Developing countries that have recently been growing more affluent and Westernized have correspondingly been growing more diabetic. In first place stand the eight Arab oil-producers and newly affluent island nations that now lead the world in national diabetes prevalences (all of them above 15%). All Latin American and Caribbean countries now have prevalences above 5%. All East and South Asian countries have prevalences above 4% except for five of the poorest countries, where prevalences remain as low as 1.6%. The high prevalences of the more rapidly developing countries are a recent phenomenon: India’s prevalence was still below 1% as recently as 1959 but is now 8%. Conversely, most sub-Saharan African countries are still poor and still have prevalences below 5%.
Those national averages conceal large internal differences that constitute further natural experiments. Around the world, urbanization results in less exercise and more supermarket food, obesity, and diabetes. Individual urban populations that thereby achieved notably high diabetes prevalences include the already mentioned Wanigela people of Papua New Guinea’s capital city (37% prevalence) and several groups of urban Aboriginal Australians (up to 33%). Both of those cases are all the more striking because diabetes was unknown among New Guineans and Australians under traditional conditions.
Thus, the Western lifestyle somehow increases the risk that those enjoying it will become diabetic. But the Western lifestyle consists of many interlinked components: which components contribute most to the risk of diabetes? While it isn’t easy to tease apart the effects of correlated influences, it appears that the three strongest risk factors are obesity and sedentary lifestyle (which you can do something about) and family history of diabetes (which you can’t do anything about). Other risk factors that you can’t control are either high or low birth weight. While diet composition surely acts at least in part by its relation to obesity, it also seems to have some independent influence: among people matched for obesity, those consuming a Mediterranean diet appear to be at lower risk than those with high intakes of sugar, saturated fatty acids, cholesterol, and triglycerides. Not exercising may create risks mainly through predisposing towards obesity, while smoking, inflammation, and high alcohol consumption appear to be independent risk factors. In short, Type-2 diabetes originates with genetic factors and possibly intra-uterine factors, which may become unmasked later in life by lifestyle factors resulting in disease symptoms.
Pima Indians and Nauru Islanders
These proofs of an environmental role in diabetes are illustrated by the tragedies of the two peoples with the highest rates of diabetes in the world: Pima Indians and Nauru Islanders. To consider the Pimas first, they survived for more than 2,000 years in the deserts of southern Arizona, using agricultural methods based on elaborate irrigation systems, supplemented by hunting and gathering. Because rainfall in the desert varies greatly from year to year, crops failed about one year in every five, forcing the Pimas then to subsist entirely on wild foods, especially wild jackrabbits and mesquite beans. Many of their preferred wild plants were high in fiber, low in fat, and released glucose only slowly, thereby constituting an ideal antidiabetic diet. After this long history of periodic but brief bouts of starvation, the Pimas experienced a more prolonged bout of starvation in the late 19th century, when white settlers diverted the headwaters of the rivers on which the Pimas depended for irrigation water. The result was crop failures and widespread starvation. Today the Pimas eat store-bought food. Observers who visited the Pimas in the early 1900s reported obesity to be rare and diabetes almost non-existent. Since the 1960s, obesity has become widespread among the Pimas, some of whom now weigh more than 300 pounds. Half of them exceed the U.S. 90th percentile for weight in relation to height. Pima women consume about 3,160 calories per day (50% over the U.S. average), 40% of which is fat. Associated with this obesity, Pimas have achieved notoriety in the diabetes literature by now having the highest frequency of diabetes in the world. Half of all Pimas over age 35, and 70% of those at ages 55 to 64, are diabetic, leading to tragically high occurrences of blindness, limb amputations, and kidney failure.
My second example is Nauru Island, a small remote tropical Pacific island colonized by Micronesians in prehistoric times. Nauru was annexed by Germany in 1888, was occupied by Australia in 1914, and eventually achieved independence in 1968 as the world’s smallest republic. However, Nauru also has a less welcome distinction as the grimly instructive site of a rarely documented phenomenon: an epidemic of a genetic disease. Our familiar epidemics of infectious diseases flare up when transmission of the infectious agent increases, and then wane when the number of susceptible potential victims falls, due both to acquired immunity of the survivors and to differential mortality of those who are genetically susceptible. An epidemic of a genetic disease flares up instead because of a rise in environmental risk factors, and then wanes when the number of susceptible potential victims falls (but only because of the preferential deaths of those who are genetically more susceptible, not because of acquired immunity; one doesn’t acquire immunity to diabetes).
The traditional lifestyle of Nauruans was based on agriculture and fishing and involved frequent episodes of starvation because of droughts and the island’s poor soils. Early European visitors nevertheless noted that Nauruans were plump, and that they admired big fat people and put girls on a diet to fatten them and so make them more attractive in their eyes. In 1906 it was discovered that most of Nauru underlying those poor soils consists of rock with the world’s highest concentration of phosphate, an essential ingredient of fertilizer. In 1922 the mining company extracting the rock finally began to pay royalties to the islanders. As a result of this new wealth, average sugar consumption by Nauruans reached a pound per day in 1927, and laborers were imported because Nauruans disliked working as miners.
During the Second World War Nauru was occupied by Japanese military forces, who imposed forced labor, reduced food rations to half a pound of pumpkin per day, and then deported most of the population to Truk, where half of them died of starvation. When the survivors returned to Nauru after the war, they regained their phosphate royalties, abandoned agriculture almost completely, and resumed shopping in supermarkets, heaping their shopping carts with big bags of sugar and eating double their recommended calorie intake. They became sedentary and came to rely on motor vehicles to travel around their little island (averaging one and a half miles in radius). Following independence in 1968, per-capita annual phosphate royalties rose to $23,000, making Nauruans among the world’s richest people. Today they are the most obese Pacific Island population, and the one with the highest average blood pressure. Their average body weight is 50% greater than that of white Australians of the same height.
Although colonial European physicians on Nauru knew how to recognize diabetes and diagnosed it there in non-Nauruan laborers, the first case in a Nauruan was not noted until 1925. The second case was recorded in 1934. After 1954, however, the disease’s prevalence rose steeply, and it became the commonest cause of non-accidental death. One-third of all Nauruans over the age of 20, two-thirds of those over age 55, and 70% of those few who survive to the age of 70 are diabetics. Within the past decade the disease’s prevalence has begun to fall, not because of mitigation of environmental risk factors (obesity and the sedentary lifestyle are as common as ever), but presumably because those who are genetically most susceptible have died. If this interpretation should prove correct, then Nauru would provide the most rapid case known to me of natural selection in a human population: an occurrence of detectable population-wide selection within less than 40 years.