Reading · Axel F Sigurdsson MD, PhD

It is Tuesday morning. He wakes to the alarm at 6:30, as he always does. Outside, there is a sharp winter cold, but the bedroom is warm and comfortable.

Breakfast requires very little effort. He opens the cupboard and takes out the cereal. He uses an app to warm up the car before going outside and driving to work. He takes the elevator up to the office. It is a sedentary job; he sits at a computer for most of the day. Food at lunchtime. There is almost no physical exertion, at least not on this particular day. Dinner is substantial, often the high point of the day, and he helps himself to a second serving.

There is nothing about this day that directly calls up the word unhealthy. This is a fairly ordinary routine for many people, and in his case it is quite clear that tomorrow will be very similar, as will the day after that.

If we look at this through the lens of the biology we have discussed in the previous three chapters, a number of things emerge that are not immediately obvious. His bodily functions and metabolism have, after all, developed over a long period of time to respond to circumstances that are nowhere to be seen in his life today.

He does get hungry shortly before noon, but it doesn’t last long. He is never cold. He is not carrying anything heavy, and he does not take many steps each day. He never really reaches into his body’s energy stores, simply because there is never any need to. Systems that were built to move between physical stress and recovery meet no such demand on this perfectly ordinary Tuesday.

Before going any further, we need to understand that there is nothing broken in him. The operating systems I discussed in the previous chapters are responding appropriately and functioning much as they evolved to function. But they were never developed to deal with the circumstances of this winter Tuesday if the days, weeks, months, and years ahead all look much the same.

Let’s begin with two aspects of this new and unfamiliar situation. Getting energy now takes no effort at all, and movement itself has become optional; daily life runs fine without it. Neither change looks like a problem in the short term. Give it thirty years, though, and the picture looks nothing like this.

Let us turn first to the acquisition of energy and what comes with it. For most of our evolutionary history, we had to work to obtain food, and it could often involve considerable physical risk. There were certainly periods of abundance, but they were usually short-lived and, in some cases, seasonal. We did, however, develop a means of storing energy, and adipose tissue played a key role in that process, as it still does today. The body’s motto was: store as much as you can while it is available, because there may be none tomorrow.

Adipose tissue has particular properties. It is not merely a storage site but also plays an important role in the body’s metabolism. It gives us a place to store energy we don't need right now. Later, when food is scarce, that energy can be released and used. Much of the metabolic dysfunction we struggle with today arises when weeks, months, and even years pass without much need to draw on the reserves we have entrusted adipose tissue to store.

Adipose tissue can expand enormously and store large amounts of energy. There are limits, however, and eventually excessive fat accumulation can lead to health problems. Fat generally accumulates first beneath the skin, which is often a relatively safe storage location. Subcutaneous adipose tissue can store considerable amounts of fat without causing metabolic problems. But when that storage system can no longer accommodate the surplus effectively, more fat begins to accumulate in places less suited to it. Examples include the liver, excess visceral fat around the abdominal organs, and lipid accumulating within skeletal muscle.

A full tank in the right place is an asset. Fat that accumulates in the wrong places, however, can begin to interfere with the tissues and organs involved and have adverse effects on metabolism and health.

I am not going to delve too deeply into the physiology here, although I may do so later. But I cannot avoid mentioning two things: insulin and leptin.

We often blame insulin for more than it deserves. The simple explanation goes something like this: insulin levels remain high, cells stop listening, and insulin resistance develops. The real physiology is more complicated.

Insulin levels and the effects of insulin are certainly part of the story. Many other factors matter too: fat accumulating in the wrong places, adipose tissue that no longer functions normally, changes in signaling pathways within the cells themselves, and, not least, genetics.

It’s a major oversimplification to think of insulin resistance as some kind of switch that can simply be turned on and off. Many physiological changes are happening at the same time, gradually, and usually over a long period.

Leptin is produced mainly in fat tissue, and its level generally reflects how much energy the body is storing in its fat tissue. When leptin production falls, the brain reads it as a warning that energy reserves are dwindling. As a result, hunger increases, and the body may even start cutting back on energy use wherever it can.

The response, though, works the opposite way when energy stores are high. Leptin production rises, and the hypothalamus receives a signal that energy stores are plentiful. For some reason, though, this message doesn’t seem to get the same attention as the one warning of impending energy shortage, and the response is weaker.

This imbalance might seem odd at first, but it really shouldn’t surprise us. For most of human history, ignoring hunger could be dangerous. Finding food could mean the difference between staying alive and not over the next few days. Excess energy stores, on the other hand, were rarely a problem and posed no acute danger. Once again, we see here that the body’s main goal is staying alive from one day to the next.

Modern life has made it possible for us to survive without engaging in physical exertion. This is quite recent in evolutionary terms, and the human body isn’t really well prepared for it. A body that no longer needs to lift, climb, walk, run, or exert itself in other ways has no reason to maintain the extra muscle mass, bone strength, and cardiovascular capacity those tasks require. This reflects the kind of prioritizing I’ve discussed in earlier chapters.

Maintaining muscle, bone, and the cardiovascular system’s capacity to respond to demand costs energy. When those capacities are rarely called upon, the signals that maintain them become weaker. When strength or physical capacity isn’t used, the body quietly lets it disappear. No alarm bells ring; it happens slowly and usually draws little attention. We simply don’t notice until the next time we need that strength, and it’s no longer there.

Doctors see the consequences of this phenomenon every day. We can store considerable energy reserves in our fat tissue and still become short of breath on exertion, or need both arms to push ourselves up from a chair. Why keep building and maintaining muscle that is hardly being used?

There is plenty of fuel on board. Something else is missing.

Skeletal muscle is not just the force that moves us and gets us from one place to another. It also plays an important role in the body’s metabolism. After a meal, skeletal muscle is one of the main destinations for glucose, taking up a large part of it from the bloodstream for use or storage. Loss of muscle mass is not only a loss of strength. It is also a loss of some of the body’s capacity to deal with incoming fuel.

We can possess abundant energy stores, enough, in fact, to live for weeks without food, while at the same time having very limited physiological reserves. First and foremost, the body tries to maintain what is needed to meet the demands repeatedly placed upon it.

The key point here is to understand that energy stores and physiological reserves are not the same thing. Our modern lifestyle can push these reserves in opposite directions. One may be well supplied while the other is almost depleted.

Decades later, he wakes again in the same warm bedroom on an ordinary Tuesday morning. He has never been hungry for more than a few hours at a time in his life. His energy stores are plentiful, and they have never really been under any serious threat. Yet his muscles are smaller than they once were. His endurance and strength are only a fraction of what they used to be. His bones have thinned. Getting up from a chair is an effort. He avoids climbing stairs; the elevator has become his best friend.

He has always had access to enough energy. There is still plenty to draw on.

For decades, however, there has been little reason to maintain the physiological reserve he once had.