Cardiovascular diseases and tumors are among the top causes of death in the world for many years[1], and adhering to healthy eating habits can delay the onset of these two types of diseases and prolong life [2]. Fatty acids are an important type of dietary nutrient, and reducing the intake of saturated and trans fatty acids and increasing the intake of polyunsaturated fatty acids, especially omega-3 fatty acids, has become the cornerstone of the global dietary guidelines [3]. However, controversy over the extent and potential risks of omega-3 fatty acids continues [4,5].
α-linolenic acid (ALA) is a common, plant-derived, essential omega-3 polyunsaturated fatty acid that may play an anti-inflammatory, cardiovascular, and cancer role [6], but may also increase tissue oxidative stress levels when consumed in large quantities[4] and the risk of cancer [7]. How is ALA related to deaths of different causes? How is tissue-level ALA linked to death? Is there a dose-response relationship? This research topic is in dire need of high-quality meta-analyses to provide evidence.
Flaxseed containing ALA, Source: Pexels
Recently, a research team led by Professor Omid Sadeghi of the Department of Community Nutrition at Tehran Medical University in Iran published a meta-analysis of prospective cohort studies in the Journal of British Medical Journal [8]. The study included data from 41 cohort studies and suggested that higher dietary ALA intake was associated with a 10% lower risk of all-cause death, an 8% lower risk of cardiovascular disease death, and an 11% lower risk of death from coronary heart disease, but associated with a 6% increased risk of cancer death.
There is a dose-response association between increased dietary intake of ALA and decreased risk of all-cause and coronary heart disease-related death, and elevated levels of ALA in the blood are associated with a decreased risk of all-cause, coronary heart disease-related death.
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The researchers screened 12,873 studies published before April 30, 2021, sifted out 41 prospective cohort studies that analyzed dietary intake or tissue-level ALA associated with risk of death, and extracted their data to calculate the relative risk of death at the highest intake/tissue level of ALA compared to the lowest intake/tissue level among trial participants.
Research flowcharts
A meta-analysis of 41 trials including 1197564 participants showed a 10% reduction in ALA intake (median 1.59 g) and an all-cause mortality risk (relative risk 0.90; 95% CI 0.83-0.97; I2=77.8%), an 8% lower risk of cardiovascular disease death (relative risk 0.92; 95% CI 0.86-0.99; I2=48.2%) and an 8% lower risk of cardiovascular disease death compared with lower ALA intake (median 0.73 g; 95% CI 0.86-0.99% I2=48.2%) A 11% reduction in the risk of death from coronary heart disease (relative risk 0.89; 95% CI 0.81-0.97; I2=5.6%) was significantly correlated.
This means that higher dietary ALA intake can result in 113 fewer all-cause deaths, 33 fewer deaths from cardiovascular disease, and 23 fewer deaths from coronary heart disease per 10,000 person-years.
Association of ALA dietary intake/tissue levels and risk of all-cause death
Dose-response analysis suggested that an increase of 1 g of ALA per day (equivalent to 1 tablespoon of canola oil or 14.2 g of walnuts) was significantly associated with a 5% reduction in the risk of all-cause mortality (relative risk of 0.95; 95% CI 0.91 to 0.99; I2=76.2%), and a 5% reduction in the risk of cardiovascular disease death (relative risk of 0.95; 95% CI 0.91 to 0.98; I2=30.7%). This equates to a 57 reduction in all-cause deaths and a 21 reduction in cardiovascular disease deaths per 10,000 person-years.
Association of ALA dietary intake/tissue levels and risk of cardiovascular death
In the case of tissue-level ALA, each standard deviation increase in blood ALA concentration was significantly associated with an 8% reduction in the risk of coronary heart disease (relative risk 0.92; 95% CI 0.86 to 0.98; I2=37.1%). Blood ALA levels and risk of all-cause death, serum plasma ALA levels, and cardiovascular disease death risk were also inversely associated.
Association of ALA dietary intake/tissue levels and risk of death from coronary heart disease
One more scoop of rapeseed oil and a few more walnuts a day may reduce the risk of all-cause death by 5%!
It sounds so easy to achieve, but while the situation seems to be a good situation and the ALA aura continues to increase, the potential risks cannot be ignored. The higher-dose intake group of ALA compared with the low-dose ingestion group may be associated with a 6% increased risk of cancer death (relative risk 1.06; 95% CI 1.02-1.11; I2=3.8%).
Association of ALA dietary intake/tissue levels and risk of cancer death
The researchers also subdivided the association between ALA intake and the risk of death in different dose ranges, and the results suggested that ALA intake of 1.0-2.5 g/day may be associated with a reduced risk of death from coronary heart disease, and ALA intake of 2.25-3.0 g/day may be associated with a reduced risk of cardiovascular disease death.
However, given the possible association between higher ALA intake and risk of cancer death, it is not yet possible to recommend the optimal daily dose of ALA based on the results of this table.
Relative risk of different intakes of ALA and different causes of death
As a nutrient closely related to daily life, the increase in intake can reduce the risk of all-cause death and cardiovascular disease-related deaths, but the risk of cancer death is worrying.
The researchers also interpreted and speculated about this result, on the one hand, the association between ALA and cancer may be affected by different ALA transformants in food, and secondly, ALA is susceptible to oxidation, which in turn produces oxidized lipids, causing DNA damage.
However, there are also meta-analyses that have found that ALA-rich nut intake can reduce the risk of cancer death [9], and this study alone cannot confirm that ALA leads to an increased risk of cancer death, and the role analysis of nutrients may be affected by many confounding factors, and there is also a certain possibility of false associations caused by bias.
Therefore, the association between ALA and the increased risk of cancer death still needs to be further determined by future studies, but it is clear that the daily consumption of polyunsaturated fatty acids such as ALA is significantly better than saturated fatty acids and trans fatty acids, and do not use vegetable oils such as rapeseed oil at home because of the data in this article. Solid animal fats and trans fatty acids in fried foods are the bigger health culprits.
Prospective study pooling and large sample sizes added to the persuasive conclusions of this study, and quantitative analysis of ALA and risk of death, and testing of linear/nonlinear dose-response relationships also made the findings more intuitive and perceptible. The correlation between ALA levels and death in tissue samples is also an innovative point in this paper, and the test results are more objective than the ALA intake calculated on the dietary scale.
In addition, this study also has certain flaws. Observational studies do not provide causal associations, the effects of potential bias cannot be accurately determined, and errors can occur during ALA intake measurements, which in turn affect the correlation analysis of ALA and death risk. There is a famous saying in the meta-analysis community: Garbage in, garbage out, the criteria for grouping ALA in different studies are not the same, and the quality and heterogeneity of the included studies will also affect the reliability of the conclusions. There are also temporal and spatial differences and fluctuations in the types and proportions of meals.
Dietary research is always very interesting, but the results of a single nutrient element on the guidance of life is still quite limited, and the interaction of different types of nutrients in the body is still unknown, how much the results of the literature can have their own reference value is more need to weigh it, careful decision-making, do not fast, ketone, Mediterranean, drink oil, grazing take turns to try.
Image source: Pexels
Human exploration of the roles and mechanisms of nutrients will not cease because of the current controversy, and the truth that the sages have insight into — that the people eat for heaven — will not become obsolete. One day, human beings are expected to find their own optimal dietary patterns, and the code of longevity may be hidden between the stoves full of rice, oil and salt.
bibliography:
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[7] Simon JA, Chen YH, Bent S. The relation of alpha-linolenic acid to the risk of prostate cancer: a systematic review and meta-analysis. Am J Clin Nutr. 2009;89(5):1558S-1564S. doi:10.3945/ajcn.2009.26736E
[8] 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. BMJ. 2021;375:n2213. Published 2021 Oct 13. doi:10.1136/bmj.n2213
[9] Aune D, Keum N, Giovannucci E, et al. Nut consumption and risk of cardiovascular disease, total cancer, all-cause and cause-specific mortality: a systematic review and dose-response meta-analysis of prospective studies. BMC Med. 2016;14(1):207. Published 2016 Dec 5. doi:10.1186/s12916-016-0730-3
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