In this entry, we will continue our discussion of the “why” of obesity. I must confess that I have been a bit “taken” with my human body/automobile analogy and have neglected to even mention calories. A calorie is a unit of measurement for energy and is defined as the the amount of energy needed to increase the temperature of 1 gram (30 grams = 1 ounce) of water by 1 degree Celsius, or centigrade, which is about 2 degrees Fahrenheit. The scientific term generally used for energy units these days, the joule, is not often used in describing food energy. Also, by convention, the term calorie is used to substitute for what is really a kilocalorie, 1000 calories. In this discussion we will use the term calorie but we really mean kilocalorie- don’t worry about it. If you really want to learn more about this calorie stuff, check here.
So, as we discussed earlier, all foods consist of carhohydrates (“sugars”), proteins, and fats. Carbohydrates and proteins generate 4 calories for each gram consumed and burned up. Fats generate a whoopping 9 calories for each gram. What this means is that the body has devised a fantastic way to store energy and not add enormously to body weight. Back to the automobile analogy, it’s like the difference between gasoline and ethanol. Each gallon of gasoline generates more energy than does a gallon of ethanol (i.e., more miles per gallon). The down side of the large amount of energy stored per gram of fat is that it’s much easier to take in extra calories eating fats compared to carbohydrates and proteins.
Back to Genetic Causes of Obesity
Sorry for the digression. Let’s consider more possible mechanisms by which genetic factors can contribute to obesity. Theoretically, if there are genetic differences in absorption of foods, that would affect body weight. A number of medical disorders and drugs are well known to inhibit absorption of food, but I do not know if there is much solid scientific evidence that healthy people have widely differing capacities for absorbing foods (it would be like an automobile having a leaky gasoline tank).
What about the other side of the equation, the burn-the-calories side? Obviously, with exercise, one burns “extra” calories, but that’s not strictly-speaking a genetic effect. But, there is evidence that people can be divided into “twitchers” and “non-twitchers.” Twitchers are those people who, when sitting still are in motion- fidgeting, drumming their fingers, etc. Recent studies show that twitchers can burn up to 300 calories more per day than non-twitchers. That is an enormous difference in energy consumption; 100 calories, or the equivalent of about 1 slice of bread, translates into 1 pound of fat, more or less, per month. That’s 12 pounds in a year. So, 300 calories per day- you can figure it out! Certain medical conditions and drugs can affect the “twitch rate.” Examples include excesses of circulating thyroid hormone (caused by disease or medication) and Parkinson’s disease (tremors).
What about appetite? In my opinion, this is one of the most important genetic factors related to obesity. Appetite and the sense of satiety (fullness) are complicated biological processes, tightly controlled by the brain and gut hormones. There are well-described differences among people in the amount of food necesary to give a sense of fullness. Remember, even a small difference in caloric intake each day will translate into a large effect on body weight over time; eating 50 “extra” calories per day, everything else being equal, will result in weight gain of 60 pounds after 10 years. Depressing thought. So, some people tend to gain weight because their appetite center told them to do it. One of the best examples of this is the Pima Indians. The Pimas are a tribe, originally from the Sierra Madre region in Southern Mexico. Over many hundreds of years, many Pimas migrated north and settled in what is now the southwestern U.S., just outside Phoenix, AZ. The Pimas have been of great interest to medical scientists because they have the highest known prevalence of type 2 diabetes of any ethnic group in the world, about 50 %. They also have a very high prevalence of obesity. The Pimas have been studied, mostly by scientists from the U.S. National Institutes of Health (N.I.H.) who have had a field office in the Phoenix area for many years. the scientists have carried out a series of fascinating studies that have taught us an enormous amount about obesity and diabetes. One of the lead investigators, , Peter Bennett, had the brilliant idea of visiting the Pimas who were still living in the Sierra Madre, to determine if they had as much diabetes and obesity as the U.S. Pimas (the same genetic make-up in both geographic locations. What Dr. Bennett and his team learned was that the Sierra Madre Pimas had virtually no obesity and very little diabetes. In addition, their studies showed that the Sierra Madre Pimas actually consumed more calories each day than did the U.S. Pimas. The difference was in the amount of activity; the Sierra Madre Pimas had to work hard to survive and had dramatically higher energy expenditure than did their U.S. cousins. So here we likely have two factors at work; first the exercise side of the energy balance equation, which is not genetic, and appetite. Whatever the U.S. Pimas ate, even though it was somewhat less than the Sierra Madre Pimas, it was more than they needed to maintain a normal body weight.
Still More Genetic Influences
Finally, we need to acknowledge that there are factors that we do not understand very well. For example, in the 1970s, a medical scientist, Ethan Sims, from the University of Vermont, carried out a series of very interesting studies on Vermont prison inmates. They were the kind of studies that probably could not be carried out these days, since research ethical guidelines have greatly limited the use of prisoners for research. I do not mean to imply that Dr. Sims’ research studies were dangerous. Anyway, he found that there were innate (I can’t think of a better term at the moment) differences in rates of weight gain among individuals given the same amount of excess calories. He also found that there were large differences in the rates at which individuals lost the weight they had gained when the extra calories were discontinued. So, some people are more likely to gain weight and to have difficulty losing weight independent of the calorie intake. I do not know the specific mechanisms involved but they have to be related to either differences in innate efficiencies in burning calories or, possibly some adaptive changes to changes in caloric intake. So, even without any differences in activity levels or caloric intakes, some people are less likely to gain weight than others and less likely to retain extra weight. Those are the kind of genes most of us want to get!
- Obesity: What Do Cars and People Have in Common?
- Obesity: Twin Studies