In 1977 the Senator McGovern committee issued some dietary goals for Americans (Ref 1). The first goal was “Increase carbohydrate consumption to account for 55 to 60 percent of the energy (caloric) intake.” This recommendation did not come from any evidence related to carbohydrate. It was the inevitable consequence of setting a dietary fat guideline of 30% with protein being fairly constant at 15%.
Call me suspicious, but when a paper published 40 years later, in August 2018, concluded that the optimal intake of carbohydrate is 50-55%, I smelled a rat. The study, published in The Lancet Public Health (Ref 2), also directly contradicted the PURE study, which was published in The Lancet, in August 2017 (Ref 3). No wonder people are confused.
The study was in three parts, but all were based on population studies and so the usual limitations of these apply. The first part was a study of 15,428 adults aged 45-64 years, in four US communities, who completed a dietary questionnaire at enrolment into the Atherosclerosis Risk in Communities (ARIC) study (between 1987 and 1989). The primary outcome of interest was all-cause mortality. The second part of the study involved combining the data from the ARIC study with data from seven (multi)national population studies in a meta-analysis. The final part was an assessment of whether the substitution of animal or plant sources of fat and protein for carbohydrate affected mortality.
I will do a complete review of all parts of the study, but I wanted to break away from looking at tomorrow’s post on keto mice and diabetes (Ref 4) to highlight what I think is the major flaw in this ‘moderate carb is best’ paper. The paper has generated irresponsible global headlines, such as “Low carb diets could shorten life, study suggests” (Ref 5). As epidemics of obesity and type 2 diabetes show no sign of abating, encouraging the consumption of at least half one’s diet in the form of the one non-essential macronutrient, is unhelpful, to say the least.
The conclusion from the first part of the paper (the ARIC study) was that 50-55% of energy from carbohydrate was associated with the lowest risk of mortality. The average intake of carbohydrate in the ARIC study was 49%. The paper presented a U-Shaped curve to indicate that carbohydrate intake below 30% and above 65% was associated with the highest risk of mortality.
I wondered what kind of person would be consuming a low carbohydrate diet in the late 1980s/early 1990s (when the 2 questionnaires in a 25 year study were done). The characteristics table in the paper tells us exactly what kind of person was in the lowest carbohydrate group. They were far more likely to be: male; diabetic; and current smokers; and far less likely to be in the highest exercise category. The ARIC study would adjust for these characteristics, but, as I often say, you can’t adjust for a whole type of person.
The Appendix to the paper revealed the biggest limitation of the ARIC part of the study: The characteristics table in the main paper split the 15,428 people into equal groups (of 3,085-3,086) from the lowest to the highest carb intake. This is the objective way to review data, because there is no argument that you drew the line in a particular place to bias the finding. The appendix revealed what had been done to produce the U-shaped finding that grabbed the headlines:
The table below shows: the carb ranges subjectively selected by the researchers; the number of people that ended up in each range; and the deaths that occurred in that carb range during the 25 year follow-up. I have then calculated the death rate (before any adjustment).
The groups have been subjectively chosen – not even the carb ranges are even. Most covered a 10% range (e.g. 40-50%), but the range chosen for the ‘optimal’ group (50-55%) was just 5% wide. This placed as many as 6,097 people in one group and as few as 315 in another.
This is the single biggest issue behind the headlines.
The subjective group divisions introduced what I call “the small comparator group issue.” This came up in the recent whole grains review (Ref 6). I’ll repeat the explanation here, and build on it, as it’s crucial to understanding this paper.
If 20 children go skiing – 2 of them with autism – and 2 children die in an avalanche – 1 with autism and 1 without – the death rate for the non-autistic children is 1 in 18 (5.5%) and the death rate for the autistic children is 1 in 2 (50%). Can you see how bad (or good?) you can make things look with a small comparator group?
From subjective grouping to life expectancy headlines
For the media headlines “Low carb diets could shorten life, study suggests” (Ref 5), the researchers applied a statistical technique (called Kaplan-Meier estimates) to the ARIC data. This is entirely a statistical exercise – we don’t know when people will die. We just know how many have died so far.
This exercise resulted in the claim “we estimated that a 50-year-old participant with intake of less than 30% of energy from carbohydrate would have a projected life expectancy of 29·1 years, compared with 33·1 years for a participant who consumed 50–55% of energy from carbohydrate… Similarly, we estimated that a 50-year-old participant with high carbohydrate intake (>65% of energy from carbohydrate) would have a projected life expectancy of 32·0 years, compared with 33·1 years for a participant who consumed 50–55% of energy from carbohydrate.”
Do you see how both of these claims have used the small comparator group extremes (<30% and >65%) to make the reference group look better?
Back to the children skiing… If we were to use the data we have so far (50% of autistic children died and 5.5% of non-autistic children died) and to extrapolate this out to predict survival, life expectancy for the autistic children would look catastrophic. This is exactly what has happened with the small groups – <30% carb and >65% carb – in this study.
The data have been manipulated.
Ref 1: Carter J.P. Eating in America; Dietary Goals for the United States; Report of the Select Committee on Nutrition and Human Needs US Senate. Cambridge, MA, USA: MIT Press 1977.
Ref 2: Seidelmann SB, Claggett B, Cheng S, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health 2018. https://www.thelancet.com/journals/lanpub/article/PIIS2468-2667(18)30135-X/fulltext
Ref 3: Dehghan M, Mente A, Zhang X, et al. Associations of fats and carbohydrate intake with cardiovascular disease and mortality in 18 countries from five continents (PURE): a prospective cohort study. The Lancet 2017 doi: 10.1016/S0140-6736(17)32252-3
Ref 4: https://news.sky.com/story/keto-diets-could-increase-diabetes-risk-11465771
Ref 5: https://www.bbc.co.uk/news/health-45195474