Obesity: What Do Cars and People Have in Common?
Now we are ready to get into the “meat” (maybe the “tofu” for vegetarians?) of what causes most obesity and contributes to the development of type 2 diabetes. The big picture is that a combination of genetic and environmental factors are to blame. I use the term “environmental” very broadly to include virtually all non-genetic influences such as psychological, socioeconomic, cultural, etc.
How the Human Body Works: The Automobile Analogy
Just for the sake of our discussion, consider the human body as a fine automobile. This automobile has an engine that powers the vehicle, allowing for motion and other activities such as use of a fancy radio, sunroof, etc. The automobile engine is fueled by gasoline (or other fuels, such as ethanol). In the end, nothing much in the automobile works if the engine doesn’t deliver energy; even the battery which stores energy will give out quickly without a working engine. In the case of the human body, there is also an engine. The engine consists of organs, the heart, liver, the kidneys, etc., and more basic cellular processes which we will call the metabolic machinery. The body engine is fueled by food. Food components, protein, fat, and carbohydrate, are ingested and converted into energy for the engine. That which is not used immediately is stored, mostly in the form of fat. So, think of the body as having a fuel tank full of fat instead of gasoline. The more fat the body stores, the longer the body engine can work; with the automobile, the more gasoline in the tank, the more miles (or kilometers) the vehicle will go and/or the longer the accessories will work. Are you with me so far? It’s not all that bad an analogy. is it?
How Well Does Your Engine Perform?
So, both the human body and the automobile rely on their engines to get the job done. In the case of the automobile, the engine can be turned off and restarted whenever. For the body, the engine must never stop running or death occurs. It’s like an automobile engine that idles but never stops, always using up at least some of the energy stores (in the very short-term, the body can mobilize carbohydrates for energy, but in the long run, it’s the fat that provides the lion’s-share). When the body is on “idle,” we call that the basal state. In that situation, most of the energy stores are used to keep the brain working. Enough energy also needs to be made available to generate enough heat to keep the body warm, the heart beating, etc.
So, already you may have some ideas how genetic factors can cause obesity? For example, automobile engines differ in their performance characteristics. One gallon (about 3.8 liters) of gasoline will get a high-mileage automobile pretty far or allow a long period of idling while an eighteen-wheeler cannot go nearly so far or as long a period of time. It’s the same for the human body. Some people require more stored fuel than others to keep things going. They are the lucky ones- they can eat and eat and not put on weight. Others (like the majority of people in the U.S.) have fabulously efficient engines; they can keep things going in the basal state and with activity using very little stored energy.
The actual mechanisms that determine how well individuals use their stored energy are not well understood but there are definite person-to-person differences. This gets into the metabolic rate, which determines how fast the body engine is running. Some people just “idle” faster, using up those fat stores more quickly. We can affect the metabolic rate with drugs (I didn’t say we should). For example, giving people large doses of thyroid hormone, really revs things up and as you would predict, causes weight loss.
Next entry, I will continue the causation discussion, looking at other ways genetic factors can affect the rate at which the body burns up stored fuel.