April 12, 2016

expert reaction to study on vegetable oil, cholesterol levels, and risk of heart disease

Various components of diet have been linked to a range of diseases and the contribution of different types of fats to heart disease is the subject of a paper published in The BMJ. The authors reanalysed results from a trial which replaced saturated fats with vegetable oils, and they report that this led to cholesterol reduction but didn’t have a protective effect in terms of heart disease.

Prof. Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:

“This report on the Minnesota Coronary Experiment is a re-analysis of a large dietary trial that was completed in 1973 in mentally ill hospital patients. It aimed to demonstrate that replacing saturated fatty acids in the diet with linoleic acid would reduce coronary heart disease (CHD). The participants on average were 52 years old and had normal body weight, blood pressure and serum cholesterol levels. The dietary intervention involved replacing a large amount of dietary fat with corn oil or corn oil margarine. Corn oil contains 50% linoleic acid and is relatively deficient in alpha-linolenic acid (0.9%) compared to other commonly used oils such as soybean oil (7.3%) and rapeseed oil (9.4%). The experimental diet provided 13% of the energy from linoleic acid compared to about 4% in the control group. This is well above the amount currently recommended which is 5% in the UK. Compliance to the dietary intervention appeared good as blood cholesterol levels fell on average by about 13%. This would be predicted to reduce the relative risk of death from cardiovascular disease by about 7.5% and fatal and non-fatal events by about 15%. However, as the average absolute risk of cardiovascular events was low because of the age of the participants, the study lacked statistical power to detect differences between treatments. It is, thus, not surprising that no differences in cardiovascular disease incidence or mortality were reported in the original study.

“This present report is based on a sub-group on 2355 of the original 9570 participants on whom blood cholesterol measurements were available. The paper focuses on all-cause mortality and found no overall difference between groups. The paper reports that the greater the fall in blood cholesterol in the study, the greater the risk of death. However, this occurred in both experimental and control groups and so it is not an effect that can be attributed to the experimental diet. It is well known that illness can result in falls in blood cholesterol. The authors tried to adjust for this by adjusting for changes in weight and blood pressure but the effect, although attenuated, remained. Caution is needed in translating this tentative finding in mentally ill patients to the general population or other groups.

“While I concur with the conclusion that the study provides no evidence to support replacing saturated fatty acids with linoleic acid for prevention of cardiovascular disease, I have some concerns about its potential impact in the media. The first concern is that patients may be discouraged from taking effective cholesterol lowering medications because the paper implies that reducing cholesterol increases the risk of death.  However, this is not the case as well-designed trials using statins show risk of death is lowered proportionately to the reduction in serum cholesterol.

“My second concern is that this paper may be misinterpreted to suggest that current dietary advice given to the public is wrong. It would be regrettable if the paper spawned headlines suggesting that eating lots of fatty meat products and butter is good for you! Science has moved on since the 1970s and so have dietary guidelines. Current UK (see the Eatwell Guide) and US dietary guidelines focus on providing an adequate intake of nutrients as well as preventing chronic disease. They do this by advocating healthy diets such as a Mediterranean dietary pattern. Here the dietary fat is provided mainly by monounsaturated fatty acids (olive oil, rapeseed or high oleic sunflower seed oil) and saturated fatty acid intake accounts for no more than 10% of the energy intake. The diet also contains plenty of fruit and vegetables and a balanced supply of polyunsaturated fatty acids from both vegetable sources and fish. A large randomized controlled trial (Estruch et al. 2013) conducted in healthy older people has shown that following such a diet does reduce the incidence of cardiovascular disease compared with the type of advice used in the Minnesota Coronary Experiment.”

Prof. Jeremy Pearson, Associate Medical Director, British Heart Foundation (BHF), said:

“We know that having too much cholesterol in your blood can increase your risk of cardiovascular disease, including heart attack and stroke, which is why managing our cholesterol level is crucial.

“This is an interesting study which shows that decreasing your intake of saturated fat can have a positive impact in helping lower cholesterol. However more research and longer studies are needed to assess whether or not eating less saturated fat can reduce your risk of cardiovascular death.

“In the meantime, it is advised to eat a balanced diet rich in fish, fruit, vegetables and whole grains to maintain a healthy heart.”

‘Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73)’ by Ramsden et al. published in The BMJ on Tuesday 12th April. 

Declared interests

Prof. Tom Sanders: “Prof Tom Sanders is a Scientific Governor of the charity British Nutrition Foundation, member of the scientific advisory committee of the Natural Hydration Council (which promotes the drinking of water), and honorary Nutritional Director of the charity HEART UK. Prof. Tom Sanders is now emeritus but when he was doing research at King’s College London, the following applied: Tom does not hold any grants or have any consultancies with companies involved in the production or marketing of sugar-sweetened drinks. In reference to previous funding to Tom’s institution: £4.5 million was donated to King’s College London by Tate & Lyle in 2006; this funding finished in 2011. This money was given to the College and was in recognition of the discovery of the artificial sweetener sucralose by Prof. Hough at the Queen Elizabeth College (QEC), which merged with King’s College London. The Tate & Lyle grant paid for the Clinical Research Centre at St Thomas’ that is run by the Guy’s & St Thomas’ Trust, it was not used to fund research on sugar. Tate & Lyle sold their sugar interests to American Sugar so the brand Tate & Lyle still exists but it is no longer linked to the company Tate & Lyle PLC, which gave the money to King’s College London in 2006. Tom also used to work for Ajinomoto on aspartame about 8 years ago.  Tom was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain. Tom has previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and Tom is a member of the Programme Advisory Committee of the Malaysian Palm Oil Board. In the past Tom has acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC. Tom’s research on fats was funded by Public Health England/Food Standards Agency.”

Prof. Jeremy Pearson: No conflicts of interest