This article first appeared in the St. Louis Beacon, Jan. 21, 2009 - Watching Barack Obama’s inauguration, I could not help but think of energy. For the 2 million Americans who witnessed in person, it must have been cold, standing for hours in 17 degree chill, waiting for the beginning of a brighter day. I know I shivered, just watching them shiver on television.
Most of us, when we think about energy, think of gasoline and electricity, of the energy used to power our cars, to light and heat our houses. Only when we have been exercising, or shivering from cold, do we think of energy as powering our own bodies, or warming them. But energy is just as essential to powering and heating your body as it is to powering and heating your car, and for the same reason. Muscles, like car engines, use chemical energy to do work, contracting muscles to do the work of living. A breath, a blink, a heartbeat, a shivering muscle — all are achieved by contracting muscles.
Where do we get the chemical energy that powers all this shivering? By eating. The things we eat, what we loosely call “food,” are made of proteins, carbohydrates and fats, molecules rich in chemical energy. Your body breaks these big molecules down, releasing from them the chemical energy that fuels its activities, including shivering. Energy is at the very core of living.
It thus should come as no surprise that eating is a primal drive in all animals. Birds will migrate thousands of miles to reach the food supplies needed to carry them through a long winter. Caribou will migrate in large herds nearly 800 miles a year looking for new sources of grass, scarce in the cold Canadian tundra. Even early humans seem to have migrated for food before they invented agriculture.
These early human hunter-gatherers ate a wide variety of things, but appear to have been more gatherers than hunters. Examination of fossilized teeth of ancestral humans reveals that they ate a diet of plants, seeds, fruits, roots and nuts. When they did eat animals, it was more as scavengers than as predators, and the bone marrow was the preferred part of the animal, rather than its meat.
The first real hunters were members of the Homo erectus species that lived about 1.5 million years ago. They lived in caves, hunted, and knew how to make and control fire, which they appear to have used to cook the animals they killed. Even these hunters ate a diet that was quite varied, changing with the seasons and from year-to-year.
They, and we, must eat a balanced diet of carbohydrates, proteins, lipids, vitamins, essential minerals and fiber to maintain our health. Your body cannot manufacture six of the amino acids it needs to build proteins, so it is essential that you get these essential amino acids from the food you eat. Without the proper amount of protein in your diet, your body can’t grow and repair itself.
Carbohydrates and lipids are energy foods. Your body breaks these down, releasing chemical energy to fuel metabolism, to power running and talking -- and shivering. You need to eat enough of these foods to do what you do, but not so much that you gain excessive weight. Fiber consists of substances, such as cellulose, that our bodies cannot breakdown. They travel through the body undigested. Fiber doesn’t offer any nutritional value but aids the body in the digestion process, keeping food moving through the digestive system. Lack of adequate fiber in modern diets has been linked to higher incidences of colon cancer and heart disease.
We humans are omnivores, meaning we can eat a broad range of plant and animal tissues—but not all of us choose to do so. Each of us makes choices about what we eat. Some people, for example, don’t like spinach and love steak, while others select vegetables and choose not to eat meat.
People who make this particular dietary choice, mirroring that of our early human ancestors, are called vegetarians. Some become vegetarians because they judge it a more healthy diet — plants are lower in saturated fats, which are linked to heart disease and obesity, and high in vitamins, minerals and fiber. Others make the choice for ethical reasons, sensitive to the animal rights issues associated with livestock agriculture. Still others simply don’t like meat.
The diets of vegetarians are as varied as the reasons for becoming one. Some vegetarians are selective in what animals products are eaten, eliminating red meat and/or poultry from their diets while still eating fish. Others avoid all meat, including poultry and fish, but still eat animal “products” such as eggs and/or dairy products like yogurt, milk, cheese and ice cream.
The vegan diet is the most extreme form of a vegetarian diet. Vegans avoid all animal proteins, eating neither meat nor animal by-products. They don’t eat red meat, poultry, fish, eggs or milk. Instead, they obtain all nutrients from grains, vegetables, fruits, legumes, nuts and seeds. The vegan diet is very challenging, because no single fruit, vegetable or grain contains all six of the essential amino acids which humans require in their diet. Vegetal foods must be eaten in particular combinations to provide this necessary balance. For example, beans and rice eaten together provide a balanced diet, but neither food does when eaten alone. For calcium not obtained from milk, vegans must eat green leafy vegetables like broccoli or spinach.
Nutritional guidelines recommend that a person should eat 0.8 grams of protein per kilogram (2.2 pounds) of body weight. A typical omnivore human diet contains about 16 percent protein. It is important to understand that not all of this protein has to come from hamburgers.
A vegetarian diet can be almost as rich in protein. A vegetarian diet that includes eggs and dairy contains about 13 percent protein, while a typical vegan diet contains a little less, between 11 and 12 percent protein, adequate for most people.
Regardless of how you feel about eating meat, an extremely beneficial aspect of a vegetarian or vegan diet, one that even meat-eaters would do well to incorporate into their own diets, is the high fiber content of most plant foods. The human digestive system cannot break down and digest the cellulose fibers found in plants, with the consequence that they pass completely through the human digestive system, greatly facilitating its work.
In fact, only a limited number of microorganisms and one group of animals possess the enzymes necessary to digest cellulose. Plant-eating animals like cows and horses are only able to live on a diet of grass because they harbor large colonies of these microorganisms in their guts. Vegetarians, and in fact all humans, could obtain vastly more energy from the plants they eat, and indeed could live on a diet of grass like a wild horse does, if only they too could digest cellulose. You cannot gain the energy you need to power your body by eating the grass growing on a lawn— although you could get ample energy by eating corn, also a kind of grass.
What’s the difference here? It has to do with how these two plants, grass and corn, are constructed. Much of the body of both kinds of plant — the stem, roots and leaves — are constructed largely of cellulose and hemicellulose, long chains of 6- and 5- carbon sugars. While these sugars would be an excellent energy source for you, you cannot use them as food because you lack the enzyme needed to break them free of the chain. With the necessary cellulase enzyme provided by microbes living in their gut, a cow can eat grass and a termite a twig. When you eat grass or corn, none of this structural cellulose and hemicellulose is available to you as food. It passes right through your digestive system without contributing any energy.
The key difference between the two plants, the difference that lets you thrive eating corn but starve eating grass, is provided by starch, an energy storage molecule that corn plants store in their kernels. Grass has little starch, corn a lot.
Essentially all of the energy you gain from a vegan diet comes from this sort of energy storage molecule. Potatoes and other tubers store massive amounts of starch in their roots, sunflowers store oils in their seeds, beans store easily digestible protein. Indeed, fruits and seeds and nuts store copious amounts of energy-rich molecules, which is why vegans, vegetarians and all the rest of us eat them.
A strict vegan must eat different parts of the plant and different types of plants to achieve a balanced diet of carbohydrates, proteins, lipids, vitamins and minerals. Only if a vegan eats a variety of plants will his or her diet contain an adequate amount of protein, carbohydrates and lipids, with all necessary amino acids, vitamins and minerals.
Cellulose and hemicellulose fiber constitutes 40 to 60 percent of the mass of a typical plant. Thus for vegans eating only plant material, nearly half of their diets are indigestible cellulose. Can you image how much less a person would need to eat if they could obtain energy from even a portion of this biomass?
Interestingly, gene technology may provide a way to approach this problem. The necessary cellulase genes have been identified, isolated and transferred to laboratory strains of microbes. The initial attempt is to use them to convert plant biomass to ethanol, replacing gasoline as fuel for cars. It is impossible not to speculate how these same enzymes could expand the energy potential of the human diet. Very useful, on a cold Inauguration day.
George B. Johnson's "On Science" column looks at scientific issues and explains them in an accessible manner.
Johnson, Ph.D., professor emeritus of Biology at Washington University, has taught biology and genetics to undergraduates for more than 30 years. Also professor of genetics at Washington University’s School of Medicine, Johnson is a student of population genetics and evolution, renowned for his pioneering studies of genetic variability.
He has authored more than 50 scientific publications and seven texts.
As the founding director of The Living World, the education center at the St Louis Zoo, from 1987 to 1990, he was responsible for developing innovative high-tech exhibits and new educational programs.
Copyright George Johnson