Eating More Nuts Could Protect Against Type 2 Diabetes

Liam Davenport

October 17, 2017

Eating a diet rich in omega-6 polyunsaturated fatty acids (PUFAs), which are found in soybean and sunflower oils, as well as in nuts, may protect against the development of type 2 diabetes, say an international team of researchers.

Pooling data on levels of linoleic acid and its metabolite arachidonic acid in almost 40,000 individuals from 10 countries, they found that those with the highest linoleic acid levels had a 35% reduction in the risk of developing type 2 diabetes compared with those with the lowest levels.

The research was published online in Lancet Diabetes & Endocrinology on October 11.

The results indicate that a "simple change in diet" may help protect against type 2 diabetes, said lead author Jason HY Wu, PhD, the George Institute for Global Health, University of New South Wales, Sydney, Australia, in a press release from his institution.

Current US guidelines recommend that between 5% and 10% of energy consumption come from linoleic acid. However, it has been suggested that this may be harmful either because it competes with omega-3 PUFAs or due to potential harmful effects of arachidonic acid.

Dr Wu acknowledges that some scientists have theorized that omega-6 is harmful to health. "But based on this large global study, we have demonstrated little evidence for harms and indeed found that the major omega-6 fat is linked to lower risk of type 2 diabetes."

Dariush Mozaffarian, MD, Friedman School of Nutrition Science and Policy, Tufts University, Boston, Massachusetts, who was senior author on the paper, added: "This is striking evidence."

"The people involved in the study were generally healthy and were not given specific guidance on what to eat. Yet those who had the highest levels of blood omega-6 markers had a much lower chance of developing type 2 diabetes."

Linolenic-Acid Levels Inversely Associated With Type 2 Diabetes Risk

Noting a recent meta-analysis indicating that total PUFA consumption improves both glycemia and insulin resistance (PLOS Med. 2016; DOI:10.1371/journal.pmed.1002087), the team conducted a search of the MEDLINE database and relevant reference lists for studies published on the impact of linoleic acid and arachidonic acid levels on the risk of incident type 2 diabetes.

They contacted the authors of the 26 prospective cohort studies identified by the search, of whom 20 agreed to take part in a harmonized, individual-level analysis. This yielded a total of 39,740 adults from 20 cohorts in the United States, Iceland, the Netherlands, Germany, Finland, the United Kingdom, Sweden, France, Australia, and Taiwan, none of whom had type 2 diabetes at baseline.

Biomarkers were sampled between 1970 and 2010 from phospholipids, total plasma or serum, cholesterol esters, and/or adipose tissue, with measurements performed in more than two lipid compartments in six cohorts.

The mean patient age ranged from 49 to 76 years, and the mean body mass index (BMI) ranged from 23.3 kg/m2 to 28.4 kg/m2.

Over 366,073 person-years of follow-up, 4347 individuals developed type 2 diabetes.

Multivariate analysis of the pooled data indicate that linoleic-acid levels were inversely associated with the risk of developing incident type 2 diabetes on continuous analyses, at a risk ratio per interquintile range of 0.65 (< .001), and moderate heterogeneity (= .002).

The team notes that the findings were generally similar across different lipid compartments, including phospholipids, plasma, cholesterol esters, and adipose tissue.

In contrast, levels of arachidonic acid were, overall, not significantly associated with the risk of developing type 2 diabetes, at a risk ratio per interquintile range of 0.96 (= .38; P heterogeneity < .0001).

An exception was seen, however, for arachidonic acid levels in total plasma, at a risk ratio of 0.73 (= .0003).

The analysis also demonstrated that the findings were not significantly affected by taking into account potential confounding factors, such as age, BMI, sex, race, aspirin use, omega-3 PUFA levels, or variants in the fatty-acid desaturase (FADS) gene, which is strongly associated with omega-6 PUFA levels.

A "Step Forward" for Linoleic Acid and " Reassuring" Data on Arachidonic Acid

The team says: "The prevalence of type 2 diabetes is escalating rapidly around the world, so identification of dietary and other modifiable risk factors for the prevention of the disease is of clinical, scientific, and public-health importance."

They continue: "Our analysis provides novel findings that, when combined with in vitro experimental and shorter-term interventions for metabolic risk factors, suggest that linoleic acid has an additional role for prevention of type 2 diabetes in healthy populations."

In addition, "our findings do not corroborate concerns about potential harmful effects of arachidonic acid."

In an accompanying comment, Gabriele Riccardi, MD, department of clinical medicine and surgery, Federico II University, Naples, Italy, says that the findings represent "a step forward" in the evidence supporting the idea that linoleic acid protects against type 2 diabetes.

He notes: "Although this finding cannot immediately be extrapolated to linoleic-acid intake, it seems reasonable to assume that the differences in the linoleic-acid biomarker associated with this outcome would be consistent with linoleic-acid intake corresponding to one serving of nuts or one spoonful of sunflower or corn oil daily.

"Epidemiological studies have shown that this amount of linoleic acid is associated with a reduced risk of type 2 diabetes."

Moreover, given the previous concerns over whether arachidonic acid may cause harmful effects, Dr Riccardi describes the finding that arachidonic-acid biomarkers are not associated with increased type 2 diabetes risk as "reassuring in view of the potential implications for dietary advice."

"Altogether, the outcomes of the study by Wu and colleagues are fully concordant with a nested case-cohort analysis of the European EPIC cohort [PLOS Med. 2016; DOI:10.1371/journal.pmed.1002094].  Together, these studies indicate that the association between higher linoleic-acid biomarkers and reduced risk of type 2 diabetes does not represent a chance finding.

"The effect of dietary linoleic acid on diabetes incidence should now be tested in a randomized, controlled intervention trial," he concludes.

Cohorts within the Fatty Acids and Outcomes Research Consortium received direct funding from a number of sources. Unilever also provided Tufts University (Massachusetts) with a restricted grant to partly support the analysis. Dr Wu received research grants from Unilever for this study. Dr Mozaffarian received ad hoc honoraria and consulting fees from the Life Sciences Research Organization, AstraZeneca, Boston Heart Diagnostics, Global Organization for EPA and DHA Omega-3, DSM, Nutrition Impact, the Haas Avocado Board, and Pollock Communications; and chapter royalties from UpToDate. Disclosures for the coauthors are listed in the paper. Dr Riccardi has no relevant financial relationships.

Lancet Diabetes Endocrinol. Published online October 11, 2017. Article, Editorial

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