucose instability [2]. However, how does satiety impact BMR?

Well, if satiety is unregulated due to feedback mechanisms not functioning the same as they used to, this could change how the body reacts to the intake of food. If food intake is not consciously tracked, as it often is not, then it is possible for the body to under eat and not attenuate that under consumption by decreasing satiety and relating hunger cues; if, then, this occurs, chronic under consumption could lead to a loss in fat free mass as the energy is not being consumed to continue the preservation of metabolically rich tissue (muscle, bone, etc.)[2].

 

How to prevent metabolic decline with age?

Well, if we take everything above into consideration, then the logical move would be to increase physical activity, or even all out exercise. This would not only lead to temporary increases in total daily energy expenditure (TDEE) from added movement (which requires added energy), but if the exercise is done with enough intensity, it could lead to a permanent increase in basal metabolic rate (BMR) as fat free mass would either remain relatively stable or increase from a non-training state; also, post exercise oxygen consumption would increase TDEE after exercise, as well [8][9][10].

Not only would exercise, specifically resistance training (weight lifting, for example), attenuate muscle loss and possibly reverse it from a non-training state, but it would also increase insulin sensitivity, help regulate glucose, possibly aid in increasing hunger cues, and improve muscle metabolism [10]. So, all in all, resistance training is likely the best treatment, then cardiovascular exercise, and then it may be recommended to include a food journal to recall energy intake and bypass the body’s innate hunger cues in an attempt to keep weight (specifically, fat free mass) up.

 

SUMMARY

Metabolic decline does seem to occur and at a rate of 1-2% decline in basal metabolic rate per decade, and while studies are inconclusive if the loss of fat free mass is the major contributor, or the sole contributor, to this phenomenon, it is clear fat free mass loss is the biggest factor. Other potential factors include physiological changes in tissue quality, decreased brain glucose metabolism, and changes in eating behavior that would inevitably impact fat free mass degradation. Obviously, a way to mitigate and possibly increase basal metabolic rate, even in the face of aging, would be to include varied exercise from aerobic to resistance training to maintain and grow fat free mass while also increase total daily energy expenditure.
 

Writer: Nicolas Verhoeven

                                                                                                            Citations

[1] Metabolism: MedlinePlus Medical Encyclopedia. (n.d.). Retrieved from https://www.nlm.nih.gov/medlineplus/ency/article/002257.htm

[2] Roberts, S. B. (2006). Nutrition and Aging: Changes in the Regulation of Energy Metabolism With Aging. Physiological Reviews, 86(2), 651-667. Retrieved from http://physrev.physiology.org/content/86/2/651

[3] Fukagawa, N. K. (1990). Effect of age on body composition and resting metabolic rate.American Journal of Physiology, 259, e233-238. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/2382714?dopt=Abstract

[4] Nair, K. S. (2005). Aging muscle. American Journal of Clinical Nutrition, 81(5), 953-963. Retrieved from http://ajcn.nutrition.org/content/81/5/953.full

[5] Total daily energy expenditure | definition of total daily energy expenditure by Medical dictionary. (n.d.). In TheFreeDictionary.com. Retrieved from http://medical-dictionary.thefreedictionary.com/total+daily+energy+expenditure

[6] Toth, M. J. (2005). Age-related differences in skeletal muscle protein synthesis: relation to markers of immune activation. AJP: Endocrinology and Metabolism, 288(5), E883-E891. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/15613683

[7] Deschenes, M. R. (2004). Effects of Aging on Muscle Fibre Type and Size. Sports Medicine,34(12), 809-824. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/15462613

[8] Byrne, H. K., & Wilmore, J. H. (1999). THE EFFECTS OF EXERCISE TRAINING ON RESTING METABOLIC RATE IN PREVIOUSLY SEDENTARY, MODERATELY OBESE WOMEN. Medicine & Science in Sports & Exercise, 31(Supplement), S310. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11255134

[9] KNAB, A. M., SHANELY, R. A., CORBIN, K. D., JIN, F., SHA, W., & NIEMAN, D. C. (2011). A 45-Minute Vigorous Exercise Bout Increases Metabolic Rate for 14 Hours.Medicine & Science in Sports & Exercise, 43(9), 1643-1648. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/21311363

[10] Kalyani, R. R., Corriere, M., & Ferrucci, L. (2014). Age-related and disease-related muscle loss: the effect of diabetes, obesity, and other diseases. The Lancet Diabetes & Endocrinology, 2(10), 819-829. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4156923/

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