For background information, see the Introduction.
Part 1: Is It Really Low-Carb?
The study purports to evaluate "the effects on mortality of habitual low carbohydrate–high-protein diets that are thought to contribute to weight control." - ostensibly, this study evaluates low-carbohydrate/high-protein diets designed for weight control (that is, weight loss). In other words, this study purports to evaluate low-carb weight-loss diets. (I may sound repetitive here, but I have a purpose). What, then, does this study consider to be "low-carbohydrate/high-protein"?
According to the Results section:
With respect to the choice LC/HP score that relies on energy-adjusted components, at the high extreme of the distribution around 20% of energy intake was derived from proteins, whereas around 25% was derived from carbohydrates. At the low extreme of the distribution, around 10% of energy intake was derived from protein, whereas more than 50% was derived from carbohydrates.
Thus, for the purposes of this study, "low-carbohydrate/high-protein" means 25% carbohydrate and 20% protein.
Yes, you read that right: this study claims that a diet that includes 25% of its caloric intake from carbohydrate is "low-carb", and that a diet that includes 20% of its caloric intake from protein is "high-protein".
For a 2,000 kCal daily intake, these values equate to 500 kCal, or 125g, of carbohydrate per day - and bear in mind, this is the most extreme low-carb limit in the study. (Likewise, these values equate to 400 kCal, or 100g, of protein per day, for a 2,000 kCal daily intake. This is the extreme high-carb limit in the study.)
So, my first question is this: what mainstream low-carb diet in any way resembles this macro-nutrient intake?
To put the bounds of the study into perspective, here are the mean macro-nutrient intake values for the population:
In this population, the mean intake of protein was 76 g/day with standard error of the mean (s.e.m.) 0.16 g/day, the mean intake of carbohydrates was 208 g/day with s.e.m. 0.44 g/day, and the mean intake of lipids was 109 g/day (28% saturated, 15% polyunsaturated, 48% monounsaturated fatty acids and 9% other components of the lipid group) with s.e.m. 0.25 g/day.
Let's explore those numbers for carbohydrate intake: mean intake 208 g/day, s.e.m. 0.44 g/day. Standard Error on the Mean (s.e.m.) is equal to the standard deviation (σ) divided by the square root of the number of samples (total population size). The total population size is 22,944; the square root of 22,944 is 151.5. Thus:
s.e.m. = 0.44 g/day = σ / 151.5
σ = 0.44 g/day * 151.5 = 66.7 g/day
Thus, carbohydrate intake for the study was a mean of 208 g/day with a standard deviation of 66.7 g/day.
Standard Deviation is the measure of the distribution around the mean. We can probably safely assume that our population is normally distributed (bell-shaped curve, with the peak at the mean, and exactly half of the population on either side of the peak). In a normally distributed set of data, 2/3 of all data lie within 1 σ of the mean, and 90% of all data lie within 2 σ of the mean.
Therefore, given the values for mean and standard deviation, we know that, statistically speaking, 2/3 of the study participants had a carbohydrate intake between 141 and 274 g/day, and that 90% of the study had a carbohydrate intake between 75 and 340 g/day.
How accurate is that estimate? The accuracy depends on the normality of the distribution.
According to Table 1 of the study, only 552 men (5.9%) and 2,218 women (16.3%) - a total of 2,770 participants (12.1%) had a carbohydrate intake of less than 140 g/day.
Based on the mean and standard deviation above, we can estimate that since 2/3 of the study participants had a carbohydrate intake between 141 and 274 g/day, that 1/3 were outside of that span - half of which (1/6, or 16.7%) had a carbohydrate intake of less than 140 g/day.
If anything, the distribution is biased against the low-carb side of the curve - meaning that the true number of participants at any given standard deviation is actually less than predicted by the normal curve.
The two most popular mainstream low-carb diets are the Atkins Diet and Protein Power (the South Beach diet does not consider itself to be "low-carb" and is excluded here).
The Atkins Diet allows anywhere from 20g carbohydrate per day in the Induction phase to 60-90g carbohydrate per day in the Maintenance phase (with allowance for more than 90g per day for active exercisers).
Protein Power allows anywhere from 40g carbohydrate per day at the Intervention level to 80g carbohydrate per day at the Maintenance level (according to Table 13.6 of my Protein Power Lifeplan book, page 355).
That means that less than 5% of the study participants had a carbohydrate intake anywhere near what is specified by mainstream low-carbohydrate diets.
In other words, this study in no way resembles either mainstream, low-carbohydrate diet. Therefore, any conclusions to which this study might lead do not apply to mainstream low-carbohydrate diets.
Coming Soon - Part 2: Is It Really High-Protein?
