The Paleo Diet - Loren Cordain [90]
Why Exercise by Itself Doesn’t Promote Weight Loss
The idea of exercising the extra pounds away—if this is your only means of weight loss—is not terribly practical. Exercise combined with diet is no more effective than diet alone in causing weight loss. How can this be? The answer is a scientific equation: to lose a pound of fat, you need to achieve a caloric deficit of 3,500 calories.
Imagine that a mildly obese woman, weighing 154 pounds, would like to lose 30 pounds, or 105,000 calories, by walking or jogging for 3 miles (forty-five minutes) a day. On days when she walks or jogs, she expends 215 additional calories (compared to the 80 calories she expends for that same forty-five-minute period on other days). The 3-mile walk/jog causes a net deficit of 135 calories—not a lot, considering the amount of work she’s doing. At this rate, it will take her 26 days to lose 1 pound and 780 days (more than 2 years) to lose 30 pounds. Most dieters simply don’t have the patience to wait that long. (Frankly, most of us need the encouragement of seeing the scale change more rapidly to help us keep up the good work. Otherwise, it’s easy to become discouraged and give up.)
Experiments by my colleague Dr. Joe Donnelly and coworkers at the University of Nebraska at Kearney, and by Dr. David Nieman at Appalachian State University in Boone, North Carolina, have demonstrated that diet alone is just as effective as diet plus exercise in causing weight loss.
The real benefit from exercise for weight loss comes not from the modest caloric deficit that it may create, but from its ability to keep weight off once it has been lost. Dr. Rena Wing of Brown University School of Medicine in Providence, Rhode Island, reviewed a large number of exercise trials in which participants either dieted only or dieted and exercised. Reporting on the participants a year later, Dr. Wing noted, “In all of the long-term randomized trials reviewed, weight losses at follow-up were greater in diet plus exercise than in diet only.”
Why Should You Exercise?
Regular exercise, though, is great for your body. One major benefit: it improves your insulin metabolism. As I’ve discussed earlier in this book, many overweight people are insensitive to insulin, a hormone secreted by the pancreas that aids the entry of glucose from the bloodstream into all cells of the body, including the muscle cells. When muscle cells become insensitive to insulin, the pancreas responds by secreting more insulin. This, in turn, raises the normal level of insulin in the bloodstream. The resulting elevation of blood insulin levels, called “hyperinsulinemia,” is the underlying cause of the metabolic syndrome diseases. Insulin is a master hormone that influences many other critical cellular functions. An elevated level of insulin in the bloodstream encourages fat deposition and the development of obesity.
Regular exercise has been shown in clinical studies to improve the muscles’ sensitivity to insulin and to lower the level of insulin in the bloodstream. In other words, although exercise alone does not cause the large caloric deficits needed for weight loss, it sets the metabolic stage for weight loss to occur by improving your insulin metabolism—as long as you cut back on calories.
Improving your insulin sensitivity may also reduce your appetite by preventing the large swings in blood sugar levels that are a direct consequence of too much insulin secretion. When you eat a carbohydrate-heavy meal, digestive enzymes convert most of the carbohydrate to glucose, which then enters the bloodstream. Normally, the pancreas secretes just the right amount of insulin to help convert glucose into muscle and other cells of the body and to help keep the blood sugar level on an even keel. However, when your muscles are resistant to insulin’s action and the pancreas must secrete extra insulin, this drives the blood glucose level even lower. This reduction in