Activity and Obesity

We have discussed that net body weight is the sum of two opposing forces, calories in and calories burned. If a person eats more than he or she burns up through basal metabolism and additional activities, there is weight gain. Of course, for growing children it is okay to gain weight, but it should be appropriate for the gain in height. Anyway, it should be clear to you by now that a person’s activity level can have a profound effect on his weight. With genetic influences already in place, the only mechanisms to lose weight or to maintain an appropriate weight, independent of drugs and/or surgery, are related to activity and calories.

There is no doubt that steadily decreasing amounts of activity contribute to the obesity upsurge in the U.S. Why has this happened? The answer is a multifactorial one but a major contributor is technology; many tasks that in the past required intense physical labor now are performed by machines. How many farmers weed their crops by hand? How many people walk to the shopping mall and to the grocery store? How many people mow their own lawns? Of those that do, how many use a manual “push” mower? You get the point; we are an affluent people and we are good at getting other people or machines to do much of what used to be physical activity for each of us. Many of us do try to conmpensate by going to a gym or jogging, or whatever, but the statistics show that very few people have regular exercise programs long-term that amount to much in the way of caloric expeniture.

What about the children?

It’s particularly worrisome for the children. Computers and television have largely replaced outdoor play after school. In the school setting, things are as bad; most schools have cut back drastically on physical education programs because of funding formulas that favor academic classes. The five-day-a week physical education programs that used to be standard in the schools are rare indeed; many high school students have phycical education classes only as an optional activity. Certainly many students do participate in organized school sports and in non-school sponsored sports such as community baseball leagues, but fewer and fewer children are getting much exercise. What is very important to understand is that it’s not a question of children getting no exercise, but rather, less exercise than in the past.
The situation is particulary bad for teenage girls. Studies have shown that girls participate in exercise activities less and less beginning at about age 14 years; by age 18 years, fewer than 20% of girls get any meaningful physical activity on a regular basis (unless you consider turning a key to start a car and holding a cell phone to one’s ear as meaningful exercise).

The “Thrifty Gene” Theory

So, we understand a bit of why the U.S. Pimas have so much more obesity (and diabetes) than their Sierra Madre cousins- it’s large differences in activity levels, and perhaps, something to do with diet composition. That explanation still does not really explain why the U.S. Pimas are so overweight; shouldn’t the Pima brain appetite center make adjustments in appetite to compensate for the low activity level? The answer to the question is that maybe the appetite center should compensate, but it doesn’t. Many scientists who study obesity believe that over thousands of years, human genes have been selected that allow “overeating” when food is plentiful. This genetic trait would have important survival value in times of famine- all the lean and mean people would starve to death while those with excellent fat stores would be more likely to survive. This is the so-called “thrifty gene” theory. Whether this is true or not, it seems to make sense. It is also a very scary thought, since it may well be that in the twenty-first century (2001 and beyond), most people around the world do carry these thrifty genes and we don’t know how to make them go away. Maybe the Pimas’ dilemma is that they just have too much of what used to be a good thing?

Implications of the thrifty gene theory

If this thrifty gene business is true, we will need to either wait thousands of years until something happens to decrease fertility in people who carry these genes, or find other ways to deal with our misguided appetite centers (behavior modification, drugs, surgery, etc.) Maybe we should move to the Sierra Madre?

Environmental (non-genetic) causes of obesity: overview

We have spent quite a bit of time discussing genetic factors, theoretical and proven, that might contribute to the high prevalence of obesity in the U.S. In summary, it is very clear that genetic factors are very important but they cannot fully explain why we have so much obesity, and particularly why we have so much more of it recently; I don’t think we can postulate a large upsurge in obesity genes over the past 20-30 years. So, what are the non-genetic factors that might explain our rising BMIs?

Remember the Pimas?

We briefly discussed the Pimas in an earlier posting. They are an indian tribe that started out in the Sierra Madre of Southern Mexico. Over many years, many of the Pimas migrated northward to what is now the southwestern U.S. Many of them live on the Gila River Indian Reservation, just outside of Phoenix, AZ. The Pimas have the highest known prevalence of diabetes (type 2) of any genetic group- about 50%. The Pimas also have a very high prevalence of obesity, including people many people with BMIs over 40. What is particularly interesting about the Pimas is that those who are still living in the Sierra Madre show virtually no obesity and very little diabetes. Yet, the Sierra Madre Pimas consume more calories, on average, than the New Mexico Pimas. What gives? The answer is activity; the Sierra Madre Pimas have to work hard to survive and their levels of physical activity are far higher than in their U.S. cousins.

Another factor that may be important is the diet composition itself. In the Sierra Madre, the Pimas eat mostly what they grow and raise- lots of corn and occasional chicken. The New Mexico Pimas eat what most other people in the U.S. eat- lots of highly processed foods which are high in fat. I know the U.S. diet is not as healthy as the Sierra Madre diet but I am not certain if diet composition itself contributes to the high prevalence of obesity in the U.S. Pimas and non-Pimas. Certainly it is easier to consume lots of calories when one eats lots of calorie dense foods, but that is a different issue.

Up to this point we have been discussing theoretical and known genetic mechanisms that might explain why there is so much obesity around. There is also strong epidemiological evidence. For example, the more overweight the parents, the more likely the child wil be overweight and that the overweight will persist into adulthood. If only one parent is overweight, the risk for the child is increased 6.8-fold; if both parents are overweight, the risk is 8.4-fold. One might argue that such data do not prove genetic causes, since environmental factors could also be to blame. I agree. But, if we look at data from twin studies, rates for overweight in identical twins, even raised apart show nearly 100% concordance- if one twin is overweight, the other is almost certain to be overweight also. The opposite is also true- if one of an identical twin pair is slender, the other twin is almost certain to be slender as well. These relationships do not hold for fraternal (nonidentical) twins. Clearly, genetic factors contribute to obesity and are powerful forces with which to contend. Next entry we will begin a discussion of non-genetic causes, those we are most able to modify.

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.
Calories

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).

Appetite

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!