Another paper has been produced by the Harvard team, which likes to analyse data for associations between food and health. This one is entitled “Saturated Fats Compared With Unsaturated Fats and Sources of Carbohydrates in Relation to Risk of Coronary Heart Disease: A Prospective Cohort Study”.
The researchers have looked at data already collected for two large studies: the Nurses’ Health Study (84,628 women) and the Health Professionals Follow-up study (42,908 men). The nurses were followed from 1980 to 2010 and the men from 1986 to 2010. All were free from diabetes, cardiovascular disease and cancer at baseline. Diet was assessed by a food frequency questionnaire every four years i.e. not very accurately.
The study results were that there were 7,667 incidents of coronary heart disease (CHD) in 24 – 30 years of follow-up. The results reported that:
– Higher intakes of polyunsaturated fat (PUFAs) were significantly associated with a lower risk of CHD. Comparing the highest with the lowest quintile (fifth) gave a hazard ratio (HR) of 0.80 (0.73 – 0.88).
– Higher intakes of carbohydrates from whole grains were significantly associated with a lower risk of CHD. Comparing the highest with the lowest quintile gave a hazard ratio (HR) of 0.90 (0.83 – 0.98).
– Higher intakes of carbohydrates from refined starches/sugars were associated with a higher risk of CHD. HR was 1.10 (1.00 – 1.21).
The team then went on to claim that replacing 5% of energy intake from saturated fats with equivalent energy intake from PUFAs, monounsaturated fats or carbohydrates from whole grains was associated with a 25%, 15% and 9% lower risk of CHD respectively.
The overall conclusion of the study was “Our findings indicate that unsaturated fats, especially PUFAs, and/or high-quality carbohydrates can be used to replace saturated fats to reduce CHD risk.”
The first point to always remember is that this is association, not causation. Could it be the case that people who eat more fish (PUFAs) and/or eat more cous cous (whole grains) are generally healthier than those who eat fries and burger buns? This would make PUFA and whole grain consumption a MARKER of health, not a MAKER of health.
The numbers are always presented as relative risk, which is misleading and should not be done. Responsible researchers present absolute risk, full stop.
There were a total of 127,536 people studied. There were 7,667 incidents of CHD over a 24-30 year period (use 27 as an average). The chance of any person having an incident in any one year was 0.22% (about 1 in 450 people). Given that the nurses were 34-59 in 1980 (and therefore 64-89 in 2010) and the men were 40-74 in 1986 (and therefore 64 – 98 in 2010), I’m surprised that the incident rate wasn’t way higher than this. And I bet it was massively correlated with age, as heart disease always is.
Table 1, baseline information, was quite interesting. It presented basic data about the participants in the five quintiles of saturated fat (SFA) intake, from lowest to highest. This informed us that those in the highest SFA group consumed almost double the total fat of the lowest SFA group and almost double the monounsaturated fat (MUFA) of the lowest SFA group. The highest SFA group also consumed almost double the trans fat intake of the lowest SFA group. I expect this adversely affected the health of the highest SFA intake group. This also tells me that the highest intake SFA group were eating processed food – another confounding issue.
Table 1 also told us that total carbohydrate intake ranged from 34.2% of energy intake (for the highest fat intake women) to 55.1% of total energy intake (for the lowest fat intake men). The carb intake from whole grains was as low as 0.72% – 1.2% of daily calories for the women and slightly higher at 2-5.2% for men. The whole grain results have been calculated on a tiny base, therefore.
The three key results presented above (on PUFAs, whole grains and refined carbs) came from Tables 2 and 3 in the paper. Table 2 also found the following, but this wasn’t included in the findings:
– Higher intakes of total fat were significantly associated with a lower risk of CHD. Comparing the highest quintile (fifth) with the lowest quintile gave a hazard ratio (HR) of 0.88 (0.80 – 0.96).
– Higher intakes of trans fat were significantly associated with a higher risk of CHD. Comparing the highest quintile (fifth) with the lowest quintile gave a hazard ratio (HR) of 1.20 (1.09 – 1.32). This was the single most significant result of the paper, but not mentioned in the press release or the abstract.
– Higher intakes of saturated fat were (not statistically significantly) associated with a lower risk of CHD. Comparing the highest quintile (fifth) with the lowest quintile gave a hazard ratio (HR) of 0.93 (0.82 – 1.05).
The swapping sat fat for something else bit
The researchers estimated that the effect of replacing 5% of energy intake from saturated fats with equivalent energy intake from PUFAs, monounsaturated fats or carbohydrates from whole grains was associated with a 25%, 15% and 9% lower risk of CHD respectively.
First of all – that relative risk applies again. Remember that the incident rate for any person in any one year was 0.22%. Even if we take these relative risk estimates at face value, the incident rate would be 0.17%; 0.19% or 0.20% vs. 0.22%. Not exactly hold the front page now eh?
Second – replacing 5% of energy intake from saturated fats may not sound like much, but it’s huge (it’s not 5% of 9.6% – it’s taking 5% away from 9.6%). The intake of saturated fat (Table 2) ranges from 9.6% to 16.9% of total energy intake. The lowest quintile would need to more than halve saturated fat intake (from 9.6 to 4.6%) and the highest quintile would need to reduce it by approximately one third (to 11.9%).
Third – what would people actually be cutting back on if they cut saturated fat significantly? Figure 3.4, page 26, of the 2010 US dietary guidelines shows that the main sources of saturated fat are processed food: pizza; grain-based desserts; dairy desserts (ice cream); KFC; hot dogs; burgers; tortillas; candy; potato chips etc. Butter accounts for 2.9% of saturated fat sources and milk 7.3%.
Do you think health would improve if people halved their intake of saturated fat by halving their intake of this kind of junk? Of course it would. Would health improve by removing eggs, meat and dairy products from grass living animals from the diet? Quite the opposite.
The press release “Butter is not back” is therefore an inaccurate, misleading, cheap shot from a university that should know better. Doing this gave the ‘Harvard kings of association nonsense’ the headline they were looking for “Butter isn’t better than marg”.
Remember the most significant, not-reported, result in the whole paper? The one about transfats? Check out how liquid vegetable oils are processed into an unnaturally solidified butter-like substance and you’ll realise how dangerous this press release was.
What do we take away from this?
Sensible dietary advice remains unchanged – eat real food; don’t eat processed food. Butter, churned from natural food sources, is far better for you than hydrogenated (or some other solidifying process), bleached, deodorised, emulsified, coloured, gunge (a.k.a. margarine). Additionally: association studies, which selectively report relative risk, especially those that emanate from the Harvard ‘got it in for butter’ team are potentially bad for your health.