Cell and animal research on omega-3s and diabetes show that these essential fats exert genetic influences similar to diabetes drugs … without adverse side effects.
These documented “nutrigenomic” effects should help deter diabetes … but while generally positive, the evidence from human clinical and diet-health studies remains mixed.
The positive side of the scale just got added weight from two investigations published this week in the American Journal of Clinical Nutrition.
Both of these diet-health epidemiological studies link higher omega-3 intakes to lower diabetes risk.
Omega-3s and diabetes: a promising but mixed picture
The same Harvard team behind today's good news came to a negative conclusion for omega-3s earlier this year.
In that study, they compared the self-reported diets of 36,328 women in the Women's Health Study to their health status over a 16-year period.
Much to most observers' surprise, that analysis linked fishy diets to higher diabetes risk, while plant-source omega-3s were not linked to risk:
“Our data suggest an increased risk of T2D [type 2 diabetes] with the intake of long-chain omega-3 fatty acids, especially with higher intakes.” (Djoussé L, Gaziano JM, Buring JE et al. 2011)
Specifically, the risk of developing diabetes within 16 years was 44 percent higher among those who reported eating two or more servings of fish daily, and similarly elevated among the women estimated to consume two or more grams of omega-3s per day.
However, as with the Singapore findings we report today, these outcomes strain credulity … a point made by the authors of another survey-based study that linked omega-3s to higher diabetes risk (see “Diabetes-Fish Study Raises Doubts”):
“Omega-3 fats (also known as n–3 fatty acids), particularly long-chain omega-3 fats from seafood sources, alter the expression of peroxisome proliferator-activator receptor [PPAR] genes, which are involved in signaling nutrition status, and of the production of inflammatory cytokines, which are associated with T2DM [type 2 diabetes mellitus]. These findings suggest that omega-3 fatty acids could lower the risk of T2DM.” (Kaushik M et al. 2009)
In fact, the association this study found—between eating more fish than average and having a slightly higher risk of diabetes—makes little sense in light of what's known about the cell-level effects of omega-3s.
As the authors themselves noted, “…numerous studies have demonstrated beneficial effects of fish and omega-3 fatty acids on multiple risk factors associated with diabetes, and on heart disease—the major sequelae [subsequent effects] of diabetes”.
They also admitted that people with healthier lifestyles generally eat more fish and undergo more medical tests than average… which could explain the link they observed between fish intake and reporting a diagnosis of diabetes.
In addition, as the Harvard team noted, participants may have increased their consumption of fish after being diagnosed with the cardiovascular signs associated with diabetes, such as high cholesterol and triglycerides or hypertension.
In truth, no study that relies on diet surveys and uses them to make rough estimates of omega-3 intake can be as reliable as one – like the positive Harvard study – that employs blood tests.
For more on this topic, see the “Omega-3s & Metabolic Health” section of our news archive.
And surprisingly, plant-source omega-3s seemed to equal or exceed the benefits of seafood-source omega-3s.
Smaller but stronger of the two studies favors seafood omega-3s
In the smaller but more reliable of the two studies, Harvard researchers studying more than 3,000 older adults for a decade linked high blood levels of omega-3s to a substantially reduced risk of diabetes.
Their findings applied to seafood-source, “marine” omega-3s (EPA and DHA) and plant-source omega-3s (ALA) alike.
The body converts plant-source omega-3 ALA into EPA and DHA … the only omega-3s we humans actually need to survive and thrive.
But the body can only turn from one to 10 percent of dietary ALA into omega-3 EPA and DHA ... which explains the generally far greater health benefits associated with seafood and fish oils.
The Boston-based team tested blood levels of omega-3 EPA and DHA among 3,088 older men and women – mean age 75 years – and compared those levels with the volunteers' health status after 10 years (Djoussé L, Biggs ML, Lemaitre RN et al. 2011).
Surprisingly, compared with high levels of seafood-source omega-3s, the Harvard team linked high blood levels of plant-source omega-3 ALA to a slightly greater risk reduction … a 43 percent cut in the risk of developing diabetes within 10 years, versus a 36 percent drop.
Importantly, higher omega-3 levels were still linked to lower diabetes risk after the results were adjusted to account for factors like weight and exercise habits.
Singapore study favors plant-source omega-3s … but relies on shakier data
A larger, Singapore-based study published simultaneously in the same journal linked high intake of plant-source omega-3 ALA to a 22 percent cut in diabetes risk during one decade of follow up.
Surprisingly, no risk reduction was seen among those with the highest estimated intakes of marine omega-3s.
In the Singapore study, an international team led by Andrew Odegaard of the University of Minnesota linked plant-source omega-3s – but not marine omega-3s – to reduced diabetes risk over time.
Odegaard's group recruited 43,176 healthy, ethnically Chinese Singaporeans of both genders, aged 45 to 74, and then questioned them about their eating habits and followed them for 10 years.
Their analysis linked high ALA intake to a 22 percent drop in diabetes risk … after accounting for weight, exercise, and other factors.
However, as the authors wrote, omega-3s from fish were not tied to any reduction in diabetes risk:
“Omega-3 FAs [fatty acids] from marine sources were not associated with [lower or higher] diabetes risk, whereas non-marine omega-3 FA intake was strongly associated [with reduced diabetes risk].” (Brostow DP et al. 2011)
Interestingly, neither omega-6 fat intake nor the omega-6/omega-3 intake ratio was associated with any change in the risk of developing type 2 diabetes.
Still, this large Singapore study is likely less reliable than the smaller Harvard investigation.
Why? Instead of measuring people's omega-3 blood levels, the Singapore study authors could only estimate omega-3 intakes, based people's responses to diet questionnaires.
The sheer size of the Singapore study adds statistical strength to its outcomes … but this advantage is significantly undercut by use of a much less reliable methodology.
Is omega-3 ALA really responsible … or just a marker for healthy lifestyle?
Why didn't omega-3 from fish reduce diabetes risk in the large Singapore-based study?
Unlike EPA and DHA from seafood, some lab research suggests that omega-3 ALA might improve body cells' sensitivity to insulin, the hormone that regulates blood sugar.
However, as lead author Andrew Odegaard told Reuters, people who consume a lot of ALA – which is concentrated in flaxseed, walnuts, canola oil, and dark, leafy greens – are likelier to have healthier, plant-rich diets, hence healthier lifestyles overall.
He added that while his team tried to adjust their analysis to account for those variables, it's feasible that ALA intake is a “marker” for other anti-diabetes factors.
Odegaard noted that if fish is deep-fried and served alongside unhealthful fare like French fries and sweet soda, this could easily cancel out any beneficial effect from omega-3s.
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