Feb 26 2009

Want To Lose Weight? Reduce Calories.

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As a scientific skeptic I am careful to emphasize that my current position on any scientific question is tentatively based on available evidence. If new data comes in that warrants a change in my position, I will happily change it. Not only is this position scientific, it has the advantage of not tying you to a position that might be wrong. I try to become as emotionally dispassionate about specific conclusions as possible – it is only the validity of the process that I value.

But I admit I cannot help but feel a sense of satisfaction when new data supports a position I have taken on a controversial topic (a guilty pleasure, I suppose).  In this case a new study published in the New England Journal of Medicine comparing various weight loss diets supports my contention that weight loss is all about calories in vs calories out.

There has been a tremendous amount of popular marketing of diets claiming that the proportion of macronutrients – fats, protein, and carbohydrates – is important to weight loss and other health factors. For example, Atkins has made low-carb diets extremely popular. The problem has always been that the scientific data simply did not support any such claims. This did not stop the press from declaring that “Atkins is vindicated” every time a small and worthless pilot study came out showing possible short term advantage but no long term advantage, and overall very small differences in various diets. The weight of the evidence supported the conclusion that weight loss is achieved by burning more calories than are consumed, regardless of the source of those calories.

The authors of this new study summarize prior research thusly:

Small samples, underrepresentation of men, limited generalizability, a lack of blinded ascertainment of the outcome, a lack of data on adherence to assigned diets, and a large loss to follow-up limit the interpretation of many weight-loss trials. The novelty of the diet, media attention, and the enthusiasm of the researchers could affect the adherence of participants to any type of diet.

The thread of plausibility that many miracle diet advocates cling to is that their diets reduced hunger so that you could more easily curb calorie intake. This is certainly plausible, but again the problem was simply that the data did not show a big effect. I was willing to accept that maybe there was a short term effect from very low carb diets, but no study showed a persistent advantage beyond 6 months. In most of these studies at the end of a year everyone was in a similar place, with a small amount of weight loss and any differences between diets gone or diminishing.

Clearly what was needed to resolve any lingering controversy was a large study with a long follow up. That is what this current study does. Here are the methods:

We randomly assigned 811 overweight adults to one of four diets; the targeted percentages of energy derived from fat, protein, and carbohydrates in the four diets were 20, 15, and 65%; 20, 25, and 55%; 40, 15, and 45%; and 40, 25, and 35%. The diets consisted of similar foods and met guidelines for cardiovascular health. The participants were offered group and individual instructional sessions for 2 years. The primary outcome was the change in body weight after 2 years in two-by-two factorial comparisons of low fat versus high fat and average protein versus high protein and in the comparison of highest and lowest carbohydrate content.

A two year follow up is significant and long enough that any short term effects (on the order of months) should be worked out the data by then. After all, what people want and what is important for health is sustained long term weight control.  The results are easy to summarize – no difference in any of the groups. Low fat vs high fat, low carbs vs high carbs, low protein vs high protein – no difference. Weight loss correlated only with compliance with the diet and calorie restriction. The authors conclude:

Reduced-calorie diets result in clinically meaningful weight loss regardless of which macronutrients they emphasize.

The study also found that calorie restriction and weight loss correlated with an improved lipid profile.  This is not surprising and is consistent with prior studies.

This is one of those issues, however, where I have encountered surprising emotionality. It is amazing how ideological people can become when it comes to food and weight. Especially if someone has tried a diet and they believe it worked for them – they become hardened true believers and passionately spread the word about their amazing discovery. They may quietly forget their fervor when the weight returns, however (which is does in the vast majority of cases).

Given the wishy-washy nature of prior evidence, this large and long follow-up study should put to rest the hype over macronutrient-based diets. I know it won’t, but it should. No one wants to hear the truth we all know in our hearts – if you want to lose weight you have to eat less and move more. The macronutrient claims are largely a distraction and public health would best be served (but not the self-help book industry) if we move past them.

This study does not address what should now be the real focus – how to maintain a permanent change in habits to effect a more healthy or appealing weight? Only about 5% of the population can seem to do this. Will power alone (the preferred method by default) is notoriously ineffective.  Some basic principles have been established – long term weight control correlates with sustainable changes in habits (not short-term diets) and correlates strongly with exercise. Common sense dictates others – healthy habits are more likely to be sustainable if they are easy and appealing.

There is no magic solution.  I do think that fad diets are an unfortunate distraction for many people. Hopefully this study will help reduce the distraction. Unfortunately, such appealing distractions are very profitable.

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