My patient, Mrs. Withers (not her real name), was forty-five and morbidly obese. "I swear I've cut my calories down to almost nothing," she told me, "but I haven't lost a pound! I eat the exact same thing every day: a banana for breakfast, a turkey sandwich on wheat bread for lunch, and a piece of fish or steak for dinner. And no snacks in between! I used to eat twice as much, easily. I just don't know what I'm doing wrong!"
She was literally on the verge of tears. I fully understood her frustration, as well as why her attempts at dieting hadn't worked. Most of my patients who try to lose weight long-term ultimately fail, though not for lack of trying. Both physiologic and psychological mechanisms are in place that make sustained weight loss incredibly difficult. But, as I told Mrs. Withers, if you understand some of the mechanisms you're up against in trying to lose weight, you'll be able to leverage that understanding into weight loss strategies that work.
BACKGROUND
Body Mass Index (BMI) is now the standard measurement used to assess body fat composition, calculated by dividing your weight in kilograms by your height in meters squared (calculate yours here). The National Institute of Health now defines anyone with a BMI>25 as overweight and anyone with a BMI>30 as obese (although it's worth noting BMI fails as a good measure of obesity for people who are extremely muscular). Unfortunately, more than 50% of Americans have BMI's over 25. Overweight and obesity have clearly reached epidemic proportions.
Having a BMI>30 is statistically associated with numerous diseases such as diabetes and hypertension and, even more importantly, with an increased risk of death. Unfortunately, studies also show that 90-95% of overweight and obese people who successfully lose weight subsequently regain it within 5 years. Given the great value placed on thinness in American society and the health risks of obesity, it's not surprising that so many people continually turn to fad diets popularized in the mainstream press in efforts to lose weight.
WHAT CAUSES OBESITY?
The first law of thermodynamics, which states that the amount of energy stored in your body = your energy intake - your energy expenditure, applies to all biologic systems. In humans, mechanisms keep this difference very close to zero, but small differences over a long period of time can have a large cumulative effect. On average, between the ages of 25 and 55, Americans eat only 0.3% in excess of what they burn but this results in a weight gain of 20 lbs. over that time!
Body fat content is highly regulated. In recent years, evidence has accumulated supporting the set point theory of body weight. Mechanisms have been identified that enable the body to "defend" its usual weight, whether that weight is excessive or not. Small decreases in body weight cause your body to lower its energy expenditure even if you cut your calories, which explains how people like Mrs. Withers can cut down their food intake and literally not lose a pound. Further, studies have shown that if you were once overweight, in order to maintain your new lighter weight you have to eat fewer calories than a person who weighs what you do now but who was never obese! Clearly, some people seem to have an increased propensity to store fat and to maintain a lower resting metabolic rate, making weight loss more difficult.
When I explained all this to Mrs. Withers, she heaved out a heavy sigh and asked, "What about the Atkins diet? Doesn't that work?"
THE LOW-CARBOHYDRATE DIET (ATKINS SAYS...)
The low-carbohydrate ketogenic diet was first devised by an English surgeon, William Harvey, over a century ago, and has been "rediscovered" several times throughout the last hundred years by different authors: in 1953 by Pennington, in 1961 by Taller, in 1967 by Stillman, and most recently (and for the second time) in 1992 by Atkins. Essentially, the low-carbohydrate diet is a protein-sparing modified fast (PSMF) in which calories from carbohydrates are replaced by calories from fat and/or protein. In Dr. Atkins' New Diet Revolution, Atkins advances the following claims:
- On the ketogenic diet, you can eat an unlimited number of calories and still lose weight.
- Ketosis reduces hunger.
- High insulin levels cause obesity. If plasma levels of insulin are reduced by eating a ketogenic diet, this will lead to weight loss.
None of these claims, however, is true. Let's review them one at a time.
On the ketogenic diet, you can eat an unlimited number of calories and still lose weight. This would seem to violate the first law of thermodynamics. There is evidence that when people set out to eat a low-carbohydrate diet, they lose weight because in fact they end up eating fewer calories. It's been shown that the accelerated short-term weight loss seen in the low-carbohydrate diet over the first 10 days is due to loss of water.
Ketosis reduces hunger. Of the few well-controlled studies that are available, none show a reduction of hunger as a direct result of ketosis. However, there is evidence that when about 500 kcal/day are consumed on a low-carbohydrate diet (an extremely low number of calories to eat per day!), hunger is indeed reduced. Possible causes might include the high protein content of low-carbohydrate diets, some particular characteristic of the protein-sparing modified fast as of yet unidentified, or the monotony of such a low-calorie diet. The last possibility is supported by evidence that hunger is greater when people consume a preferred food than when they consume a less preferred food, and that people consume more food at meals with several food choices than at meals with only one choice.
High insulin levels cause obesity. There is in fact far more evidence to the converse, that obesity causes high insulin levels.
POTENTIAL HEALTH RISKS OF THE LOW-CARBOHYDRATE DIET
Substituting fat for carbohydrate in the low-carbohydrate diet would seem to put people at risk for developing high cholesterol. However, multiple studies have shown that on low calorie diets, if weight loss ensues, cholesterol profiles improve—or at least do not worsen—regardless of what kind of food you eat. This suggests that the beneficial effect of weight loss on cholesterol is more important than the detrimental effect of eating a relatively increased percentage of calories from fat.
POTENTIAL BENEFITS OF THE LOW-CARBOHYDRATE DIET
While ketogenesis itself has been shown not to affect weight loss, there is evidence that the higher intake of protein that typically occurs with the low-carbohydrate diet might. Studies have shown a consistent, spontaneous reduction of total calories consumed by people eating protein-laden diets compared to people eating carbohydrate-laden diets—by approximately 470 calories/day. There seems to be two mechanisms at work in the protein-laden diet to cause greater weight loss than in carbohydrate-laden diets:
- Eating foods rich in protein seems to cause people to feel more full and to suppress even the following day's calorie intake more than eating the same amount of food rich in carbohydrates.
- A low calorie, protein-laden diet reduces your resting metabolic rate to a lesser degree than a low calorie, carbohydrate-laden diet. In other words, when you cut your calories down you'll have a higher resting metabolic rate if the calories you do eat come predominantly from protein instead of carbohydrate. And having a higher resting metabolic rate means you burn more calories just sitting around.
I told Mrs. Withers that some of my patients do, in fact, find their hunger reduced when they eat a low-carbohydrate diet—while others do not. "How can you tell which you'll be?" she asked me.
"You just have to try it and see," I replied. But even those who do lose weight on a low-carbohydrate diet most often find the diet too boring, I warned her, and eventually go off it and regain the weight they'd lost.
She became despondent again. "So what can I do?"
PUTTING IT ALL TOGETHER
