Should the current guidelines around dietary fat be reassessed?

Matt Whitaker – Research and Trainer in Public Health/25 October 2016

In a previous blog I mentioned how the current recommendations around dietary fat are not supported by a strong evidence base (1). This blog aims to address this and increase public knowledge of this matter. This blog is written in response to a recent review by Dr Zoë Harcombe (2) questioning when the national dietary guidelines around fat and saturated fat are going to be updated.

Current NICE lipid guidance 2014 recommends total fat intake of ≤ 30% of total calories with saturated fats at ≤ 7% and dietary cholesterol is less than 300 mg/day. It also recommends saturated fats are replaced by monounsaturated and polyunsaturated fatty acids (3). These guidelines are derived from the National Advisory Committee on Nutritional Education in 1983 (10) and have not changed since. However public health has changed since… for the worse!

In the UK, in 1972, 2.7% of men and 2.7% of women were obese with 23% of men and 13.9% of women classed as overweight. By 1999, obesity rates had risen to 22.6% of men and 25.8% of women with 49.2% of men and 36.3% of women categorised as overweight (4). The Health and Social Care Information centre recently reported 65% of men and 58% of women are now classed as overweight or obese (5). In addition to this the incident rate of diabetes within the UK has increased from 1.42% in 1980 to 6.1% in 2015 (6).

Let’s take a step back and consider why these recommendations were set. They were generated based on the diet-heart hypothesis, which is split into three parts:

* Dietary saturated fat increases blood cholesterol levels
* Increased blood cholesterol levels increase risk of cardiovascular disease (CVD)
* Dietary saturated fat must therefore cause CVD

This hypothesis was grounded on several animal studies based on the impact of animal food versus plant foods on the aorta and arteries; with fatty deposit accumulation observed following the consumption of animal products but not plant products in rabbits (2). This research led to several human studies, which concluded that the cholesterol content of all natural diets had no impact on serum cholesterol or in the development of atherosclerosis (7).

Despite this, findings in animals studies led to human observational trials in men (8) which suggested the saturated fat content in one’s diet is positively correlated with serum cholesterol. These theories provided the context and rationale for The Seven Countries Study (9) which was an observational study that concluded total fat content was not associated with coronary heart disease (CHD) but saturated fat was strongly correlated with CHD in cross country comparison.

It is worth noting, observational studies provide researchers with potential hypotheses that should then be clarified in randomised controlled trials (RCT’s) before conclusions are drawn and recommendations are made. This was not the case with the diet-heart hypothesis which saw both types of study running in parallel. In addition, RCTs mainly concentrated on men who had already suffered from a heart attack, questioning generalisability to the population. Despite the lack of evidence, particularly in women or otherwise healthy individuals the low fat, low saturated fat diet was advocated in 1983 and remains to be so.

So what did the research show leading up to 1983? Surely there was some robust RCT evidence before the Government set their dietary recommendations to the then population of 56.33 million people in the UK….

A systematic review and meta-analysis (1) examining the evidence surrounding dietary fat intake and CHD on evidence available in 1983 concluded out of the six prospective cohort studies available only one (Seven Countries Study) offered support for the saturated fat but not total fat guidelines. The other five studies showed NO significant findings in relation to total or saturated fat intakes and all-cause mortality or deaths from CHD. So in short the UK dietary guidelines around fat and saturated fat intake were supported exclusively from the Seven Countries study (9) which had several substantial limitations, stated in the discussed review (2), mainly the fact that Ancel keys pre-selected the included countries to match his hypothesis, tiny sample sizes of these countries had diets analysed and of course correlation does not equal causation. Guidelines ignored the other five studies which showed no significant relationship between fat and saturated fat intake on all-cause mortality and CHD.

A second systematic review and meta-analysis has been conducted (10) examining all evidence currently available (so everything in the first review up to 1983, plus all relevant studies published since). Guess what this review concluded? Despite the lack of evidence available when the guidelines were released there has been an abundance since and we should all relax and have that low fat yoghurt! ….Nope, again there was shown to be no link between total dietary fat intake and saturated fat intake on CHD mortality. This review analysed 39 different reports, four of these showed significant findings linking fat to CHD and CVD events. Of these four, one was related solely to trans fats (11) and one (12) was heavily criticised for ‘cherry picking’ studies to influence results by avoiding two studies that would have made findings non-significant (13). Another showed significant results in just one out of 12 in the analysis relating to a 14% reduction in CVD events (14) and the fourth showed statistically significant results in just one RCT out of eight with a 17% reduction in CVD events (15). Both of these latter studies were referring to saturated fat, not total fat intake and reported their findings on relative risk, not overall risk (See last week’s blogs for limitations on this – Parts 1 and 2). The recent NICE Lipid Guidance identified 14 RCTs but were unable to find any evidence that interventions that reduced total or saturated fat, or increased polyunsaturated fat, reduced primary or secondary CVD – however they still made a recommendation that total fat intake is 30% or less, saturated fats 7% or less, dietary cholesterol is less than 300 mg/day and saturated fats are replaced by MUFA & PUFA (3)! The tiny body of observational evidence showing links between saturated fat intakes and CVD is heavily flawed and evidence is yet to be shown in RCT studies.

When the dietary guidelines were introduced back in 1983 it was acknowledged that the current evidence base was not conclusive but there was a ‘strong consensus of opinion’ (16). As mentioned in last week’s blog this not an acceptable evidence source, particularly when discussing the nation’s health!

So I end where I began – Should the current guidelines around dietary fat be reassessed?

Any questions, feedback and/or suggestions would be most welcome, please email me at –

Matthew.Whitaker@xperthealth.org.uk

References

1. Harcombe Z, Baker JS, Cooper SM, Davies B, Sculthorpe N, DiNicolantonio JJ, 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 Heart. 2015;2(1):e000196. doi: 10.1136/openhrt-2014-.

2. Harcombe Z. Dietary fat guidelines have no evidence base: where next for public health nutritional advice? British Journal of Sports Medicine. 2016.

3. National Institute for Health and Clinical Excellence. Cardiovascular risk assessment and the modification of blood lipids for the primary and secondary prevention of cardiovascular disease.: https://www.nice.org.uk/guidance/cg181/resources/guidance-lipid-modification-cardiovascular-risk-assessment-and-the-modification-of-blood-lipids-for-the-primary-and-secondary-prevention-of-cardiovascular-disease-pdf; July 2014.

4. Wadsworth M, Kuh D, Richards M, Hardy R. Cohort Profile: The 1946 National Birth Cohort (MRC National Survey of Health and Development). Int J Epidemiol. 2006;35(1):49-54.

5. The Health and Social Care Information Centre. Statistics on Obesity, Physical Activity and Diet. 2016.

6. Diabetes UK. Diabetes Facts and stats June 2015. 2015;22.

7. Keys A AJ. The relationship of the diet to the development of

atherosclerosis in man. In: National Research Council DoMS, ed. Symposium on

atherosclerosis. Washington: National Academy of Sciences – National Research

Council. 1954:181–96.

8. Keys A FF, Scardi V, et al.,. Studies on serum cholesterol and other characteristics of clinically healthy men in Naples. . Arch Int Med. 1954;93:328–36.

9. Keys A, Menotti A, Aravanis C, Blackburn H, Djordevič BS, Buzina R, et al. The seven countries study: 2,289 deaths in 15 years. Preventive Medicine. 1984;13(2):141-54.

10. Harcombe Z, Baker JS, DiNicolantonio JJ, Grace F, Davies B. Evidence from randomised controlled trials does not support current dietary fat guidelines: a systematic review and meta-analysis. Open Heart. 2016;3(2).

11. Chowdhury R WS, Kunutsor S, et al. Association of dietary, circulating and supplement fatty acids with coronary risk: a systematic review and meta-analysis. . Ann Intern Med. 2014;160:398-406.

12. Mozaffarian D, Micha R, Wallace S. Effects on coronary heart disease of increasing polyunsaturated fat in place of saturated fat: a systematic review and meta-analysis of randomized controlled trials. PLoS medicine. 2010;7(3):e1000252.

13. Ravnskov U, DiNicolantonio JJ, Harcombe Z, Kummerow FA, Okuyama H, Worm N. The questionable benefits of exchanging saturated fat with polyunsaturated fat. Mayo Clin Proc. 2014;89(4):451-3.

14. Hooper L, Summerbell CD, Thompson R, Sills D, Roberts FG, Moore H, et al. Reduced or modified dietary fat for preventing cardiovascular disease. The Cochrane database of systematic reviews. 2011(7):CD002137.

15. Hooper L, Martin N, Abdelhamid A, Davey Smith G. Reduction in saturated fat intake for cardiovascular disease. The Cochrane database of systematic reviews. 2015;6:CD011737.

16. National Advisory Committee on Nutritional Education. A discussion paper on proposals for nutritional guidelines for health education in Britain. 1983.

 

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