Like many of my physician colleagues, I have been accused of harping on a single weight loss message. Weight loss is merely a question of calories in and calories out we usually say. Just make your calorie balance negative and all will be well. You keep on arguing with us. ‘I swear I don’t eat that much, I watch my calories but I’m not losing weight,’ you respond. We then roll our eyes and complain about you in the doctor’s lounge (just kidding).
In our defense, there has never been any convincing research to say that this is not true. More importantly, the first law of thermodynamics argues that it is. A recent article in the Journal of the American Medical Association may change our collective professional views somewhat. The article is getting a lot of press. A recent article in the New York Times discussed the study but left out some very important details. Unfortunately, this and many of the other reports leave out a great deal of important information. This leads them to overdramatize the results. I think it would be helpful to explain the article to you in its entirety to help you understand the data more completely. I’ll try to give you the entire picture without the media spin.
The study had two phases and started with 32 participants. In the first “run-in” phase, subjects were placed on a balanced diet designed to get them to lose 12.5% of their body weight. Only 21 subjects that were able to lose that much weight and be included in the second “test” phase where they were placed on one of three diets:
1. Low fat diet. This diet was designed to reflect conventional recommendations to reduce fat and increase whole grains, fruits, and vegetables.
2. Low glycemic index diet. Replaced grains and starchy vegetables with sources of healthful fat and low-glycemic index vegetables, legumes, and fruits.
3. Very low carbohydrate diet. This diet restricted carbohydrates to about 10% of calorie intake.
The study was designed to measure metabolism using radio-isotope measurements, activity level using an accelerometer, hormones, insulin sensitivity, cholesterol, and participant reported measures of hunger and well-being.
The most interesting finding of the study was that total metabolism (total calories burned per day) decreased in all groups after dieting but to a greater extent in the low fat diet group (about 423 calories/day). It is well known that when you diet, your metabolism slows down but it was somewhat surprising to see the differences among the groups and the extent to which it slowed. The group that ate the very low carb diet had the smallest drop in their metabolism (about 97 cal/d). In other words, this diet had less of an effect on the slowing of metabolism such that you would have to eat 300 fewer calories to maintain the same weight loss on a low fat diet as comparede to a low carbohydrate diet. The low glycemic index diet was in between (297 cal/d). These differences were not accounted for by increased exercise among the dieters as measured by an accelerometer.
The hormone leptin (which I talk a lot about in my book) is a hormone that, when elevated, triggers appetite and affects metabolism to help gain weight. In the low-fat diet, levels were the highest while they were the lowest in the low carb diet. Insulin effectiveness, HDL cholesterol, and triglycerides were all better in the low carb diet, following the same trend as the numbers for metabolism. There were no differences in the subjects sensation of hunger.
It appears that a calorie is not necessarily a calorie with respect to metabolic responses to dieting. All dieting leads to a reflex decrease in metabolism that will favor weight gain if you’re not careful with controlling your calories. With that said, it appears that a diet that is lower in carbs will cause less of a drop in metabolism and help maintain weight loss better in the short term.
This explains why people who are on low fat diets have a harder time losing weight initially than those on low carbohydrate diets. It also helps to explain the explosion in obesity in our country. About 40 years ago, dietary fats were found to be associated with heart disease and were regualted to be decreased significantly in our food. The food industry replaced those calories with carbohydrates which would lead to the metabolic changes noted above and make weight loss harder.
1. Even though this is a seemingly exciting and powerful study that sheds light on a very important part of weight loss, it is important not to get carried away and draw too many conclusions from the results. This was a small study over a small period of time that didn’t include exercise as part of the regimen. It is not clear if the differences among the groups would persist over time. It is possible that the differences could equal out or reverse entirely if the diets are maintained for a even a short time longer. Most of us want to be successful for more than just one month.
2. The calorie intake was strictly monitored and controlled by the research coordinators. This means that it is not clear how effective the diet would be if subjects were left to do it on our own.
3. Only subjects that were succesfull at losing weight during the run-in phase were entered into the test phase of the study. This could bias the results significantly in any number of ways.
4. It’s not clear what the diet’s effects were on body composition. Maybe the metabolic increases came from decreasing muscle mass and not fat. What good is it to lose weight if you become more flabby? This would also have an impact on future metabolism which is largely determined by lean muscle mass.
5. On a more cautionary, note, one of the things the researchers measured was 24-hour cortisol excretion which is a marker of physiologic stress. It was significantly higher in the low carb group. This indicates that a low carb diet may be putting the body under stress. The values were not high relative to normal but they were significantly higher in the low carb group. Additionally, C-reactive protein was higher indicating a pro-inflammatory response to the diet. These are possibly important indicators that even though the diet may be better for losing weight in the short term, it may not be good for general over-all health in the long term.
I think this study supports but does not clearly prove some of the things I have been discussing in previous posts about the dangers of sugar loaded foods and beverages. A diet that is specifically low in fat may be less effective at weight loss because of the resultant larger decrease in metabolism. You should be careful when buying foods that are advertised as low in fat. They usually make up for the lower fat by increasing the sugars (click here for more on this). If you’re looking to lose weight, you should cut out the carbs first but you still have to keep track of your overall calorie intake. It is important to note that it still is just an equation of calories in and calories out. It just turns out the the type of calories in may have an impact on the amount of calories out.
With that said, I worry that people will think that they can cut out carbs and eat whatever else they like. Remember that, in this study, overall calories were still rigidly controlled. This is something I cannot stress enough. If you’re not counting, you’re probably not losing. The authors still conclude, and I strongly agree, that sustained and effective weight loss requires long-term changes in lifestyle. Even if a low-carb diet is more effective in the short-term, the weight will come back if the change is not permanent.