단맛 당기는 이유일까…뇌 키운 건 단백질보다 당분, 논쟁적 가설 나와

Around 400,000 years ago, the human ancestor known as Lucy from Africa had a brain weighing just 300 grams, similar to a modern-day chimpanzee. However, about 30,000 years later, the modern human brain (Homo sapiens) had grown to weigh a massive 1,500 grams. To support the activity of such a large brain, considerable energy was required.

Additionally, human brains, despite making up only 2% of our body weight, consume 20% of our total energy intake, even more for growing children. Traditionally, the academic community attributed brain capacity growth to the consumption of meat (protein and fat) and the discovery of cooking. However, this explanation has its flaws, as the brain primarily uses sugar, not protein, for energy.

An international team of researchers from the University of Sydney in Australia and the University of Glasgow in the UK published a study in the journal Science, suggesting that sugars from fruits, honey, and starch may have played a more significant role in human brain evolution than previously thought. Their analysis of human nutritional and metabolic models showed that over half of the total energy required by the human body came from carbohydrates, with the majority coming from fruits and honey during the early stages of human evolution, and starch later on.

Today, our craving for sweet foods can be traced back to this evolutionary history. A 2017 study by researchers at New York University sparked interest after discovering that fruit-eating primates have larger brains on average than leaf-eating primates and even larger brains than omnivorous primates that eat meat. Dr. Jennifer Brandel-Miller, the lead author of the current study from the University of Sydney, nutrition department, said she was inspired by the research, likening it to turning on a light bulb in her mind.

She pointed out that converting protein to sugar in our bodies is not only energy inefficient but also insufficient to meet the high demand for sugar during pregnancy and lactation. The researchers divided the 4 million-year evolution of our ancestors into six stages and created models of their hominin diets based on fossil records.

They calculated the total sugar needs of key organs and tissues, as well as the additional sugar requirements during pregnancy and lactation. Their findings suggest that early humans primarily obtained sugar from fruits and honey in the African tropical rainforest, with natural sugars accounting for over two-thirds of their total energy intake.

The discovery of fire around 100,000 years ago allowed humans to cook roots and grains, providing a secondary burst of energy to further drive brain growth. However, as humans migrated to cooler, drier regions after leaving Africa, carbohydrate intake became insufficient, leading to selective evolution of genes that help better absorb sugars.

For example, humans have up to 20 amylase genes involved in starch digestion, compared to just one in chimpanzees. The researchers point out that these genes that raise blood sugar levels may also contribute to modern-day conditions like type 2 diabetes and cardiovascular diseases. However, this study is not without its limitations, as it is based on assumptions and not definitive evidence.

Critics, such as Dr. Matthew Spornheimer from Colorado State University, argue that the study presents a valuable analysis based on unverified assumptions. They note that while the role of diet in human evolution is speculative, objective evidence like eye color, tooth shape, and isotopic ratios suggest that early humans primarily consumed fruits for sugar.

The researchers acknowledge that while our ancestors likely had a diet rich in fruits, the exact nature of their diet remains uncertain.