Dale Schoeller: OK, part 2. I'm going to start to provide more answers to that question, so I'll address it over the next number of slides, but I will say it's not just the water loss. That water loss difference really only continues for the first 1-2 weeks in the diet. Thereafter, the difference in weight loss suggests that something else is going on.

This is just reiteration of our caloric balance equation. The energy that goes in the diet has to equal the energy that goes out in energy expenditure, plus the change in body energy stores. Based on that, we can predict how much weight a person should lose when they go on a diet.

In healthy but overweight individuals who are on a nutrient-sufficient diet, the amount of weight they lose over a period of 6, 12, and 24 weeks averages 80% of fat by weight. We know how many calories there are in a kilogram of fat, so we can look at those rates of weight loss over that dietary period, dietary restriction period, and predict that if a person decreases their energy intake 1,500 calories per day below their expenditure, they should be losing about 3 pounds per week. At the other end of the scale, if they're only decreasing their intake by about 100 calories per day, they'll only be losing about one tenth of a pound per week.

Knowing that, let's look at the results of a couple of published studies of weight loss that I've referred to earlier. One is the outpatient study in adolescents by Sondike, and the other is the inpatient study in which individuals were fed only 1,000 calories per day.

Shown here in the graph are the rates of weight loss in pounds per week for the 6-week inpatient study and for a 24-week outpatient study. You'll notice that again, the low-carb diet only shows the greater weight loss in the outpatients. This suggests again that it's not just the water loss, and also that the rates of weight loss are slower in the outpatient setting than in the inpatient setting.

Well, if we're going to conserve energy and obey the laws of thermodynamics, that means that the inpatient study, which was 1,000 calories per day, must not be being met in terms of the intakes in the outpatient study. The outpatients must be eating more than 1,000 calories per day, or else they'd be losing weight more rapidly.

This suggests that the differences in weight loss in the inpatient and the outpatient study are largely due to how well a person complies with the energy restriction. The differences that we've been talking about between a low-carb diet and a low-fat diet are due not to just the water loss, but are actually due to differences in compliance with the reduced caloric intake strategy of a diet.

So, why is it that patients are a little bit more compliant on a low-carb diet?

We don't know all the answers, but we do have some evidence, again, from the literature. One is that short-term satiety on a high-protein diet is greater. This was a study that was done in 35 preschool children, so we don't have to extrapolate for adults, which also shows similar results, by the way. But in the study, they provided a controlled lunch, following a snack that was either high in protein or high in carbohydrates. The children who consumed the lunch, an ad libitum lunch, following the snack that was high in protein ate less.

So there's something about these low-carb diets that is good, and it's probably that they're a little bit higher in protein than the average low-fat diet that is prescribed. So if you're going to prescribe a diet, do pay attention to the protein intake.

The other advantage of a low-carb diet is that it restricts the food choices a person has. Many studies have shown that the more variety on the plate in front of you, the more you're likely to eat. Nowhere is this more obvious than in a restaurant setting. If you are on some sort of restricted diet which says I cannot eat food X, Y, and Z, in this case, carbohydrate diets, on that plate in front of you, you're not going to eat the bread, you're not going to eat the dessert, because that's high-carb. You're not going to eat the potatoes along with it. So there are fewer calories on your plate while eating out. Many studies have shown that the people who eat out more, because we're overserved in almost every restaurant we go to, tend to gain more weight.

The other advantage of a low-carb diet, that may help people stay on that diet, is they're often given keto sticks so that they can test their urine daily to detect if they've overconsumed carbohydrate. That's a good form of biofeedback, and it helps a person with a daily compliance to a diet. So, the bottom line here is that these low-carbohydrate diets work because people eat fewer calories.

However, in the 1 long-term study that has been done in adults, that decrease, or that difference, the rapid weight loss that is seen in the first 12-24 weeks, dissipates when you start to look at these people after another 6 months, so there's no difference in weight loss on the typical low-fat diet or a low-carb diet.

Are there any other limitations to the low-carb diet?

Well, there is one that is very important for the adolescent. The low-carb diet suggests somewhere between 20 and 90 g of carbohydrate per day, depending upon which phase you're in. That has been shown in many studies to be insufficient for participation in sports. Physical activity and sports participation is very important to many adolescents. By cutting out the carbs, you're going to be cutting out their ability to perform.

It's also been known that the incidence of headache, probably due to the dehydration and low blood glucose and lethargy related to the low blood glucose, increases on the low-carb diet.

Finally, as we answered a little earlier in the talk, the low intake of whole grains and fortified grain products that accompany a low-carb intake provide a diet that's low in B vitamins and many minerals, so that a vitamin and mineral supplement is needed.

When you get to the bottom of the list, you have to ask yourself, why are we putting adolescents, or for that matter anyone, on this rollercoaster of weight gain -- followed by weight loss, followed by going back to bad dietary habits and regaining weight? We can avoid all of these negative consequences by better instilling a family approach, as Dr. Katz has indicated, which emphasizes good skills for controlling dietary intake and for increasing physical activity.

But let's return to the mystique of the low-carb diet again. As indicated by the published evidence, there are indeed differences in weight loss on a low-carb diet. During that first week, however, they're due only to increased water loss -- no health advantages there at all. The idea of a metabolic advantage on this low-carb diet, so you can eat all you want and still lose weight, is not supported by any evidence of measured energy expenditure. The differences in weight loss at 12 or 24 weeks that have been reported in the literature are due to consuming fewer calories on the low-carb diet.

So why not just consume fewer calories on a healthy diet? And, finally, the differences that are reported in many of the short-term studies have not been shown to continue when the patients are followed out to a 1-year interval.