Intelligence is an energy-intensive luxury – we have been reminded of this through the emergence of artificial intelligence. The human brain is no exception.
Although the brain makes up only about two percent of its body weight, it uses a fifth of the body’s energy at rest – compared to less than half in other primates. A five-year-old child spends 66 percent of his or her energy needs on the brain.
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How did our ancestors manage to finance the energy bills for their uniquely large brains? Eating more meat is often cited as the answer. But there’s one catch: the brain relies on a type of fuel not found in meat: glucose.
Our new study, published in Science, shows that carbohydrates have accounted for more than half of our total energy needs over four million years of evolution. The discovery provides important clues as to why we crave sweet foods today – and how we can eat healthier.
The role of meat
Many anthropologists have credited the eating of meat with stimulating the development of a large brain. After all, it required tools to dismember carcasses and gain access to the fat-rich bone marrow inside the skeleton.
Protein and fat in fruits and leaves are diluted and require hours of chewing, while animal foods are dense sources that can be consumed quickly. Bone marrow is a rich source of essential fatty acids.
But humans don’t actually need more protein as a proportion of their energy than other primates. Our ever-larger brains and high reproduction rates required carbohydrate calories (found in plants but not in meat), while our taller and heavier bodies needed fat calories to move their large muscles.
Although the body can synthesize glucose from precursors such as amino acids, the process is limited and energetically inefficient. There are also limits to using only protein as fuel – for example, it can lead to a type of poisoning known as “rabbit starvation”.
A minimum amount of carbohydrates in the diet was necessary. Our new study shows that the sugar in fruit and honey was the source throughout much of evolution.
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Modeling Ancient Diets
We modeled the diets of hominins – the group made up of humans and our immediate ancestors – over four million years of evolution.
First, we calculated the total need for glucose in organs and tissues that use it as a primary energy source. Besides the brain, red blood cells and kidneys require glucose.
Then we took reproductive needs into account. The fetus and placenta use glucose not only as an energy source, but also as a structural component of growing tissues. Synthesis of DNA, RNA, and nerve cell membranes requires glucose. During breastfeeding, women consume about 80 grams of glucose each day to produce the sugar in their breast milk.
Then we modeled the availability of macronutrients – carbohydrates, fats and proteins – from food, starting with the tiny, early monkey known as Lucy (Australopithecus afarensis).
This early ancestor of ours walked on two legs and was probably a specialist in ripe fruit, as chimpanzees are today. Over two-thirds of her energy came from naturally occurring sugars. In fact, some researchers believe that eating fruit – a diet characterized by fruit – was what kick-started the development of large brains. In tropical forests, it took good cognition to remember when and where the best fruits ripened – and strategic thinking to beat birds and other competitors.
We finished with the familiar dietary composition of modern hunter-gatherers in hot climates. In six steps, we increased the proportion of animal foods – from five per cent of calories to 35–50 per cent.
About one million years ago, mastering fire allowed boiled starch – which, unlike raw starch, can be easily digested into glucose – to replace some of the sugar. Relatively recently, about 100,000 years ago, starch in cereals became more available with the help of millstones and fireplaces.
Lessons for Modern Diets
Did early hominins get enough carbohydrates to meet the invariable needs of the brain and other tissues? Yes, if you were a man – but barely if you were a pregnant woman.
When we abandoned tropical environments in favor of cold and dry areas, carbohydrate intake became restrictive. Plants were plentiful, along with protein and bone marrow – but fruit and honey were seasonal.
We speculate that limited access to carbohydrates was selected for genes that cause higher blood sugar levels. This would improve the fetus’s growth and future survival. Today, the same genes probably predispose us to type 2 diabetes and cardiovascular disease. Low-carbohydrate diets may therefore be useful in specific clinical contexts.
But our findings provide an evolutionary explanation for why healthy people need about half of their energy as carbohydrates. They also give us insight into why humans crave sweetness – a pleasant signal on the tongue that encouraged food that fueled both body and mind millions of years ago.
Sugars are basically highly reactive molecules, but in nature they are bound to antioxidants and other compounds that reduce damage inside the cell. Ideally, we should consume them in that form – like fruit – rather than refined sugar.
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