June 8, 2015

expert reaction to study investigating dietary saturated fat, unsaturated fat, blood pressure and LDL cholesterol levels

Publishing in American Journal of Clinical Nutrition, a research team has attempted to analyse the effects of replacing saturated fat with unsaturated fat, and they report that the switch brought about beneficial effects in terms of lowering blood pressure and levels of ‘bad’ cholesterol.

Dr Ian Johnson, nutrition researcher and Emeritus Fellow, Institute of Food Research, said:

“This was a large, well-controlled experiment in which the diets of healthy volunteers were modified in a way that enabled the effects of replacing saturated fats with different types of unsaturated fats to be investigated, independently of other aspects of lifestyle. Such studies are notoriously difficult and expensive to perform, and the authors are to be congratulated.

“The results showed that replacement of saturated fat with monounsaturated and polyunsaturated fats had potentially beneficial effects on blood cholesterol, consistent with a reduced risk of heart disease. The investigation provides welcome support for current public health guidelines on the consumption of saturated fat, and should help to refute some of the often ill-informed criticisms of such advice that have appeared in the media recently.”

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

“The finding of a 10% reduction in LDL cholesterol by the replacement of saturated fat by MUFA or PUFA is in line with previous research (1,2) but the estimates of this change on cardiovascular disease risk are exaggerated and might be interpreted as suggesting that replacing saturated with unsaturated fatty acids is more effective than drug treatment. We know that statins lower LDL cholesterol by about 1 mmol/L (30%) and this results in a risk reduction of 10 and 20% for fatal and non-fatal cardiovascular disease respectively (3). This paper reports a reduction (about 10%) in LDL cholesterol of 0.3-0.4 mmol, being slightly greater on the polyunsaturated fat than the monounsaturated fat rich diet, which would predict risk reductions of 3-4% in fatal and 6-8% in non-fatal cardiovascular disease risk, not a 17-20% reduction as claimed in the press release.

“A few previous studies had suggested that saturated fat caused damage to the arteries by disturbing endothelial function. This study was designed specifically to test this and in this respect there were no differences between treatments, which is in agreement with two other large UK studies (2,4).  This would suggest that replacing saturated fat with other types of fat has no effect on endothelial function.

“This study measured many risk factors and there is a chance that some differences might turn out significantly different by play of chance. The report of a higher night-time blood pressure on the saturated fat diet is novel but should be treated with caution because the difference could be by play of chance. While a 20% difference in usual blood pressure is associated with a doubling of risk, the risk predictions made are unsound as overall there were no significant differences in 24 h blood pressure (differences were about 2 mm Hg, which is well below the level which this study was statistically powered to detect).”

1. Jebb SA, Lovegrove JA, Griffin BA, Frost GS, Moore CS, Chatfield MD, Bluck LJ, Williams CM, Sanders TA. Effect of changing the amount and type of fat and carbohydrate on insulin sensitivity and cardiovascular risk: the RISCK (Reading, Imperial, Surrey, Cambridge, and Kings) trial. Am J Clin Nutr 2010;92(4):748-58.
2. Reidlinger DP, Darzi J, Hall WL, Seed PT, Chowienczyk PJ, Sanders TA. How effective are current dietary guidelines for cardiovascular disease prevention in healthy middle-aged and older men and women? A randomized controlled trial. Am J Clin Nutr 2015;101(5):922-30.
3. Baigent C, Blackwell L, Emberson J, Holland LE, Reith C, Bhala N, Peto R, Barnes EH, Keech A, Simes J, et al. Efficacy and safety of more intensive lowering of LDL cholesterol: a meta-analysis of data from 170,000 participants in 26 randomised trials. Lancet 2010;376(9753):1670-81.
4. Sanders TA, Lewis FJ, Goff LM, Chowienczyk PJ. SFAs do not impair endothelial function and arterial stiffness. Am J Clin Nutr 2013;98(3):677-83.

‘Replacement of saturated with unsaturated fats had no impact on vascular function but beneficial effects on lipid biomarkers, E-selectin, and blood pressure: results from the randomized, controlled Dietary Intervention and VAScular function (DIVAS) study’ by Katerina Vafeiadou et al. was published in the American Journal of Clinical Nutrition. 

Declared interests

Dr Ian Johnson: “Dr Ian Johnson, BSc, PhD is a physiologist and nutrition researcher at the Institute of Food Research in Norwich, where he is an Emeritus Fellow. He is a member of the SACN working group on carbohydrates and health, and a member of the Scientific Advisory Committee of the British Nutrition Foundation. He currently holds no other research funding or consultancies.”

Prof. Tom Sanders is a Scientific Governor of the charity British Nutrition Foundation, member of the scientific advisory committee of the Natural Hydration Council, and honorary Nutritional Director of the charity HEART UK, a member of the Programme Advisory Board of the Malaysian Palm Oil Board.