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1970s advice to cut dietary fat not evidence-based

‘Incomprehensible’ guidance advising fat reduction ‘should not have been introduced’

Louise Prime

Tuesday, 10 February 2015

Telling everyone to reduce their fat intake to benefit their heart health should never have happened, UK and US researchers stated this morning after conducting a new analysis* of the trials evidence available when the advice was issued – in 1977 in the US, and 1983 in the UK. They said they found it “incomprehensible” that such contrary results from a small number of unhealthy men should have formed the basis of guidance that the whole population should reduce their dietary fat to no more than 30% of their total energy consumption, and saturated fats to 10%. However, the author of an editorial** in the same issue of Open Heart warned that we shouldn’t ignore other evidence supporting a link between fat consumption and heart disease.

Researchers in Scotland, Wales and the US conducted a systematic review and meta-analysis of six randomised controlled trials, published prior to 1983, that examined the relationship between dietary fat, serum cholesterol and the development of coronary heart disease (CHD). Five of the six RCTs focused exclusively on secondary prevention; five of the six did not examine either a total fat consumption of 30%, or a saturated fat consumption of 10% of energy intake; and none included women.

The trials included a total of 2,467 men, among whom there were 740 deaths, 423 of them from CHD. There were no differences in all-cause mortality, and only non-significant differences in CHD mortality, as a result of the dietary interventions. Although there were significantly higher reductions in mean serum cholesterol levels in the intervention groups than in control groups, this did not result in significant differences in either CHD or all-cause mortality. None of the six original trials’ authors concluded that their findings merited dietary recommendations.

The authors of this meta-analysis said the fact that reductions in serum cholesterol were not associated with lower mortality “undermines the role of serum cholesterol levels as an intermediary to the development of CHD and contravenes the theory that reducing dietary fat generally and saturated fat particularly potentiates a reduction in CHD”.

They went on: “Government dietary fat recommendations were untested in any trial prior to being introduced … It seems incomprehensible that dietary advice was introduced for 220 million Americans and 56 million UK citizens, given the contrary results from a small number of unhealthy men.” They concluded: “Dietary advice not merely needs review; it should not have been introduced.”

In a linked editorial, Rahul Bahl, of the Royal Berkshire NHS Foundation Trust, warned that just because the original guidance was based on limited evidence (and was acknowledged to be so at the time), this does not mean that dietary fat is not a true risk factor – although, he said, overreliance on fat as the “main dietary villain” has distracted attention from the risks from other nutrients, such as carbohydrates. He pointed to the existence of epidemiological and ecological evidence suggesting a link between dietary fat and heart disease, and added that public policies don’t generally require randomised controlled trial evidence – so it would be “unusual” to withdraw this one just because of an absence of such evidence.

* Zoë Harcombe, et al. Evidence from randomised controlled trials did not support the introduction of dietary fat guidelines in 1977 and 1983: a systematic review and meta-analysis. Open Heart2015;2: doi:10.1136/openhrt-2014-000196

** Rahul Bahl. The evidence base for fat guidelines: a balanced diet. Open Heart2015;2: doi:10.1136/openhrt-2014-000229

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