Why Is Sugar So Hard to Resist? Sweetness, the Brain and an Industry Built on Both
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A newborn given a drop of sugar water smiles and relaxes, and given something bitter grimaces, before it has learned anything about food at all. The preference is built in, because for the whole of human history sweetness meant ripe fruit and honey, rare, safe and full of energy, and the animal that sought it out fed itself better. The same wiring now meets a world in which sweetness is the cheapest ingredient there is, added to bread, sauces and cereal by an industry that knows exactly what the newborn knew, and the mismatch is why a taste that used to protect people is now a public health problem.
What sugar is
Sugars are small carbohydrates, and the ones in food are a few: glucose, the fuel every cell burns and the form in which the blood carries energy; fructose, the sugar of fruit and honey, sweeter than glucose and handled almost entirely by the liver; and sucrose, table sugar, which is one glucose bonded to one fructose and is split in the gut. Lactose in milk and maltose in malt are the others. All are digested to glucose and fructose within minutes and absorbed, and a sugary drink raises blood glucose faster than almost any other food, which triggers a surge of insulin to clear it and, an hour or two later, a dip that feels like hunger. Starch is chains of glucose and is broken down more slowly; fibre slows everything and is why an apple and its juice do different things to the same body.
Why it tastes good
Sweetness is detected by a single receptor on the tongue, and its signal goes to the brain's reward system, where it triggers a release of dopamine in the same circuit that responds to every other pleasure, food, sex, drugs, money. The response is fast and does not need the sugar to be absorbed; the taste alone is enough, which is why artificial sweeteners give some of the pleasure and why a mouthful of sugar water swilled and spat improves an athlete's performance. The reward is amplified by fat, which is why the combination in ice cream, chocolate and pastry is more compelling than either alone, and it is not blunted by fullness in the way that the appetite for savoury food is, which is why there is always room for dessert. Whether sugar is addictive in the clinical sense is argued; rats given intermittent access binge, show withdrawal and relapse, and the brain changes in people who eat a great deal of it resemble those of drug users, but the effects are weaker and most people can stop.
What it does
The harm is mostly a matter of quantity. Sugar itself is not toxic, and a body that gets a little in fruit handles it easily; the trouble is that a fifth of the calories in a typical Western diet now come from added sugar, and that quantity does several things:
- •Supplies energy without nutrients or fullness, so that it is eaten on top of everything else and drives weight gain, especially in drinks, which the body does not register as food
- •Overloads the liver with fructose, which it turns into fat, contributing to fatty liver disease and to raised blood fats
- •Feeds the bacteria on teeth, whose acid dissolves enamel; tooth decay is the commonest chronic disease of childhood worldwide and is almost entirely sugar
- •Raises the risk of type 2 diabetes, through weight and through repeated surges of glucose and insulin
- •Crowds out the foods that would have been eaten instead
From luxury to commodity
Sugar cane was domesticated in New Guinea about 10,000 years ago, crystallised sugar was made in India by 500 AD, and it reached medieval Europe through the Arabs as a spice and a medicine, sold by the ounce alongside pepper. The Atlantic changed that. From the sixteenth century the Portuguese, Spanish, Dutch, French and British planted cane across Brazil and the Caribbean, and the labour that cut and boiled it was enslaved Africans, some two thirds of the twelve million carried across the ocean going to sugar; the plantation, the slave ship and the sugar bowl were one system. Consumption in Britain rose twentyfold in the eighteenth century as sugar went into tea, and beet sugar, developed in Napoleonic Europe to beat the British blockade, made it a temperate crop. By 1900 it was cheap, by 1970 high-fructose corn syrup had made it cheaper, and the food industry found that adding it to almost anything sold more of it.
What is being done
The World Health Organisation, which uses the British spelling of its own name in its official documents alongside the other, recommends that free sugars, those added or in juice, supply less than ten percent of energy and ideally under five, which is about six teaspoons a day; the average Briton eats twice that and the average American more. Taxes on sugary drinks, introduced in Mexico in 2014 and Britain in 2018, have cut consumption and, in Britain, pushed manufacturers to reformulate so that the sugar in soft drinks fell by a third before the tax was even collected. Labelling, limits on advertising to children and the removal of sweets from checkouts are the other levers. The individual lever is the drink: the largest single source of added sugar in most diets is what people swallow between meals, and the newborn's smile, which was right for a hundred thousand years, is the thing to distrust.
The takeaway
Sugar is hard to resist because sweetness evolved as the signal for rare, safe energy and the brain rewards the taste with dopamine before the body has absorbed anything, a preference that fat amplifies and fullness does not switch off. In the quantities the modern diet supplies, a fifth of calories, it drives weight gain, fatty liver, diabetes and tooth decay, and a food that was a spice in 1400 became a commodity through slave plantations, beet sugar and corn syrup, and is now being pushed back by taxes and reformulation.