OFFICE OF SCIENCE OUTREACH
DEPARTMENT OF BIOLOGY
Obesity, Type 2 Diabetes, and Fructose
Part 5: Our Evolutionary History
Why should things be the way they are? We like sugar so much, and yet fructose converts us into fat-storing machines and leads toward Type 2 Diabetes. And, once we've gained more weight than we like, it's hard to get rid of it. We diet, but then gain the weight back again. It's as if our bodies have some kind of "setpoint" of some particular weight, and won't let us go below it.
There may actually be logical reasons for these phenomena.
Let's think of this historically. Refer back to the graph of sugar consumption in Britain and the US -- shown again below. To its right is an extrapolation for several centuries earlier. This brings us to an interesting poing: sugar was a rare commodity even as late as 1800.
What if we extrapolate backwards a few more years? This allows us to indicate a couple of important events. First, sugar cane was known in tropical India for quite some time, but was a fairly well-guarded secret. The Persians discovered it in the mid-600's BC, upon invasion of India. This led, eventually, to growing sugar cane in Spain, where it was discovered by Christian Europeans during the Crusades. Sugar was still not widely used, perhaps because international sea travel and trade had been developed, making it difficult to grow cane in the tropics and transport sugar to Europe.
It is worthwhile extrapolating back even further.
We tend to think in terms of our current environment. But, this environment is out of tune with our genetic heritage, which was tuned to several hundred-thousand years of hunter/gatherer culture. In terms of human existence, even agriculture is a new invention. To understand our physiology, we must think of those millenia our ancestors spent living in roving bands of hunters and gatherers.
Hunting was a precarious business. Here are a couple of images of what it looked like:
http://gojko.net/images/11Prehistoric-hunting.jpg Diorama at the Chapin Mesa Archeological Museum, Mesa Verde National Park
Whether 15,000 years ago, hunting wooly rhinoceras in Europe, or only a few thousand years ago hunting bison in North America, the scenario was the same. Hunting was dangerous. The weapons were, perhaps, top-of-the-line, but for thousands of years the best-available was a sharp stick. The invention of chipped rock for spearpoints was a major technological breakthrough. It is easy to imagine that hunting was not always successful.
Foraging (gathering) has its own difficulties. Plant growth depends on rain and sun, which do not always cooperate with humans. There were certain to have been times when the foraging was poor. In particular, the end of the growing season would presage entry into months of scarcity, whether the dry season in equatorial regions, or winter closer to the poles. These months of scarcity would be, and sometimes are called "The Starving Time."
Survival would depend not only on ingenuity, finding food and hunting. It would also depend on a very sophisticated physiology. If you find tubers, you gorge on starch. Your blood-glucose level goes up, and you need that insulin response to store the glucose as glycogen, and convert some of it to fat. If your hunt is successful, you gorge on meat. You need that glucagon response to convert amino acids from the meat into glucose for your nervous system, and you need to use the animal's fat as your source of energy.
And, when neither the hunt nor the foraging is successful, you need that physiological down-turn of metabolic rate, so that you don't use too quickly what fat you have been lucky enough to store.
And what about fructose?
The only natural source of fructose would be fruits. Perhaps, in tropical jungles, one or another species of plant would be in its fruit-bearing state at any given time. But in the open forests and grasslands (where human fossils have been found), and in more polar climates, plants would bear fruit primarily at the end of the growing season. Many plants would bear fruit around the same time, much as we see in North America with our fall harvests. At this time, foraging would be easy, and you would gorge on fruit.
In this context, the curious metabolism of fructose makes sense. The fruits' glucose gets stored as glycogen, as usual. The fructose gets shunted directly into the formation of fat. Should your hunting be successful, and you have additional dietary fat, the fructose from your fruit-gorging shunts that fat into storage. Thus are you prepared for The Starving Time.
It is plausible that those who happened to have this quirk of metabolism were more likely to survive. If fructose led them to store more fat just before food became scarce, they would have more fat reserves to fall back on. It is plausible that those who did not have this quirk of metabolism -- say, those who produced hexokinase in their livers, rather than glucokinase and fructokinase -- did not build up their fat reserves quite so much, and were more likely to starve when times were tough. It would seem, therefore, that we are the descendents of the tribes with this curious quirk of fructose metabolism.
We can conclude the following:
- For most of our evolutionary history, fructose was available only infrequently
- When fructose was available, its abundance preceded times of scarcity
- Therefore, it was advantageous for fructose to be a physiological cue to store fat
And now? Naturally, our physiology is the same. Sugar has been abundant only for a tiny fraction of humanity's existence. Perhaps, if our current eating patterns continue for another few thousand years, those of us who still use fructose as a cue to store fat...well, we will die out, leaving fewer offspring than those who are, for one reason or another, unable to handle fructose any differently from glucose. But that would take a long time, and much suffering.
It is simpler, and more humane, to recognize our physiology, and make our food choices accordingly.
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last updated: August 24, 2010