Low carb, high protein diet found to increase mortality

Not good news for people who think low carbs is the answer. A recently published study has shown an increase in mortality associated with low-carb/high-protein diets.

Studies looking at the comparative effectiveness of different diets have shown that basically all diets are equivalent. (Equivalently bad, that is. All the diets tested produced moderate declines in weight, but adherence was low.) However, no real studies have examined what the mortality benefits or hazards might be associated with the low-carb/high-protein diets that are currently de rigeur.

Trichopoulou et al., publishing in the European Journal of Clinical Nutrition, have produced one of the first studies to look at this issue. (Happily it is not behind a subscription wall.)

They followed roughly 22,000 adult Greek citizens as part of the European Prospective Investigation into Cancer and nutrition (EPIC) from 1993 to 2003. During that period, there were 455 deaths in the study. They used information about the diets of the participants to calculate relative mortality rates for particular diets.

Just to cross the t's and dot the i's, they also control sex, age, years of schooling, smoking, BMI, physical activity, ethanol intake, and (in the data I am going to talk about) energy intake. (Someone always asks about that in comments, so I thought I would put it at the top.)

Generally what they found is in line with other studies about diet and mortality:

As presented in previous publications (Trichopoulou et al., 2003), the results with respect to non-nutritional variables were mostly in line with expectations, in showing that mortality was higher among men than among women, increased sharply with age, declined with increasing years of schooling, higher physical activity and alcohol intake and increased with smoking and total energy intake. (Link in the original.)

To quantify how much carbs and protein a participant is had, they reference mortality to what is called a low-carb/high protein ratio (LC/HP). This ratio is defined using the decile of each participants carbohydrate and protein intake relative to the rest of the study population:

[F]or each participant, ascending decile of protein intake and descending decile of carbohydrate intake were added to create an additive LC/HP using, alternatively, absolute and energy-adjusted carbohydrate and protein values. Thus, a subject with LC/HP score 2 is one with very high consumption of carbohydrates and very low consumption of protein, whereas a subject with score 20 is one with very low consumption of carbohydrates and very high consumption of protein.

The reason they use this kind of obscure measure is because one of the problems you have in a study like this one is total energy intake. Any change in diet composition is likely to cause a change in total calories consumed, and total energy consumption is also correlated with mortality. They argue that by using this measure, they have found one that does not correlate with total energy intake and is therefore more valid. (You can choose to buy that or not depending on preference.)

What they found when they compare mortality to LC/HP -- controlling for the factors listed above -- is a substantial increase in mortality for a relatively small change in diet:

An increase in the LC/HP score (energy-adjusted components) by two units was associated with an increase in mortality by 8% (95% confidence interval (CI), 3-13%). Therefore, a realistic increase in the LC/HP score by five units (corresponding to, e.g., an increase of protein intake by about 15 g/day and a decrease of carbohydrate intake by about 50 g/day) was associated with a 22% increase in overall mortality (CI, 9-36%). (Emphasis mine.)

Just for reference, a McDonald's cheeseburger has about 15 g of protein, and 2.5 cups of Cheerios (about a bowl) has about 50 g of carbohydrates.

To get a sense of typical diets in Western countries, they also mention these comparisons:

Very-low-carbohydrate diets typically contain less than 10% carbohydrates, 25-35% proteins and 55-65% lipids. For comparison, the average American diet contains 35% lipids (85 g/day), 50% carbohydrates (275 g/day) and 15% protein (83 g/day) (CDC, 2004). In Great Britain, the mean intake, as a percentage of total energy, is about 35% lipids, of about 48% carbohydrates and about 17% protein (Swan, 2004).

It is also reasonable to ask in a mortality study, what kind of deaths increased? Well in this study, they showed increases in deaths from all causes, but the only statistically significant increase in deaths was from cardiovascular causes.

What to make of this study?

Well, first there are a some caveats.

1) The people in this study were not explicitly on low-carb diets, so in order to apply this research to them we would have to extrapolate the findings. Further, most low-carb diets involve substantially less carbs than even the lowest decile of carb consumption in this study. The authors, however, argue that it is fair to make that extrapolation:

In our study population, consumption of carbohydrates, even at the low extreme of the distribution, was higher than that advocated by the prescribed low-carbohydrate diets and few individuals consumed more than 20% of their energy from proteins. Nevertheless, it is unlikely that at the extremes of the low-carbohydrate-high-protein intake distribution there would be a reversal of the trend evident in our study population. Indeed, many of contemporary public health policies rely on extrapolations, so that if something is detrimental at a certain exposure level, its effect is likely to be more detrimental at a more extreme level.

2) The Greek diet is very high in monounsaturated fats -- from the olive oil, and this makes their diet different than say American or British diets. Monounsaturated fats have been shown to have some positive health effects. However, the authors argue that differences in fat intake were controlled for in the analysis.

3) Finally, the study did not at all address the issue of supplements, and these could affect the outcome.

In spite of those caveats, however, I would use this research as evidence to caution anyone from embarking on a low-anything high-anything diet -- including low-carb high-protein.

Extreme diets in any form are unwise because too much of anything is not a good thing. Dieting is a tough business, but the evidence shows that good diet is still -- in spite all the lame hype associated with alternatives -- based on healthy food like fruits and vegetables and lots and lots of exercise.

Sorry. The world sucks that way.