Why are we so drawn to sugar?

A brain that needs sugar to function

The human brain is an extraordinarily 'expensive' organ in terms of energy. Although it accounts for only around 2% of our body weight, it consumes an enormous share of the glucose the body uses: an adult devotes approximately 100 to 120 grams of glucose per day solely to keeping the brain running. That figure is the key to understanding this entire evolutionary story.

A four-million-year experiment

Researchers from the universities of Sydney and Glasgow, led by nutrition professor Jennie Brand-Miller, published a paper in Science (2026) that models the glucose requirements of hominins over approximately four million years. They reconstructed the diet of our ancestors by combining carbon and nitrogen isotope analysis of fossil bones with the relationship between brain size, body size, and type of diet.

Their starting point is revealing: the brain of our ancestors grew from around 300 grams in Homo habilis to nearly 1,500 grams in modern humans. That spectacular growth required fuel, and that fuel was, to a large extent, glucose.

In the beginning, a predominantly plant-based diet

The authors take chimpanzees — our closest relatives, who in the same environment choose the same fruits as we do — as their first reference point. In their highly varied diet (tender leaves, flowers, insects, honey, nuts, eggs, and even some meat, since they also hunt), the majority of energy — around 76% — comes from carbohydrates.

About four million years ago, our earliest ancestors would have had a similar diet: it is estimated that barely 5% of energy came from meat and 95% from plant foods. With the emergence of the genus Homo, the brain grew, animal-sourced foods gained in importance and, along with them, greater technical capability: extracting marrow from bones, cracking hard fruits, grinding seeds, and, later, mastering fire.

Neanderthals ate more carbohydrates than we do

One of the most striking findings of the study is that Neanderthals, Denisovans, and Homo sapiens may have consumed a proportion of carbohydrates higher than that of many industrialised countries today. In other words, the romantic idea of a past 'without sugar' does not fit with what these models suggest. Sweetness — in the form of ripe fruit and honey — was part of our diet from the very beginning.

Why women and children need even more

The developing brain is particularly demanding. If an adult devotes around 100–120 grams of glucose per day to their brain, a pregnant or breastfeeding woman may need around 230 grams, and a five-year-old child between 150 and 200. In childhood, the brain can consume up to two thirds of the basal metabolic rate. It is no coincidence, therefore, that the taste for sweetness is so deeply ingrained in us: for almost the entirety of our history, finding natural sugar was a survival advantage.

Where did that sugar come from

Here a crucial nuance emerges. The sugar our ancestors consumed did not come from a soft drink or a biscuit. It came from ripe fruit, from honey — a highly prized delicacy among hunter-gatherers — and, later on, from tubers, roots, and seeds whose starch only became truly usable thanks to cooking. These were foods that arrived alongside fibre, water, vitamins, and minerals, and that also required real effort to obtain.

Why this does NOT justify today's sugar

And here we reach the point that matters most to me as a physician. The fact that our attraction to sweetness has such deep evolutionary roots explains why we find it so hard to resist — it does not make it harmless. In fact, the authors themselves acknowledge an important limitation of their work: the model does not include the stage we are living through now, one of an overabundance of added sugars that our bodies never encountered in nature.

The problem is not fruit. The problem is that we have separated sweetness from its natural packaging. Today we obtain sugar quickly, cheaply, and without the fibre that would slow its absorption: sugary drinks, ultra-processed foods, and products that do not even taste sweet to us. This excess of free sugar is associated with obesity, insulin resistance, type 2 diabetes, and fatty liver disease. Our biology is wired to seek out sugar, but not to handle the quantities that are now within arm's reach on every street corner.

What I want you to take away

Our fondness for sugar is not a failure of willpower: it is a legacy of millions of years in which natural sugar helped build our brains. Understanding this should relieve us of guilt, but it should also give us a compass. The World Health Organization recommends keeping free sugar intake below 10% of total calories, and ideally below 5%.

In practice: enjoy whole fruit rather than juice, save added sugar for occasional moments, be wary of sugary drinks, and learn to read food labels. The goal is not to fear sugar, but to restore it to its rightful place. I am Dr. Florian Vallecillo, and I find it fascinating that the very same molecule that helped make us human is today one of our greatest metabolic challenges.