Copyright © 2009
by Daniel Matthew Korn

All Rights Reserved

It is almost universally assumed that if we get enough physical activity in our daily lives and eat properly we are likely to be lean and healthy. We worry that children spend too much time watching television and playing computer games. However, even people who have jobs that are just as physically active as those typical of workers hundreds of years ago are becoming overweight in large numbers. Many people who have been successful losing weight by exercising find that at some point their weight loss slows and then stops. This occurs despite any change in diet or exercise regimen and is known as a plateau among fitness trainers; researchers have not found an answer for why this happens. We will begin by reexamining the link between exercise and weight maintenance or weight loss to see if we are missing something.

To track changes in obesity over time, we need a way of measuring to what degree people are overweight. The measurement most frequently used by scientists studying obesity is the Body Mass Index (BMI). It is a ratio calculated by dividing a person’s weight in kilograms by the square of his or her height in meters. A score of 25 to 29.9 is generally considered to be overweight, 30 to 39.9 obese, and 40 or over severely obese. These scores are not perfect, and they can incorrectly classify athletic men with a large amount of muscle mass but very little fat as overweight. However, the BMI has been in use longer than other measurements such as body fat percentage or the waist-to-height ratio, and it is still the measurement most frequently used in obesity studies.

In December, 1953, Dr. Hans Kraus, M.D.…[claimed that] the affluent lifestyle of 20th century America was making life so easy and effortless that American adults and children were rapidly losing muscle tone…Americans would have to engage in regular exercise to attain a state of physical fitness comparable to that of an earlier era, when Americans walked for transportation, worked on farms, and accomplished most activities…through manual labor.¹

The Council had been established under President Eisenhower due to his justifiable concern that the United States economy and technology were leading to an American population that was less physically fit than its European counterparts. As a veteran of both World Wars who had risen to the highest military position, Eisenhower was intimately aware of the importance of physical strength to an infantry soldier. However, the number of adults in the U.S. who were severely obese (BMI ≥ 40) during his presidency in the 1950s was so small that it would not be tracked for another 30 years. The percentage of American adults who were obese (BMI ≥ 30) in the fifties was 9.7%,² over 70% less than the percentage today, which is 34% according to the National Center for Health Statistics. In 1900, the number was about 1 in 150, or less than 1%.³ Most of the obesity increase the United States experienced in the twentieth century occurred long after work and travel were no longer a source of regular exercise for most people, not concurrently with the decrease in activity levels.

Actually, it is not certain that people in the United States are much less physically active than people in less technologically developed societies. In fact, a recent study shows that levels of energy expenditure of women in rural Nigeria and metropolitan Chicago are not that different. Yet, despite performing similar amounts of physical activity, the women in Nigeria are much thinner than the women in Chicago. More interestingly, even within each group of women, the ones who are the most active are not significantly thinner. Since they are not getting more exercise than their American counterparts, it must be that something other than exercise is keeping Nigerian women thin and giving the United States a high rate of people who are overweight.⁴

Despite the fact that exercise does not explain why Nigerian women are thin, it is nevertheless possible that it can help us stay lean when we are exposed to the modern Western diet. In our society, there are still jobs which involve a very high level of physical activity. While many crops are planted and harvested mechanically, most fruits and vegetables are planted and picked by hand, as they have been for thousands of years. For this reason, the fruit and vegetable farms of my native state of California require massive amounts of labor.

These farming jobs are some of the hardest and lowest paying available in California. Teenagers and low income Americans would much rather work in retail or fast food jobs. Working on the farms involves staying outdoors all day and engaging in manual labor. There are very few tools, other than those that are manually powered. This is the intense nature of work that humans engaged in for thousands of years, during which our ancestors were all lean. If an increasingly sedentary lifestyle were responsible for the weight gain of the American population, we would expect agricultural workers to be the thinnest people in California. However, this is not the case. In fact, a study of agricultural workers found “that 81% of male workers and 76% of female workers were either overweight or obese.”⁵

We can conclude that physical activity level by itself does not seem to keep Nigerian women thin, nor does it prevent California’s agricultural workers from becoming overweight. Later, we are going to see that genetics play a role in allowing people to become overweight when they are exposed to specific conditions, but they are not the cause of obesity. Because of genetic differences, two people who get exactly the same diet and level of exercise will not necessarily weigh the same amount, so one could argue that the women in Nigeria may have some genetic trait that allows them to stay thin despite a low physical activity level. However, this argument would predict that Nigerian women would stay thin when they immigrated to the United States, and this is not the case. Thus, physical activity and genetics are not the primary reasons why women in Nigeria are lean. There is something present in the United States that is not present in Nigeria which is causing these women to become overweight when they immigrate, or there is something present in Nigeria whose absence in the United States leads to weight gain, or both.

Similarly, the ancestors of agricultural workers in California were all lean. Their physical activity level was not too different from what is experienced on farms today. Yet the ancestors were thin, while the descendants are becoming overweight. The only differences that can explain the rising level of obesity are the diets people eat and the environment in which they live.

While exercise may not be what is keeping people in Nigeria lean, and it is not preventing California’s agricultural workers from becoming overweight, it is possible it can help people who are overweight become lean again in conjunction with diet. After all, we frequently hear that a combination of diet and exercise is the key to weight loss. So let us examine whether diet and exercise together are more effective than diet alone, jointly comprising the best method of weight loss.

The United States is filled with gyms. For the most part, people are not going to those gyms because they want to be healthy; they are going because they want to be thin and toned. The prevailing wisdom says that eating fewer calories and burning even more through exercise will make us thin or keep us thin.

We saw that when the President’s Council on Physical Fitness was established Americans were already leading very sedentary lives, yet our lack of exercise had not caused us to become overweight the way we are now. So the idea that lack of exercise alone is responsible for weight gain does not take into account that Americans used to be much thinner without exercising strenuously or leading very active lives. A U.S. Government study of the largest and best controlled experiments studying the effects of exercise and diet concluded that:

While physical inactivity appears to be a strong contributing factor to…obesity, studies on the effectiveness of [exercise] in promoting weight loss have been less than encouraging. Because physical activity increases energy expenditure, it is reasonable to assume that the combination of diet plus exercise would be more effective than dieting alone for weight loss. A recent review of 13 controlled studies found that this is rarely the case. Only two of the 13 studies reported statistically significant differences in weight loss from the diet plus exercise regimen compared to diet alone. The conclusion from this review was that ‘…exercise does not significantly increase initial weight loss over and above that obtained with diet only.’⁶

The study did find that exercise has positive health benefits. However, lack of exercise does not seem to be the main cause of weight gain, and exercise alone does not seem to be the key to long-term weight loss. Many people who experience weight loss from exercising eventually “hit a plateau,” a point at which their weight loss stops. Later on, we will see something that can explain why these plateaus occur. We will also see why, although moderate exercise, along with another factor, offers some benefit in weight loss, too much exercise can actually decrease that benefit and the other health benefits of exercise.

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Citations:

¹ Prager, Felice. "The Early History of the President’s Council on Physical Fitness." Rewind the Fifties. 3/31/2009 http://www.loti.com/sixties_history/The_Eisenhower_Years.htm.

² CDC, National Center for Health Statistics, National Health and Nutrition Examination Survey. Health, United States, 2002. Flegal et. al. JAMA. 2002;288:1723-7. NIH

³ National Institutes of Health, National Heart, Lung, and Blood Institute, Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults, 1998.

⁴ Ebersole, Kara E.. "Energy Expenditure and Adiposity in Nigerian and African-American Women." Obesity: A Research Journal. 3/31/2009

⁵ Rodriguez, Michael. "Health of Migrant Farmworkers in California." California Research Bureau. 3/31/2009 http://www.library.ca.gov/crb/03/05/03-005.pdf.

⁶ Welk , Gregory J. and Steven N. Blair. "Physical Activity Protects against the Health Risks of Obesity." The President's Council on Physical Fitness and Sports Research Digest. 3/31/2009 http://www.fitness.gov/digest_dec2000.htm