C&B Notes

Causation But Only By Genetic Mutation

Nutritionists have had a difficult time replicating the results from a 1970s study that suggested widespread health benefits from increased intake of omega-3 fatty acids.  A new report argues that these original conclusions, which relied on surface-level causation analysis, were incomplete and could not be extrapolated to populations beyond native Inuits.

As the Inuit people spread across the Arctic, they developed one of the most extreme diets on Earth. They didn’t farm fruits, vegetables or grains. There weren’t many wild plants to forage, aside from the occasional patch of berries on the tundra. For the most part, the Inuit ate what they could hunt, and they mostly hunted at sea, catching whales, seals and fish. Western scientists have long been fascinated by their distinctly un-Western diet. Despite eating so much fatty meat and fish, the Inuit didn’t have a lot of heart attacks.

In the 1970s, Danish researchers studying Inuit metabolism proposed that omega-3 fatty acids found in fish were protective.  Those conclusions eventually led to the recommendation that Westerners eat more fish to help prevent heart disease and sent tens of millions scrambling for fish oil pills.  Today, at least 10 percent of Americans regularly take fish oil supplements.  But recent trials have failed to confirm that the pills prevent heart attacks or stroke.  And now the story has an intriguing new twist.

A study published last week in the journal Science reported that the ancestors of the Inuit evolved unique genetic adaptations for metabolizing omega-3s and other fatty acids.  Those gene variants had drastic effects on Inuit bodies, reducing their heights and weights.  Rasmus Nielsen, a geneticist at the University of California, Berkeley, and an author of the new study, said that the discovery raised questions about whether omega-3 fats really were protective for everyone, despite decades of health advice.  “The same diet may have different effects on different people,” he said.

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It’s possible that with so much extra omega-3 in their diet, the Inuit evolved a way to bring blood levels of fatty acids back into a healthy balance.  “It seems that a genetic adaptation has counteracted the high intake of omega-3 fatty acids,” said Marit E. Jorgensen, an author of the new study from the University of South Denmark.  The adaptation did more than just change blood levels of fatty acids, the scientists found.  Inuit who carried two copies of the variant gene were on average an inch shorter and 10 pounds lighter than those without a copy.  “That’s quite extreme,” said Dr. Nielsen.

Indeed, it’s rare to find a single gene that can influence height and weight so drastically.  In recent years, scientists have run a number of large studies pinpointing hundreds of genes that affect height and weight, but each one played a minuscule role in the variation from person to person. Those studies missed this influential gene variant because they focused mostly on people of European ancestry.  So Dr. Nielsen and his colleagues also investigated how it affects Europeans.  As it turns out, the gene variant is linked to a drastic drop in height and weight in that population, too.

The idea that the Inuit adapted to eating fatty food was very plausible, said Anthony G. Comuzzie, a geneticist at the Texas Biomedical Research Institute in San Antonio who was not involved in the study. But he cautioned that natural selection might not have favored the FADS variant but a neighboring, as yet unknown piece of DNA that conferred evolutionary advantages.  As that gene spread through the Inuit population, the FADS variant might simply have been passed down with it.  Dr. Nielsen and his colleagues are planning to investigate the long-term health effects of the gene variants they’ve found.  They may help explain why some of us metabolize fats more effectively than others, and why omega-3s haven’t been the heart panacea once hoped.