Palate and Possibility

Chowing down on a cheeseburger one day. Supping on sushi the next. It's the American way. But while our palates may find plenty of pleasure in the global village, our metabolic machinery is still pretty much stuck in the stone age. That means you may not always get the nutrition you think you're getting out of the food you eat.

Take the case of sushi: fish and rice wrapped in nori—a seaweed that is a type of red algae, Porphyra—common in the cold waters around Japan. Inhabiting an island nation, the Japanese have been harvesting and consuming Porphyra for food at least since the eighth century, first as a paste, later dried and pressed into paperlike sheets. Nori farms now dot the coast of Japan and produce over a billion dollars worth of the seaweed annually. It supplies the Japanese diet with various B vitamins—essential to the function of the nervous system—as well as protein, iron, calcium, and magnesium.

But only people of Japanese ancestry can get what they need out of nori, an international team of researchers discovered a few years ago. For everyone else, its particular polysaccharide makeup renders the fiber-rich marine carbohydrate impervious to digestion. The Japanese have the specific carbohydrate-active enzymes needed to break down the fiber of Porphyra. Non-Japanese North Americans do not.

The human genome provides us with all of our possibilities, but one area where it notably falls short is the ability to dismantle carbohydrate polysaccharides, the starches we call dietary fiber. We just don't carry many genes for carbohydrate-active enzymes. After all, carbohydrates have made up much of the human diet only since the agricultural revolution 10,000 years ago—the blink of an evolutionary eye.

For carbohydrates, we consign much of the task to our microbiome. If it's healthy enough and we have the right array of gut bacteria, our microbiome essentially functions as a major metabolic organ, extracting and processing nutrients from food.

Even then, only people of Japanese descent harbor in their microbiome the gut bacteria that can tackle nori, and their microbiome actually acquired the algae-degrading enzymes by surreptitious means—from marine bacteria that contaminate the red algae. At some point in history, given the continuous availability and consumption of nori in Japan, the genes for carbo-active enzymes jumped from the bugs sneaking into the body on nori to the bacteria of the microbiome. Because nori is not traditionally roasted, its hitchhiking bacteria could survive to make contact with human gut microbes and "update" them with the right enzymes.

Consider that as Exhibit A of how humans adapt to the diverse environments they inhabit, whether the frozen arctic, tropical rainforests, or temperate islands. The need to survive on the food available in a particular habitat applies pressure on our genetic apparatus to find a way. Once a solution appears, that genetic feature spreads through the area's population by natural selection, as those who can adapt and flourish are likely to leave more offspring. The end result is some stunning differences around the world in how effectively bodies process specific foods.

Fatty fish may be one of them. There's recent evidence that only the Inuit can consume high levels of fats from fish and marine mammals—while being spared heart disease and the consequences of obesity, such as insulin resistance and frank diabetes. University of California at Berkeley biologist Rasmus Nielsen and colleagues discovered that the Inuit population of Greenland carries a mutation in the genes involved in fat metabolism that enabled their Siberian ancestors to survive in their cold habitat 20,000 years ago by consuming fatty seal and whale meat. That same mutation occurs only in 2 percent of Europeans and 15 percent of Han Chinese.

The genetic mutation shuts off production of the enzymes that normally allow the body to desaturate fatty acids, compensating for the Inuits' high intake of omega-3 polyunsaturated fats from cold-water fish, the team reported in Science.

That the Inuit can consume— with cardiovascular and metabolic impunity—so much fat does not mean the rest of us are so privileged. Even though the original evidence of the value of omega-3s in fish came from studies of the Inuit, "we can't extrapolate from them," Nielsen insists, because they are genetically unusual.

"We know that a low intake of omega-3 fats is not good for you," he says. But a high intake of them might not be beneficial either. "There's a great deal of debate about that right now in the scientific community," Nielsen notes. "The jury is still out."

What surprised Nielsen and his colleagues was that the genetic change that conferred metabolic protection from fat on the Inuit also consigns them to short stature. "Human growth hormone production is somewhat regulated by fatty acid composition," he explains. The fat-gene shift shaves off an average of two centimeters of height—one of the strongest effects on height ever encountered.

Although our genetic makeup is pretty much fixed and highly conservative, some parts of the genome, such as those involved in fatty acid metabolism, seem to be responsive to dietary influence. Fatty acids are used in many body processes, and they are vital components of brain cells. It's important for the body to be able to fine-tune their synthesis correctly. In fact, since Nielsen's study, research by others has shown that among Asians who consume a vegetarian diet, fatty acid synthesis shifts in the opposite direction—a genetic mutation enables the production of more unsaturated fats, apparently to compensate for the paucity of fat in the diet.

For most of history, changes in the human genome have been driven by pathogens. Scientists now recognize that local availability of nutrients is a significant spur to genetic change. In general, says Nielsen, what nourishes one human population nourishes them all. But there have been specialized adaptations in unusual circumstances. "Only now are we realizing that there are these genetic differences, and the study of them is in its infancy," he notes. "Different population groups are hardwired for different diets." The day is not far off when people will need to know their genetic makeup in order to choose the best way to nourish themselves.

The Big Switch

One of the most significant shifts in the human diet occurred with the change from hunting and gathering to agriculture and dairy farming, as animals were domesticated. Even today, the ability to digest the milk sugar lactose (beyond the period of infancy)—conferred by persistence of the gene for producing the enzyme lactase—is not universal. The lactase gene is preserved primarily among those of northern European descent; the continued ability to digest milk was likely necessary to get them through cold winters, when few other foods were available. Evidence suggests that the mutation arose 4,000 years ago. Cattle-herding tribes in East Africa also consume milk, and studies show that three mutations, each conferring lactose tolerance in a distinct way, arose among them 2,700 to 6,800 years ago. Dairy consumption in India and other parts of Asia generally occurs only after fermentation by bacteria to break down the lactose.