A new study has identified dietary sugars, known as glycemic carbohydrates, as a key element in human evolution, arguing they were essential to meet the energy demands created by the growth of the human brain. The research, led by Jennie Brand-Miller and collaborators, was published in the journal Science under the title "A central role for dietary sugars in human evolution."
Using mathematical modeling, the researchers found that as the human brain grew larger over the course of evolution, it generated an energy demand that diets based only on protein and fat could not meet. That is because the body's ability to convert protein into glucose, a process called gluconeogenesis, is limited.
The study notes that the human brain, red blood cells and the renal medulla of an adult all depend on a set daily amount of glucose to function. Over the course of hominin evolution, estimated brain weight grew from about 300 grams in Australopithecus afarensis to about 1,500 grams in Homo sapiens, sharply increasing that requirement. The mathematical model showed that on a diet with little or no carbohydrate, the body's capacity to generate glucose from protein alone could not keep up with an expanding brain.
How the human diet evolved over time
The study modeled the common ancestor of humans on the wild chimpanzee, which obtains roughly 95% of its energy from plants and honey. Of the carbohydrates it consumes, about 95% are soluble sugars from ripe fruit and honey.
As global cooling caused savannas to expand, evolution forced a dietary shift. Energy from simple sugars in fruit and honey was gradually replaced by cooked starches from tubers and grains. Even so, the presence of these glycemic carbohydrates remained the pillar that kept the body's carbohydrate balance positive, according to the study.
Pregnancy as an evolutionary bottleneck
The study found that pregnancy and lactation dramatically increased the need for sugar, or glucose, making it a decisive and limiting factor for the species' survival and expansion.
An adult already carries an obligatory daily glucose demand from the brain, red blood cells and renal medulla. During the reproductive period, the study found, women faced massive additional demands on top of that baseline.
The central evolutionary problem, according to the research, was the body's physical limit on manufacturing its own sugar. The maximum capacity to produce glucose from protein and fat is low relative to what pregnancy requires. Without direct carbohydrate intake, this produced a severe deficit of 85 grams of glucose per day for pregnant or lactating women.
As hominin brains kept expanding over the course of evolution, that deficit became an increasingly critical bottleneck. As a result, the availability and consumption of fast-absorbing soluble sugars in the diet became vital, the study found.
The research concludes that these sugars secured the positive carbohydrate balance pregnant and lactating individuals needed to gestate and nurse offspring with ever larger brains, a factor the authors say made the reproductive success of the human species possible.
