What really happens to metabolism at midlife

Metabolism changes at midlife start with the body's attempt to hold on to lean body mass. Here's what the pioneering work of Gilbert Forbes teaches us.

I was standing around some midlife women before an exercise class a few years ago. They complained about weight in their middle and how their bodies “turned on them.”

I think it helps all midlife women to understand the why behind body changes. And it starts with body composition.

Studies show that lean body mass tends to decline with age. This begins at age 30, as people lose 3–8% per decade. And it increases when women eventually experience lower estrogen levels starting about 2 years before the final period.

Lower estrogen also changes where fat is distributed with preferential storage in the abdomen. Body fat also increases 2-4 times.

What deepened my understanding was discovering the work of Gilbert Forbes from the University of Rochester. His pioneering work revealed that the body’s regulation system operated though “energy partitioning” between lean mass and fat mass. Basically, changes in diet and energy expenditure (or lack of it) result in changes in both fat and lean tissue. His famous quote is: 

lean body mass and body fat mass are in a sense companions

In other words, a change in one affect change in the other. They are linked and not separate entities.

During my research I found a review which extrapolates on Forbes work. In it, researchers from Switzerland define a concept called “collateral fattening" as:

a process whereby excess fat is deposited as a result of the body’s attempt to counter a deficit in lean mass through overeating.

There are three scenarios where this can happen. 

First is the lean body mass loss that happens with chronological aging. A deficit of skeletal muscle mass sends a signal to the brain to increase hunger and food intake. This not only helps to close the skeletal muscle deficit, but it also increases body fat.

The second scenario that can occur is inactivity. Without activity, muscles can atrophy, and not just as we age. For instance, one study with young males found just 5 days of bed rest resulted in over 3 pounds loss of lean body mass and a decrease in insulin sensitivity by 30%!

Last is the process of losing weight and gaining it back. With quick weight loss — about 1-2 pounds per week— about a quarter of lean mass is typically lost in addition to fat. When the diet is stopped, the body works to re-gain the lean tissue lost. So again, it increases hunger something researchers refer to as “hyperphagia.”

By the time the original lean mass is regained, the person ends up with more body fat than when they started. But here’s the catch; this happens to a greater extent in leaner individuals. This may be why some evidence points to higher cardiovascular risk in women who weight cycle.

Lean body mass benefits our bodies in other ways. Another study found that midlife women with more lean body mass have fewer vasomotor symptoms like hot flashes. Lean body mass also increases insulin sensitivity, mobility, bone density, and is linked to longevity.

The idea is not to avoid gaining fat at all costs because with age — and eventually menopause — we will gain more fat and at least part of that is protective. But it’s smart to find ways to build lean body mass, move our bodies, and step away from quick weight loss that can mess with body composition.

Our body fights to maintain lean body mass in ways we may not even be aware of. So why not help it along? I wrote a post a while back discussing the weight issue so check it out.

What have you noticed about your body composition during midlife?