How sweet it is to be human - how sugar made us smart

: 17 May 2023

Humans have big brains.

Humans are distinguished from other apes in having larger brains and an unusual life history that combines high reproductive output with slow childhood growth and exceptional longevity.

Our lineage has experienced an acceleration in metabolic rate, with diet providing energy for larger brains and faster reproduction without sacrificing maintenance and longevity (Pontzer 2016). The human brain is much bigger and more energy demanding than any other primate brain, in fact eight times bigger than predicted by our body size (Aiello and Wheller 1995).

In adults, about 20% of resting metabolic rate is devoted to the brain, even though it occupies only 2% of our body weight. The figures are much higher in newborns and children. Data collected using MRI-based 4D flow measures of cerebral blood flow shows the peak occurs in 5–6-year-old children, where a whopping 60% of BMR is devoted to cerebral energy use (Aronoff 2022). Interestingly, this age corresponds to the nadir (lowest point) in body fat.

Glucose fuels our big brains.

Our complex human brain and red blood cells evolved over millions of years to use glucose as their primary fuel (Megenthaler 2013) and convert sugars such as fructose and galactose into glucose. Of course, our bodies can make glucose in the liver and some other tissues in the process called gluconeogenesis using substrates such as glycerol (from fat) and alanine (from protein). But there is finite limit to this ability after which, in the absence of dietary sources of carbohydrate, blood glucose begins to fall. The lower blood glucose will eventually stabilise, and ketones formed from the oxidation of fat will provide part of the fuel needed by the brain. However, long-term, this is not ideal in terms of psychology and mood (Brinkworth 2009).

For most of human history, fruit and honey were the main sources of glucose equivalents (Crittenden 2011). Fruits that dried naturally (think dates) provided intense sugar hits on a seasonal basis. Human milk, the sweetest of all mammalian milk contains 7% lactose, a disaccharide of glucose and galactose. Once we mastered fire (perhaps 500,000 to 1 million years ago), cooked starch became a good source of glucose too (Wrangham Catching Fire). Importantly, glucose and other carbohydrates are not found in meat or leaves.

Our love of sweetness.

The desire for sweetness is innate in humans and new-borns consistently drink more of a sugar solution than of water (Naim and Kare, 1982). Most children love fruit, but need encouragement to enjoy vegetables that are not sweet. Today, we indulge our desire for sweetness on a grand scale, the agent being inexpensive, plentiful refined sugars. In Australia, apparent consumption (availability; around double actual consumption) peaked at about 60 kg per capita per year in the 1970s, although it has been steadily declining for 50 years, now down to ~40 kg/capita per year (Brand-Miller and Barclay). The last national dietary survey indicated Australians consume about 10% of our energy from added sugars or ~50 g per day (ABS). That corresponds to level that WHO recommends as the upper limit on free sugars, although some authorities would prefer 5%.

Sugar, not meat, made us smart.

The long-held view that hunting made us human is probably wrong, promulgated 50-60 years ago by a largely male discipline of paleobiology (de Waal 2022). It is being surpassed by the proposition that the sugars in fruit and nectar underwrote the process of becoming a social, highly intelligent primate because these foods fuelled the growth of our ever expanding, demanding brain. Finding the ripest fruit in the forest with the most highly concentrated source of glucose gave us not just energy for the brain, but also fuel for muscles during prolonged strenuous physical activity, and over eons of time, our colour vision (Osorio 1996).

Fruits were an important source of glucose and other sugars, but honey hunting, the golden viscous syrup created by bees from plant nectars was just as important. We know from anthropological studies of both African and Australian tribes that hunting honey was an activity that preoccupied both sexes (Crittenden 2017). And they found it in large amounts, kilos of it, enough to gorge and have enough left over to dilute it with water to quench one’s thirst with a refreshing sweet drink (Hawkes). And of course, if left to ferment, the psychotropic effects of this bubbly drink were wondrous.

Sugar consumption through the ages.

There are no precise figures for consumption at most times because honey was part of either a hunter-gatherer or farmer subsistence economy (Allsop and Brand-Miller). Historians and food writers assumed that it was a scarce commodity available only to a wealthy few. However, there’s a large amount of evidence that this is simply not true. Honey was sold in large units (gallons and even barrels) in a cash economy, and it was present in such abundance that mead, made from honey usually in monasteries, was a common alcoholic drink.

A reappraisal of the evidence from the Stone Age, Antiquity, the Middle Ages and early Modern times suggests that ordinary people ate much larger quantities of honey than has previously been acknowledged (Allsop 1996). Intakes at various times during history may well have rivalled our current consumption of refined sugar. There are implications therefore for the role of sugar in modern diets. Refined sugar may not have displaced more nutrient-rich items from our present-day diets but only the nutritionally comparable food, honey. And in my view, 50 g per day is entirely compatible with an enjoyable, safe and healthy diet.

Professor Jennie Brand-Miller AO, FAA is recognised for her work on carbohydrates in health and disease, particularly the application of the glycaemic index of foods to diabetes and obesity. She is an Officer of the Order of Australia and Fellow of the Australian Academy of Science. Her research has been translated into bestselling books in 12 languages (The Glucose Revolution, 3.5 million copies worldwide). She compiled the first tables of composition of Australian Aboriginal bush foods - the largest wild food database in the world – and has a passionate interest in human evolution and paleolithic nutrition. Jennie is also a proud recipient of bilateral Nucleus® bionic ears.