The study, funded by the Abadan University of Medical Sciences in Iran, showed that dietary ALA intake is associated with a reduced risk of mortality from all causes, CVD (cardiovascular disease) and CHD (coronary heart disease), and a slightly higher risk of cancer mortality. Higher blood levels of ALA correlated to a reduced risk of all cause and CHD mortality only.
“In this systemic review and meta-analysis, we found that a higher intake of ALA was significantly associated with a 10%, 8% and 11% lower risk of mortality from all causes, CVD and CHD, respectively,” wrote the researchers, in the BMJ.
Conversely, they found that higher ALA intake was linked to a slightly higher risk of death from cancer, but said that this could be due to the existence of trans forms of ALA in certain foods and warranted further research.
ALA: the most common dietary omega 3
Alpha-linolenic acid (ALA) is an omega-3 polyunsaturated fatty acid readily available in plants such as soybean, nuts, canola oils and flaxseed. It is metabolised to EPA (eicosapentaenoic acid) and, to a lesser extent, DHA (docosahexaenoic acid), both of which have anti-inflammatory properties. In addition to its role in the production of EPA and DHA, ALA might also have a preventative effect against CVD and some cancers.
Previous studies had already established a link between a high ALA intake and a lower risk of fatal CHD. However, no study had examined the association between ALA and the risk of all cause mortality. Moreover, previous studies had mainly focused on dietary intake of ALA, rather than on the tissue biomarkers of ALA, and no information was available on the dose-response relationship between ALA intake and the risk of mortality.
To address these knowledge gaps, an international team of researchers analysed the results of 41 studies published between 1991 and 2021 on the associations between ALA and risk of death from all causes, cardiovascular disease and cancer.
Together, these studies involved around 120,000 participants aged between 18 and 98 years who were monitored for between two and 32 years, and they accounted for factors such as age, weight, smoking status, alcohol consumption and physical activity.
The researchers found that a high intake of ALA was associated with a 10%, 8%, and 11% lower risk of mortality from all causes, CVD, and CHD, respectively. This is equivalent to 113 fewer deaths per 10,000 person years for all causes, 33 fewer CVD deaths, and 23 fewer CHD deaths.
A higher intake of ALA, however, was associated with a slightly higher risk of cancer mortality, equivalent to 63 extra cancer deaths for the highest compared with lowest levels of ALA intake.
The researchers speculated that the detrimental effect of ALA intake in relation to cancer could be due to the existence of trans forms of ALA in certain foods. ALA is susceptible to oxidation and can produce oxidative species such as oxylipins, which can induce DNA damage, increase the risk of cancer and have an adverse impact on cancer mortality, they said.
Other studies have found that high nut intakes are associated with reduced all cause, CVD and cancer mortality, which suggests that the observed association between ALA and cancer mortality could differ by food source, ventured the researchers.
“Additional studies might over time provide extra evidence to help determine the association more definitively and determine whether particular food sources of ALA have a differential impact on cancer mortality,” they wrote.
Dose-response relationship and tissue ALA
A linear dose-response effect was found for dietary ALA intake and CHD and CVD mortality; a 1g per day increase in ALA intake (equivalent to one tablespoon of canola oil or 0.5-1.25 oz of walnut) was associated with a 5% lower risk of cardiovascular disease mortality.
In their analysis of tissue biomarkers, the researchers found that higher blood levels of ALA were associated with lower risks of all cause death and CVD.
“We found a significant inverse association between blood levels of ALA and risk of all cause mortality. Additionally, each 1 SD increment in blood and total plasma or serum levels of ALA was associated with a reduced risk of CHD and CVD mortality,” wrote the researchers.
However, no association was found between tissue levels of ALA and cancer mortality.
Due to the observational design of the studies, causality could not be established. Nevertheless, the authors said their work adds to evidence supporting the potential health benefits of polyunsaturated fatty acids.
They concluded: “Further studies should examine the association between ALA and a wider range of causes of death to provide a more comprehensive assessment of the potential health effects of ALA as well as to examine whether specific foods rich in ALA are differentially associated with mortality from cancer and other causes.”
Authors: Naghshi S., Aune D., Beyene J., Mobarak S., Asadi M., Sadeghi O.
“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”