Why We Run: A Natural History - Bernd Heinrich (2002)
Chapter 15. The Fitness of Being Out of Shape
Nature has neither kernel nor shell; she is everything at once.
While studying the temperature-dependent endurance physiology of hawk moths at UCLA, I raised hundreds of these animals from caterpillars, feeding them fresh tobacco plants that I grew in the greenhouse. The big green larvae, plump and slow creatures that they were, molted into seemingly comatose, limbless brown pupae that lay motionless like mummies for about two weeks. After the two-week interim, each animal was totally transformed into a moth with wings, legs, and a tubular tongue as long as its body, curled up neatly into a roll under its head. The moth slipped out of its pupal shell, crawled a few inches up onto a twig, and dangled there limply for a couple of hours while it gradually expanded and hardened its wings and the rest of its exoskeleton. By evening the new moth would begin to shiver a minute or two to get its huge set of powerful muscles hot enough to contract rapidly and strongly enough for flight, then lift off on whirring wings to execute a superbly coordinated behavior. At the end of warm-up and during flight, these muscles contracted at 40 times per second, requiring an energy expenditure with the herculean aerobic rate of about four times the O2 max of a pronghorn antelope running all out. Incredibly, this high rate of aerobic metabolism appeared with essentially no training (some muscle contraction in the pupal stage is possible), and as far as we know, exercise does not increase the moth’s already high aerobic metabolic rate. Similarly, many birds leave the nest flying, requiring very little if any training or conditioning, but maturation instead.
It seems intuitively obvious that being instantly in shape, as these moths and many birds are, might be advantageous in short-lived animals that do not have years or months to get ready for their life tasks. These hawk moths feed on nectar and can live for several weeks. Some other hawk moths, who don’t feed at all, live only a few days as adults. Saturniid moths are born even without mouth parts and they starve as their onboard energy reserves run out after a couple of nights of flight activity; they can’t waste time and energy in training.
Whenever I contemplated the difficult and often excruciating task of getting in shape for a race, I thought of the moths. Why do we get out of shape? Why is it easy for the moths and difficult for us to become paragons of aerobic fitness, in terms of both power output and motor coordination? I knew runners who could run faster and farther than I could with little training. Did I lack talent? Was my natural state to be clumsy and out of shape?
In our evolutionary history, we may have been forced to be nearly continually active in order to survive so that, in contrast to the moths, we may never have had the necessity to deal with the effects of prolonged idleness. That idea is supported by data from bones and bears: our bones become brittle and weak if they don’t receive normal everyday stress such as from walking. Bears, who lay comatose for six months at a time in hibernation, suffer no such bone deterioration from inactivity. Similarly, if running had been a constant in our lives throughout evolutionary history, it may now be required as a supplement for optimum health, analogous to vitamins, which provide chemicals that our bodies have not evolved to produce because they have always been present in the food of our normal diet. We need to take exercise, and vitamins, when our normal life styles, and diet, are at odds with the ancestral conditions that shaped us. Alternately, or additionally, perhaps being able to be out of shape is adaptive because it permits neuromuscular flexibility. We can retrain our bodies, for example, from a lean runner to a weight lifter. Moths cannot. They are wired up and muscled up to do one specific thing extremely well, but at the cost that they can’t remake themselves.
One cost of aerobic running fitness is loss of explosive muscular strength. When untrained, I normally bound up three stairs at a time, but I know I’m becoming trained for long-distance running when I can do only two at a time. The loss of explosive power that occurs with aerobic conditioning is thought to involve a conversion of fast-twitch, anaerobic muscle fibers to slow-twitch muscle fiber characteristics. That is, with aerobic training we lose sprint speed. Conversely, we can specifically train sprint speed, but it is at the cost of endurance. Similar trade-offs apply to being fat, another aspect of being out of shape.
When we need food, having the speed and mobility to chase it down is obviously advantageous. Once we have the food, it is adaptive to conserve what we have laboriously won. It may then be advantageous not only to slow down, but also to convert food to fat and to keep it in fat. Reducing mobility and becoming physically lazy would serve that purpose. Once most animals are fat, they do indeed reduce mobility and are likely to become even more fat if food is still available. Birds are an exception, because after they fatten up they quickly use that fat as fuel for migration or for shivering on cold nights. However, they don’t get fat at any time, even with food available. They lay on fat at specific times for specific tasks.
Fat is the body’s bank account. The currency is calories. For us, it is an insurance policy that’s taken out for lean times up ahead. The best proof for the fitness of fat can be found in animals inhabiting a highly seasonal environment. In the Maine woods, the moose, porcupines, and snowshoe hares that browse year-round on the readily available buds and twigs are always lean. They have no need to carry energy stores, and they are not programmed to store calories for possible food shortages. They can stay lean, as is necessary to outrun predators. For most animals, continually available food translates to never being fat because there is then no need for fat but advantages to being thin. On the other hand, those animals, including woodchucks, bears, raccoons, and skunks, whose food supply disappears in winter become obese in the fall, when, given a chance, they feed as if there were no tomorrow. They’ve been programmed to feed precisely because the tomorrow they anticipate is one of lean times. Similarly, to our body, fasting is a danger signal that says, “Food getting scarce—stock up if you possibly can.” Thus, I predict that when trying to lose fat, a very gradual caloric adjustment, to try to sneak up on the body so it won’t notice, is probably preferable to fasting.
Prairie dog, fattened up for hibernation and carrying more food, from a photo by Stephen G. Maka
Some animals, such as migrant birds that can double their body weight in about two weeks, lay on fat to an extent that outstrips even the best of human capacities. We don’t know how they do it, but there are clues. Not surprisingly, physiological regulation of appetite is a big factor, and it involves chemical signals circulating in the blood that affect the appetite centers in the brain, and these agents are influenced by environmental cues. Bears are veritable gluttons all summer and fall, but in winter and spring they don’t eat or drink, even if food is available. If bears were hungry in winter they would not sit still, and winter-active bears did not survive to pass on their genes, because they exhausted their fat reserves too early. Even after coming out of hibernation following a half-year fast, bears’ appetites are initially still physiologically suppressed by their blood-borne chemical signals. The animals are thinned down considerably from nourishing themselves and, in the case of females, their cubs also. Appetite suppression in early spring is adaptive in them, because there is then generally still not much food available. The harvest months, when appetite appropriately peaks, are in late summer and fall.
Humans, like bears and birds, are obviously also capable of laying up fat stores, which suggests that our ancestors very likely experienced times when food was abundant, followed by times when food was scarce. Aside from seasonal changes of food availability, we could not predict famine or hunting success. Further, our ability to fatten at any time tells us that those who survived to be our ancestors were capable of fattening when opportunity arose. The benefits of storing fat are especially great for women, who have to sustain energy balance through pregnancy and lactation. Not surprisingly, even now women universally, on average, have more body fat than men and put it on more easily.
We can only speculate about why nature favored fatness in us despite its associated costs, but the examples of other animals give clues. Fatness would have been particularly useful when, as in hibernators, huge energy demands coincided with decreased mobility. In human history, women were either pregnant or lactating virtually continuously. The energetic demands of childbearing were tremendous. Also, women with children could not move around as easily as men. In a world of often limited resources, fatness was associated with fertility, and most likely considered sexy.
In females, the distribution of fat might have evolved to accentuate its presence, so that it could be shown off. Although a slim body might signal potential mobility and hence capacity to secure resources, a plump one signals something of even more direct reproductive value, namely potential success in childrearing. In the past, in most societies, food shortages were virtually inevitable. Now that food production and distribution systems have made nutrition predictable and reliable in many countries, fatness is no longer adaptive there, and hence it is no longer prized as a signal of success. Our natural tendencies are not necessarily measures of goodness. Science can help us recognize biological biases, and our values can then be engaged to either overcome or favor them.
It is possible that there were regional differences in costs and benefits of putting on fat, and that these differences would be reflected in the present. It has been proposed, for example, that Polynesian people of the South Pacific tend to be more genetically prone to be heavy than many others. The thousands of Oceanic Islands were colonized by a specific subgroup of survivors—people who had been adrift for months at sea. During these long-distance movements that resulted in chance colonizations, those leaving on their journeys with the largest buffer of energy reserves (like those of migrating birds) would have lived longer, and possibly more likely reached land, than those starting off lean. They then passed on their genes to their descendants. Regardless of whether the ability to be fat was later useful, it would still be passed on as a survival trait. In parts of Oceania fatness is even considered a mark of beauty.
To train for an ultramarathon, I made the rational choice to try to train my fat-burning, as opposed to fat-building, capacity to a maximum. Of course, I needed to have fat to burn, but I needed to have a very thin cushion of fat even though my body would naturally try to fight that tendency and put on too much.
My natural body weight—the weight I’ve had since high school and even now at age sixty—is right around 160 pounds, with almost no obvious fat. For my five-foot-eight height, I’m chunky for a distance runner. For example, Olympic and frequent Boston and New York City Marathon winner Bill Rodgers is the same height as I am but weighs 36 pounds less. Frank Shorter, Olympic marathon winner in 1972, is 2 inches taller than I and weighs 26 pounds less. I needed to reduce my weight. But how? Restricting my calorie intake would cause a conflict within my body. Some initial weight loss might be easy, but my body would wake up and defend what it perceived as its normal mass by restricting energy expenditure, to try to hang on to calories, which in the past were always hard won. My body would not be concerned with winning a marathon. All it knows about is long-term survival. Dieting can reduce metabolic rate by as much as 45 percent, making one weak, sluggish, and slow, while reducing weight only slightly. I needed to lose weight, but not at the cost of reducing my ability to have the high energy expenditure necessary for running fast. How could I induce my body to maintain a weight like Shorter’s, rather than my usual 160 pounds, while still maintaining the capacity for a high rate of work output?