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A systematic review and meta-analysis published in The BMJ looks at dietary intake and biomarkers of alpha linolenic acid and risk of all cause, cardiovascular, and cancer mortality.
Prof Tom Sanders, Professor Emeritus of Nutrition and Dietetics, King’s College London, said:
“Previous research has shown the dietary intake of omega-3 fatty acids to be linked to a lower risk of cardiovascular disease(CVD) but this effect has been only clearly seen with long-chain omega-3 fatty acids notable docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA, which are found in animal tissues especially in oily fish, not from the parent fatty acid alpha linolenic acid (ALA) which can only be synthesised by plants but not mammals). The findings of this report will be of interest to vegetarians and vegans who depend on ALA as their source of omega-3 essential fatty acids.
“This present report is a systematic review of prospective studies examining the relationship between ALA in diet or levels in blood or tissues with risk of all-cause mortality, cardiovascular and cancer mortality. It reports that a higher intake of ALA on entry to the studies were associated with a lower risk of all-cause mortality and cardiovascular mortality in the future. There were trends for differences in coronary heart disease and cancer mortality, but those overall effect were not statistically significant. The authors have tried to adjust for confounding influences such as the extent of obesity and the intake of other nutrients. But they were unable to adjust for the overall dietary pattern which has already been shown with better life-expectancy (e.g. plant based diets such as a Mediterranean diet).
“The authors conclude that an additional 1g ALA per day was associated with a 5% lower risk of all-cause mortality and suggest this could be provided by a tablespoon (15 ml) of canola oil (rapeseed oil) or the equivalent of a handful of walnuts. However, this conclusion is based on an associations that may not be causal. Several large randomized controlled trials have already tested whether an increase ALA reduce risk of death from CVD and have failed to show benefit. It does not mean that eating 1g more ALA will reduce risk of dying and the evidence from intervention trials shows no evidence to support the claim.
“The intake of ALA is relatively high in the UK as rapeseed oil is the cheapest and most abundant oil on the market and grown in the UK.”
“Current dietary guidelines for healthy eating (e.g. Netherlands dietary guidelines and the Lancet Commission on Planetary Health) do advocate eating a handful of nuts, although you need to be aware that many people are allergic to nuts.”
Prof Gunter Kuhnle, Professor of Nutrition and Food Science, University of Reading, said:
“This is a very well and thoroughly conducted analysis of the evidence currently available. The authors found that the consumption of alpha-linoleic acid (ALA), an acid found in a range of foods such as nuts, flaxseed or pumpkin seeds, is associated with a very small reduction of risk for premature death and death from heart disease, but a slightly higher risk of cancer. The change in risk is fairly small – about 5% for every gramme of ALA per day (approximately 7 walnuts) – and even with high consumption the reduction is not more than 10%.
“There are a lot of uncertainties: first of all, different ways to measure ALA have been used – from chemical analysis of tissues to self-reported food intake. This makes it difficult to derive any dietary recommendations from the data, as ALA can be present in a range of foods such as walnuts, linseed or pumpkin seeds. Often people who consume a lot of nuts and seeds also have an overall healthier lifestyle, and that might contribute to the results.
“Nuts and seeds can be part of a healthy diet, but the results of this study do not provide strong evidence for (or against) their consumption.”
Dr Duane Mellor, Registered dietitian and Lead for nutrition and evidence based medicine, Aston Medical School, Aston University, said:
“This paper represents a systematic review and meta-analysis of the data looking at a plant source of the omega-3 fatty acid – Alpha Linoleic Acid (ALA), the research about the effects of different fats on early death and heart disease and the accompanying debate has been going on for decades. This is a thorough analysis but has the limitation that all of the studies used were observational so do not demonstrate causality. Also due to the nature of many epidemiological studies people were often only asked about their diets once at the start of the study before their health was checked several years later. This study did not though, only look at dietary data it looked for markers in the blood of ALA which could make the measurements a little more reliable.
“There is a challenge with this type of research as it is focusing on one fatty acid, although it can be found in some seed oils e.g. canola (rape seed) oil it is also found in a range of nuts and seeds as well as to a lesser extent in a wide range of other foods including dark green vegetables and even pork! The really problem with any research focusing on only one nutrient, is it will always risk reducing down the detail about how people eat and importantly the combinations of food people who are healthier and are less likely to die earlier of heart disease actually choose and eat. This could mean if we take the data on face value, it could mean public health recommendations could end up suggesting people incorrectly consume more seed oils. However, it is important to remember as ALA is a polyunsaturated fatty acid, it is not very stable and can be liable to becoming, well basically rancid, which if this happens in our bodies, it could result in release of compounds called free radicals which are associated with increased risk of disease including cancer. This might explain the association observed in this analysis of the slight increased risk of mortality linked to cancer found in this paper. To help keep ALA stable in the food we eat, it is best to consume it in whole foods such as in nuts and seeds which also contain the antioxidants such as vitamin E which help to stabilise these fatty acids. So, it is probably a good idea to include around 30g of unsalted nuts or seeds in our diet per day, but there is no need to start adding extra seed oils, just use these, like other fats sparingly when cooking.”
‘Dietary intake and biomarkers of alpha linolenic acid and risk of all cause, cardiovascular, and cancer mortality: systematic review and dose-response meta-analysis of cohort studies’ by Sina Naghshi et al. was published in The BMJ at 23:30 UK time on Wednesday 13 October.
DOI: 10.1136/bmj.n2213
Declared interests
Prof Tom Sanders: “Member of Science Committee British Nutrition Foundation, Honorary Director of Nutrition HEART UK.”
Prof Gunter Kuhnle: “No interests to declare.”
None others received.