You Can See It Coming From A Mile Away

This figure is from a 2021 cross-sectional study of 2000 individuals aged 3-96 y/o that looked at FFM (Fat Free Mass), FM (Fat Mass) and PAL (Physical Activity Level).

TLDR: Physical activity level is highly correlated with Fat Free Mass. Both are important markers for health, longevity, and functional capacity. Both peak in the 30s, decline steadily in your 40s, 50s, and 60s, and then rapidly decline in your mid 70s.

First off, what are we measuring and how?

- FFM (Fat Free Mass) was measured using doubly labeled water and is a measure of how much non fat tissue you have in your body. This includes mostly bones, muscles, organs, and blood. It is often used as a marker for muscle mass which is also correlated with strength. In this study they used:

- PAL (Physical Activity level) was measured using doubly labeled water and was calculated by using the measured resting energy expenditure and the total energy expenditure measurements. This reflects how active a person 

- FM (Fat Mass) was measured using doubly labeled water and is exactly as it sounds, the mass of fat on your body. Well named. Note that with DWI we are not able to differentiate between VAT (Visceral Adipose Tissue) and SubQ (Subcutaneous) fat. Of note, this would be a helpful distinction as SubQ fat tends to be rather inert where as that pesky VAT is quite inflammatory and metabolically damaging. A DEXA scan or MRI is the scan you want to tell VAT from SubQ fat.

A short note on DLW method: "doubly labelled water method for the assessment of energy expenditure was first published in 1955, application in humans started in 1982, and it has become the gold standard for human energy requirement under daily living conditions. The method involves enriching the body water of a subject with heavy hydrogen (2H) and heavy oxygen (18O), and then determining the difference in washout kinetics between both isotopes, being a function of carbon dioxide production." (Westerterp et al 2017) To learn more about this methodology there is a great paper HERE. Though you can also google some great explanation videos.

Secondly, why is FFM important? First, let's discuss what it is. Fat free mass is a measure of the amount of non-fat in your body. This includes bones, organs, blood, and muscles. Often FFM is used as a surrogate for muscle mass. As a side note: in my own patients I prefer to us ALMI (appendicular lean muscle mass index) derived via a DEXA scan. The ALMI via DEXA looks at the non-fat tissue and non bone tissue in the arms and legs thus eliminates the organ weight variable as well as the bone mass variable. In this study though they had FFM so they used it. Other studies show that the more muscle mass you have and the more strength you have (muscle mass and strength are correlated but not the same) the longer you live and the higher your functional capacity, interpreted as increases in longevity and health span.

This study begs the chicken and the egg question. Does a higher physical activity level cause higher fat free mass, does having a higher fat free mass cause higher physical activity levels, or is there a third variable that is the real driver we need to be focusing on to get both the PAL and FFM higher? The authors of this study feel that it is the PAL that is pushing the higher FFM. However, since this is a cross-sectional observation study there can not be any causal inferences made.

The take home message is this: Just by looking at this graphic you can tell that as you age you are going to start a slow decline of muscle mass in your 30s which will then markedly accelerate in your mid 70s. This trend is also reflected in your physical activity. By knowing this is coming, you can start preparing for it now, building reserve, delaying decline, and working diligently to preserve your functional capacity and quality of life.

LINK TO PAPER

Westerterp KR, Yamada Y, Sagayama H, Ainslie PN, Andersen LF, Anderson LJ, Arab L, Baddou I, Bedu-Addo K, Blaak EE, Blanc S, Bonomi AG, Bouten CVC, Bovet P, Buchowski MS, Butte NF, Camps SGJA, Close GL, Cooper JA, Das SK, Cooper R, Dugas LR, Ekelund U, Entringer S, Forrester T, Fudge BW, Goris AH, Gurven M, Hambly C, El Hamdouchi A, Hoos MB, Hu S, Joonas N, Joosen AM, Katzmarzyk P, Kempen KP, Kimura M, Kraus WE, Kushner RF, Lambert EV, Leonard WR, Lessan N, Martin CK, Medin AC, Meijer EP, Morehen JC, Morton JP, Neuhouser ML, Nicklas TA, Ojiambo RM, Pietiläinen KH, Pitsiladis YP, Plange-Rhule J, Plasqui G, Prentice RL, Rabinovich RA, Racette SB, Raichlen DA, Ravussin E, Reynolds RM, Roberts SB, Schuit AJ, Sjödin AM, Stice E, Urlacher SS, Valenti G, Van Etten LM, Van Mil EA, Wells JCK, Wilson G, Wood BM, Yanovski J, Yoshida T, Zhang X, Murphy-Alford AJ, Loechl CU, Luke AH, Pontzer H, Rood J, Schoeller DA, Wong WW, Speakman JR; International Atomic Energy Agency Doubly Labeled Water database group. Physical activity and fat-free mass during growth and in later life. Am J Clin Nutr. 2021 Nov 8;114(5):1583-1589. doi: 10.1093/ajcn/nqab260. PMID: 34477824; PMCID: PMC8574623.