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The scientific report guiding the US dietary guidelines: is it scientific?

BMJ 2015; 351 doi: (Published 23 September 2015) Cite this as: BMJ 2015;351:h4962

Re: The scientific report guiding the US dietary guidelines: is it scientific?

A report into the methodology of the highly influential US Dietary Guidelines for Americans process was long overdue, and the over-reliance on the beliefs of those in positions of authority and under-reliance on the scientific method itself described by Nina Teicholz is troubling.

The points raised by Bonnie Liebmen of the CSPI in her rapid response above that relate to the scientific record are worth addressing.

She refers to the recent meta-analysis of saturated fat reduction trials by Hooper et al that found a 17% reduction in CVD events (but no reduction in fatal events or all-cause mortality). However, Hooper et al found in sub-group analysis that the reduction in CVD events was only seen in those trials where SFA was replaced with PUFA, and was not seen in trials where SFA was replaced with carbohydrate. [1]

Nina Teicholz correctly states that this evidence supports an increase in PUFA reducing CVD events, but does not support reducing saturated fat (as replacing SFA with other nutrients has no effect).

Further points about this study are that in many of the contributing trials where SFA was reduced, subjects were told to reduce SFA by eating less biscuits, pizza and so on, as well as meat and dairy, and to replace these foods with fish, nuts, vegetables and fruit, as well as PUFA oils and spreads.

Surely such wide-ranging "healthy eating" advice ought to have had greater benefits than a mere 17% reduction in non-fatal events. Is it plausible that the results of eating a "healthy diet" and getting adequate PUFA were actually compromised by reduction in saturated fat from dairy and meat?

This seems to be the case with regard to the risk of type 2 diabetes, which is itself a major risk factor for heart disease.

The most recent meta-analysis of epidemiological prospective cohort studies of fat consumption to exonerate saturated fat was funded by the WHO, and the CSPI spokesperson did not claim that this study was flawed or had a conflict. of interest.

This study found that higher consumption of trans-fats found in dairy was associated with a significant reduction in the incidence of type 2 diabetes (0.58, 0.46 to 0.74).[2]

In a recent prospective cohort study [n=26,930] from Malmö, Sweden, which used a 7-day food diary and a 1 hour interview as well as a food frequency questionnaire to assess dietary intake, both dairy fat consumption (including butter and cream) and intake of the shorter-chain saturated fats (4:0 – 14:0) found in dairy (coconut is richer in these fats) were associated with a significantly reduced incidence of type 2 diabetes over 14 years of follow up. The association between red meat and type 2 diabetes was non-significant with regard to fatty red meat.[3]

In a separate analysis of the same Malmö cohort, it was found that adherence to dietary recommendations to limit saturated fat to 14% or less of energy was associated with a 15% increased risk of T2D in men and a slightly smaller increase in women.[4]

The recent BMJ meta-analysis funded by the WHO also found that industrial trans fats found in partially hydrogenated oils, but not ruminant trans fats, were associated with both CHD mortality (industrial 1.18 (1.04 to 1.33) v ruminant 1.01 (0.71 to 1.43)) and CHD (industrial 1.42 (1.05 to 1.92) v ruminant 0.93 (0.73 to 1.18)).[2]

During the 1980s, the CSPI's campaign "Saturated Fat Attack" advocated the replacement of beef tallow, palm oil and coconut oil at fast food restaurants, while maintaining that industrial trans fats were comparatively benign. Macdonalds and other takeaway outlets were placed under pressure by CSPI and replaced saturated fats with hydrogenated vegetable oils that were sources of industrial trans fats. Though the CSPI reversed their position many years later, industrial trans fats linked to heart disease are still present at high levels in the US diet due in large part to this pressure, and a full ban on them will not take effect until 2018..

This story perfectly illustrates the dangers of allowing medical pressure groups to dictate dietary policy ahead of the scientific evidence, and of allowing scientific consensus against saturated fat to form around weak associations and incomplete and contradictory evidence.

[1[ Hooper L, Martin N, Abdelhamid A, Smith GD. Reduction in saturated fat intake for cardiovascular disease, The Cochrane Library, June 10 2015. DOI: 10.1002/14651858.CD011737

[2] de Souza RJ, Mente A, Maroleanu A, et al. Intake of saturated and trans unsaturated fatty acids and risk of all cause mortality, cardiovascular disease, and type 2 diabetes: systematic review and meta-analysis of observational studies. Bmj. 2015:h3978. doi:10.1136/bmj.h3978.

[3] Ericson, U, Hellstrand, S, Brunkwall, L, Schulz, C-A, Sonestedt, E, Wallström, P, et al. Food sources of fat may clarify the inconsistent role of dietary fat intake for incidence of type 2 diabetes. AJCN 2015;114.103010v1

[4] Sonestedt, E et al. A high diet quality based on dietary recommendations does not reduce the incidence of type 2 diabetes in the Malmo Diet and Cancer cohort. EADS2015 ePoster #322

Competing interests: No competing interests

25 September 2015
George D. Henderson
Human Potential Centre
Auckland University of Technology