Conventional nutritional advice varies a bit depending on who you ask, but there are a few constants. One is the importance of limiting saturated fat in favour of polyunsaturated fat. Most Governments, doctors and dieticians would therefore have us eschew foods such as red meat, dairy products and butter in favour of vegetable oils, oily fish and margarine.

This week saw the publication of the largest and most comprehensive review to assess the relationship between specific dietary fats and heart health, as well as the evidence for the supposed benefits of supplementing the diet with polyunsaturated fats [1]. The study was carried out by researchers from the University of Cambridge and MedicalResearchCouncil,University of Oxford, Imperial College London, University of Bristol, Erasmus University Medical Centre and Harvard School of Public Health, and was partly funded by the British Heart Foundation.

The study brought together two types of evidence:

1. Epidemiological evidence

Evidence where associations between different fats in the diet and risk of heart disease were assessed.

This sort of evidence itself came in two types:

a. studies where associations between dietary intake of specific fats and heart disease were assessed

b. studies where associations between body levels of specific fats (e.g. as measured in the blood) and heart disease were assessed

2. Randomised controlled trials

Where individuals were treated with specific dietary fats to see what effect this had on heart disease risk over time.

45 epidemiological and 27 randomised controlled trials were pooled in this review. The total number of subjects involved in these studies was more than 650,000.

Here’s a summary of the results:

Epidemiological studies of dietary fat and heart disease risk:

Saturated fats – No association

Monounsaturated fats – No association

Omega-3 fats – No association

Omega-6 fats – No association

Trans fatty acids – Increased risk

Epidemiological studies of body levels of specific fats and heart disease risk:

Saturated fat – No association other than one specific type of saturated fat (margaric acid) that was associated with reduced risk

Monounsaturated fat – No association

Omega-3 fats – Reduced risk

Omega-6 fats – No association other than arachidonic acid (found in meat, eggs and dairy products) which was associated with reduced risk

Trans fatty acids – No association

Randomised controlled trials of supplementation with:

Alpha-linolenic acid (a type of omega-3 fat found in plants including flaxseed/linseed) – No reduction in risk

Omgea-3 fats such as those found in oily fish – No reduction in risk

Omega-6 fats such as those found in vegetable oils – No reduction in risk

The authors of the review conclude:

…the pattern of findings from this analysis did not yield clearly supportive evidence for current cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of saturated fats. Nutritional guidelines on fatty acids and cardiovascular guidelines may require reappraisal to reflect the current evidence.

Should we be surprised?

Last year, a review published in the British Medical Journal pooled together studies in which saturated fat in the diet was replaced with polyunsaturated fats [2]. Overall, the review found no evidence of benefits when saturated fat was replaced with omega-6 fats. However, there was evidence of benefit when saturated fat was replaced with a mix of omega-6 and omega-3 fats, suggesting that omega-3 fat may be beneficial (but not omega-6).

Another major review, however, have found no good evidence that omega-3 supplementation reduced the risk of heart disease [3]. Yet another recent review found no reduced risk of heart attack when saturated fat was replaced with supposedly healthier fats in the diet [4].

Also, previously, overall the epidemiological evidence has failed to support the idea that saturated fat is linked with heart disease [5,6].

There is some variation in what the scientific literature appears to show in terms of the impact different dietary fats have on heart health. However, if I were to summarise what the evidence points to as a whole, I would say:

1. There appears to be no good evidence to fear fats found naturally in the food supply (including saturated fat)

2. Processed fats (e.g. industrially produced trans fats) should be avoided.

In other words, eat real, unprocessed food, and perhaps forget about the amount and types of fat in that food.

References:

1. Chowdhury R, et al. Association of Dietary, Circulating, and Supplement Fatty Acids

With Coronary Risk: A Systematic Review and Meta-analysis Ann Intern Med. 2014;160(6):398-406-406. doi:10.7326/M13-1788

2. Ramsden CE, et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis BMJ 2013;346:e8707

3. Skeaff CM, et al. Dietary fat and coronary heart disease: summary of evidence from prospective and randomised controlled trials. Annals of Nutrition and Metabolism 2009;55:173-201

4. Hooper L, et al. (2012). Reduced or modified dietary fat for preventing cardiovascular disease. The Cochrane Library. Published online 16 May 2012 DOI: 10.1002/14651858.CD002137.pub3

5. Mente A, et al. A Systematic Review of the Evidence Supporting a Causal Link Between Dietary Factors and Coronary Heart Disease. Arch Intern Med. 2009;169(7):659-669

6. Siri-Tarino PW, et al. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease Am J Clin Nutr 2010;91(3):535-46