Added sugar and health: what the evidence shows on weight, heart disease and teeth, and what guidelines say

Sugar has been blamed for obesity, diabetes, heart disease, fatty liver, cancer and addiction, sometimes all in the same breath. It has also been defended as just another carbohydrate whose only fault is that people eat too much of it. The research supports neither extreme.

What it does support is narrower and better documented than most readers would guess. The World Health Organization’s limit on sugar rests mainly on two bodies of evidence, one about body weight and one about teeth. The links to heart disease and early death come mostly from studies of sugary drinks. And a good deal of what is said about sugar beyond that stands on evidence that reviewers themselves grade as weak.

What counts as added sugar, and what free sugars are

Guidelines do not target all sugar. The WHO uses the term free sugars, which it defines as monosaccharides (such as glucose, fructose) and disaccharides (such as sucrose or table sugar) added to foods and drinks by the manufacturer, cook or consumer, and sugars naturally present in honey, syrups, fruit juices and fruit juice concentrates. American guidance uses the similar term added sugars.

The sugar inside whole fruit, vegetables and milk is left out. The WHO says its guideline does not refer to those sugars because there is no reported evidence of adverse effects of consuming these sugars. Much of the free sugar in a diet, by contrast, is not obvious. The WHO’s examples are a tablespoon of ketchup, which contains around 4 grams (around 1 teaspoon) of free sugars, and a can of sugar-sweetened soda, which can hold up to 40 grams, about ten teaspoons.

How much sugar the guidelines allow

Body	Limit	Strength
World Health Organization, 2015	Free sugars below 10% of daily calories	Strong recommendation
World Health Organization, 2015	Below 5% of calories, roughly 25 grams or 6 teaspoons a day	Conditional recommendation
American Heart Association, 2009	Added sugars up to 100 calories a day for most women, 150 for most men	Scientific statement

The difference between the WHO’s two tiers is a difference in evidence. The 10% limit is graded strong. The 5% figure is graded conditional, a label the WHO uses when the benefits probably outweigh the downsides but the supporting studies are thin.

Intakes run well above both. The American Heart Association reported that in 2001 to 2004, the usual intake of added sugars for Americans was 22.2 teaspoons per day. A later analysis by the US Centers for Disease Control and Prevention found that added sugar supplied about 15% of adults’ calories in 2005 to 2010, and that most adults consumed 10% or more of calories from added sugar.

What trials show about sugar and body weight

The weight evidence comes from a review led by Lisa Te Morenga and Jim Mann at the University of Otago in New Zealand and published in The BMJ. They gathered 30 randomized trials and 38 long-term cohort studies, and the WHO guideline cites the same two results they reported.

In trials where adults ate as they pleased apart from being asked to change their sugar intake, reduced intake of dietary sugars was associated with a decrease in body weight of about 0.8 kilograms. Asking people to eat more sugar produced a gain of about the same size. Some of these trials ran for as little as two weeks, so the figures describe a short-run shift, less than two pounds either way.

The most instructive result came from a third group of trials, in which sugar was swapped for other carbohydrates such as starch while calories were held equal. There, weight did not change. The authors concluded that the change in body fatness that occurs with modifying intakes seems to be mediated via changes in energy intakes. Sugar, on this evidence, adds weight because it adds calories that are easy to consume, and a calorie of sugar is no more fattening than a calorie of bread.

Children’s trials were less clear. They mostly involved advice to cut sugary foods and drinks, and the children did not follow it well; these trials showed no overall change in body weight. In cohort studies that tracked what children drank, however, those with the highest intake of sugary drinks had about 55% higher odds of being overweight a year later than those with the lowest.

Sugar, heart disease and early death

Evidence here is observational, and most of it concerns drinks.

Quanhe Yang and colleagues at the US Centers for Disease Control and Prevention followed 11,733 adults from a national survey for a median of nearly 15 years. Compared with adults who got less than a tenth of their calories from added sugar, those who got between a tenth and a quarter were about 30% more likely to die of cardiovascular disease, and those who got a quarter or more were between two and three times as likely. The study counted 831 cardiovascular deaths, so the top estimate is imprecise.

The largest study of drinks came from Harvard. Vasanti Malik and colleagues tracked more than 118,000 health professionals for about three decades and reported that consumption of sugar-sweetened beverages was associated with a higher risk of total mortality, with a graded rise as intake went up. People who had two or more a day had death rates about a fifth higher than those who had less than one a month, and deaths from cardiovascular disease about a third higher.

Sugary drinks keep appearing in this research partly because they are easy to measure and partly because they deliver a large dose quickly. Whether sugar eaten in solid food carries the same risk is much less certain.

Sugar and tooth decay

The dental case is older and simpler. Bacteria in the mouth ferment sugars into acid, and acid dissolves enamel. Paula Moynihan and Sarah Kelly at Newcastle University reviewed 55 studies for the WHO. Of the studies, 42 out of 50 of those in children and 5 out of 5 in adults reported at least one positive association between sugars and caries, the dental term for decay. They judged the evidence that decay is lower when free sugars stay under 10% of calories to be of moderate quality.

The stricter 5% target leans on an unusual source. Sugar almost vanished from some national diets during the Second World War, and tooth decay fell with it. The WHO describes this natural experiment as the basis for its conditional recommendation, and Moynihan and Kelly rated the evidence for the 5% threshold as very low quality.

What a review of 73 meta-analyses found on sugar and disease

In 2023 a team at Sichuan University in China took stock of the whole field in what is called an umbrella review, a review of existing meta-analyses. Yin Huang and colleagues collected 73 meta-analyses covering 83 health outcomes. Sugar intake showed harmful associations with 45 of them, from obesity and gout to heart disease and several cancers.

The breadth of that list is less impressive than it looks. The reviewers graded the evidence for each outcome, and the best grades they highlight are moderate. Moderate quality evidence suggested that the highest versus lowest dietary sugar consumption was associated with increased body weight in the case of sugary drinks, and with fat building up in the liver and muscle in the case of added sugars. The links they highlight to heart disease, gout, early death and pancreatic cancer were rated low quality. Of the 73 meta-analyses, 58 were about sugar-sweetened beverages.

The authors nonetheless endorsed a limit. They concluded that reducing free or added sugars to below 25 g/day (approximately 6 teaspoons/day), and keeping sugary drinks to less than one serving a week, was advisable, which matches the WHO’s stricter tier.

Limits of the evidence on sugar and health

Almost all of the disease evidence rests on asking people what they eat. Sugar intake is hard to measure, and food questionnaires are blunt instruments even when answered honestly. Heavy consumers of sugary soda also tend to differ from light consumers in income, smoking, exercise and the rest of their diet. Studies adjust for these differences and cannot remove them entirely, so an association with heart disease does not show that the sugar caused it.

The trials have the opposite weakness. They can show cause, but they are short, they measure weight and blood markers, and none has run long enough to count heart attacks. The Otago reviewers also noted potential bias in some trials and considerable variation between them.

The picture that results is uneven. That sugar adds weight by adding calories is supported by trials. That sugary drinks go with more heart disease and earlier death is supported by large cohorts.

That sugar in ordinary amounts of solid food harms health beyond its calories has not been shown.