When it comes to the popular "Paleo diets," it can be hard to know how exactly to eat like a caveman. Which cave? A good diet may depend on where your ancestors' digs were located.
And other crucial shifts have occurred over the millennia, both in our food and in ourselves. Some scientists contend that the biggest and perhaps most damaging change in our diets happened only in the last century with the explosion of a type of fat called omega-6.
New evidence is showing that humankind don't all contend with this fat equally, thanks to the way different human populations have evolved and adapted to local food sources over 50,000 years.
Understanding these differences might help scientists offer better nutritional guidance and reduce malnutrition in developing countries.
Two new papers published this month point out that people of African descent are much more likely to have a genetic variant that allows them to make brain-building omega-3 fatty acids from plants such as flaxseeds - perhaps an advantage in the past. But that same genetic variant also likely renders its carriers more vulnerable to ill effects from the much more abundant omega-6 fats, which are found in processed foods, vegetable oils, and factory-farmed fish and poultry.
Even though these omega-6 fats are "polyunsaturated," and considered healthy by the American Heart Association, "they may be more readily converted to compounds that promote excess inflammation in people of African descent," said Floyd Chilton, director of the Center for Botanical Lipids and Inflammatory Disease Prevention at Wake Forest Baptist Medical Center.
"This is more than important," said Joseph Hibbeln, nutritional neuroscientist at National Institutes of Health. "We feel this is the greatest dietary transformation in the history of Homo sapiens."
While our diets have changed considerably since the Stone Age, the most radical change occurred in just the last 70 years, Hibbeln said, with an explosion in the proportion of omega-6 fatty acids in the Western diet.
"This has critical public health implications," he said. "What we're eating now is discordant with what mammals were eating for the last 25 million years."
Both omega-3 and omega-6 fats are considered polyunsaturated, which for decades was equated with health benefits, unlike the much-maligned saturated fats found in cream, butter and red meat. But not all polyunsaturated fats are alike, said Chilton. They are chemically different and do different jobs in the body.
Omega-3 fatty acids are critical for the development and maintenance of the brain. Omega-6 fatty acids are important for building up the body's immune system, but a number of researchers, including Chilton and Hibbeln, think excessive amounts contribute to a kind of chronic inflammation that can increase the risk of allergies, arthritis, heart disease, diabetes, and possibly cancers and other diseases.
Why do our genes matter? Because most of the omega-6 fats in food can trigger such excess inflammation only when our bodies convert it into a "long chain" form.
The genes controlling this conversion come in two varieties, one of which does the job much more efficiently than the other. And those of African descent are more likely to have this "converter" form of the gene, Chilton said.
Chilton reached this conclusion through two studies, one of which was published in the journal BMC Genetics and will be published this month in the British Journal of Nutrition. He looked at 329 healthy people and an additional 395 diabetic patients, and found that in their samples, African Americans had much higher blood levels of dangerous "long chain" omega-6 fatty acids than did those of European descent. At the same time, he found that African Americans were more likely to carry the efficient converter genes compared with Americans of European ancestry sampled from the same geographic regions.
He bolstered this by looking at catalogs of genetic information taken from different world populations, including one called HapMap and another called the Human Genome Diversity Panel. The latter included individuals from Africa, where Chilton found almost all had two copies of the efficient converter gene (we all carry two copies of most of our genes).
In the U.S. samples, he found that close to 80 percent of African Americans and only 45 percent of white Americans carried two copies of the efficient converter genes.
While there are several reasons for such differences across populations, Chilton said it might come back to evolution.
During the time when all humans lived in Africa, it was crucially important to make all the "long chain" forms of omega-3 and omega-6 fats from plant sources, since meat and fish might not have been reliable sources of food.
Then, around 50,000 years ago, some people moved into Europe, Asia, and points beyond, where the food supply changed dramatically. People can get complete, ready-to-use omega-3 fats from fish and from animals that eat fish, so the need to do this fat conversion would have lessened. The pressure to have good converter genes would have been even less for those who raised animals and had a steady source of animal fats.
A similar food-related genetic difference among living people involves the enzyme that breaks down the milk sugar lactose, allowing some people to drink milk without getting sick. "These traits quite closely mirror each other," Chilton said.
Until about 7,000 years ago, everyone was lactose intolerant, the latest estimates show. But in certain groups who were raising cattle, a new gene linked to breaking down lactose spread fast.
Chilton says he'd like to apply this new understanding of genetics and dietary needs to combating malnutrition and disease in Africa. There, many children suffer poor brain and immune system development because their diets are all starch and almost no fats or proteins. Malnutrition, he said, is not just about starving, but about starving for crucial nutrients. "My dream is to create therapeutic foods for malnourished children."
Since most African children are good converters, supplements with flaxseed oil, with the building blocks of omega-3's, might work just as well as fish or fish oil, and much more cheaply. In contrast, fish or fish oil might be required for children in Asia and Central America, where fewer have the genes needed to convert flaxseed oil to these essential nutrients.
The right supplements could help them build up better immune systems, as well, which could provide protection from HIV and tuberculosis, Chilton said.
The trouble here in America is that the typical diet is bloated with omega-6 fatty acids.
These used to make up about 1 percent of the total calorie intake in the early 20th century, but that's ballooned to almost 10 percent thanks mostly to the explosion of corn and soy oils, both in food and in animal feed.
Chilton made waves several years ago when he showed that farmed tilapia, which is mostly fed with cheap, omega-6-rich commodities, has very little of the omega-3 fatty acids that are supposed to make fish a good nutritional choice. In terms of fats, he said, they are worse than several kinds of meats that are considered bad.