Have a close look at the graph that heads this post (you can click on it to make it larger). Looking at the graph is like looking at the way your life and body composition will evolve until you die. This sequence is for women, but the one for men is similar except that it ends sooner, has fewer subjects, and men gain less fat than women.
The graph is divided into a series of columns. Each column shows the size of the sample, the method used to determine lean and fat tissue weight. The bars show the weight change for the sample in each column, sorted by the average initial age of the subjects and their initial weight at the beginning of the follow-up period. The average length of the follow-up period is shown for each cohort.
The cross-hatched area is the absolute weight gain in kg of fat during the follow-up period for each cohort. The dark area is the absolute weight gain in kg of lean weight. Lean weight is all weight other than fat, so it would include muscle, organ, skeletal and neural tissues (this is slightly complicated by the fact that neurons contain fat and protein and is less dense than lean tissues).
From the age of 23 to 28, women gain lean tissue and lose fat. From 25 to about 38, they gain both. During the younger years, the gain in lean is at the expense of fat and this is true also at younger ages, but not shown. Once the initial age of the female cohorts reach the age of 31, a progressive gain in fat begins and a loss of lean starts, which continues through out the remaining years of life. The same pattern begins for men at the age of 44. There are no exceptions for either females or males, though the averages hide some important individual distinctions that I will discuss later.
This pattern supports the point I made in my Why We Get Fatter and Fatter series of posts: once you pass the age of reproduction, evolution doesn’t care about you.
To put the last point a bit more elegantly, the pattern strongly supports the genetic pleiotrophy theory; the genetic expressions that support reproduction are not strongly selected in post-reproductive years to keep you alive. Your genes may turn against you because their job is to prepare you for reproduction rather than survival beyond that age. Once you reproduce and raise your last group of children, evolution has done its job.
The individuals in the many studies aggregated in the graph differ genetically and live their lives in a wide variety of ways. Yet, the pattern is universal. You progressively gain fat and lose lean until you die. There is a catastrophic loss of lean tissue for women at 50 years of age and a simultaneous loss of lean and fat at age 75 or 80. This starts earlier for men at the age of 50, with a catastrophic loss of lean at 75 years. The pattern holds for all non-humans as well. Once you hit an age where you lose both fat and lean tissue weight, there is no turning back.
A progressive loss of lean leading into a catastrophic loss of lean and fat signals the end is near. This is the fundamental dynamic of life.
I think the point is made. In my following posts, I will go into the mechanisms behind this a bit and what to do about staving off this unalterable dynamic cascade into aging and death.
Reference: Roubenoff and Hughes, Sarcopenia: current concepts, Journal of Gerontology, Vol. 55A, No. 12, 2000.