What happens to sugar in your body?

Your body breaks sugar down into simple sugars, such as glucose and fructose, and uses them for fuel. Insulin, a hormone made by the pancreas (an organ behind the stomach), helps glucose get into cells ^[1]. Studies link eating a lot of added sugars with weight gain, type 2 diabetes and other conditions ^[3][4]. Most of that evidence shows an association, not proof of cause ^[4].

How does the body digest sugar?

Foods hold different sugars. Table sugar (sucrose) and milk sugar (lactose) are split by enzymes into single sugars ^[2]. The two you hear about most are glucose and fructose.

Glucose goes into the blood. The rise in blood glucose "quickly triggers secretion of insulin by the pancreas" ^[2]. Insulin "helps glucose get into your cells to be used for energy" ^[1]. After a meal, insulin also helps the liver store glucose as glycogen, a stored form of glucose ^[2].

Fructose takes a different route. It is handled mainly by the liver, and to a lesser extent by the small intestine and kidneys ^[2]. A review of liver research notes that the first step of fructose breakdown is not slowed by the body's usual feedback controls ^[8].

What does research link to added sugars?

Added sugars are sugars added to foods during processing ^[3]. They are different from the sugars found naturally in whole fruit or plain milk ^[6]. The CDC says too much added sugar "can contribute to health problems such as weight gain and obesity," and links it to type 2 diabetes and heart disease ^[3].

The broadest source here is a 2023 umbrella review in The BMJ. (An umbrella review gathers many earlier meta-analyses, which are studies that pool results from other studies.) It covered 73 meta-analyses and 83 health outcomes. The authors concluded that "high dietary sugar consumption is generally more harmful than beneficial for health, especially in cardiometabolic disease" ^[4]. They also reported that no high-quality evidence was found. Most of the evidence is observational, meaning researchers watch what people eat and what happens to them instead of running a controlled test. The authors called most of it low quality ^[4].

Does sugar cause fatty liver?

Fatty liver disease (MASLD) means extra fat builds up in the liver. The NIDDK (a US National Institutes of Health institute) says it is not caused by heavy alcohol use and is more common in people with obesity and type 2 diabetes ^[13].

Some trials in people found that fructose raised fat production in the liver and glucose did not ^[8]. But the same review says human intervention studies on sugar and fructose are "limited and contradictory" for fatty liver ^[8]. It also notes it was unclear whether some effects came from extra calories rather than fructose itself ^[8]. In short: sugary drinks are linked to fatty liver in observational studies, and the exact role of sugar is still being studied.

What does sugar do to your teeth?

The CDC explains that cavities are caused by bacteria in the mouth. "You feed the bacteria every time you eat or drink," and the bacteria "produce acid which starts to dissolve the outer enamel layer of your teeth" ^[9]. The CDC lists drinking or eating sugar, especially between meals, as a risk factor ^[9].

Does sugar cause inflammation?

The evidence is mixed. A 2018 meta-analysis in Nutrients pooled 13 trials with 1,141 people. It found no difference in a common inflammation marker (CRP, short for C-reactive protein) between fructose and glucose, or between high-fructose corn syrup and sucrose. The authors rated the evidence low quality and wrote that it "does not support the hypothesis" that fructose adds more to subclinical inflammation than other sugars ^[10].

A 2022 meta-analysis of controlled feeding trials found that the food source mattered. When these foods replaced other foods at the same calories, mixed sources that included sugar-sweetened drinks raised CRP (moderate certainty). When fruit was added to the diet, fruit lowered CRP ^[11]. The authors say the food source seems to matter, and that effects may depend on whether calories were held steady ^[11]. So the trials do not give one simple answer on sugar and inflammation.

Is sugar addictive?

A 2016 review in the European Journal of Nutrition looked at the science. Its authors found "little evidence to support sugar addiction in humans" ^[12]. In animal studies, addiction-like behavior such as bingeing happened only when sugar was available on and off, not all the time ^[12]. This review covers addiction claims. It does not rule out that people enjoy or crave sweet foods.

What do health agencies say about how much added sugar to eat?

Agencies differ in wording:

• The 2025-2030 Dietary Guidelines for Americans say "added sugars are not part of eating real foods and are not recommended." They encourage avoiding added sugars entirely, especially for children. They allow the sugars found naturally in whole fruit and plain dairy ^[6].
• The WHO recommends cutting free sugars to less than 10% of daily calories. It calls this a "strong" recommendation. It says going below 5%, about 25 grams or 6 teaspoons a day, would bring more benefit, and calls that a "conditional" recommendation ^[5].
• The CDC reports that US adults age 20 and older averaged 17 teaspoons of added sugar a day in 2017-2018. This is an intake average, not a recommendation ^[3].

The bottom line

The body turns sugar into glucose and fructose, and insulin and the liver handle them. The BMJ umbrella review linked high sugar intake with worse heart and metabolic outcomes, but the evidence was mostly observational and low quality ^[4]. Trials show a mixed picture on inflammation ^[10][11], and a 2016 review found little evidence for sugar addiction in humans ^[12]. The WHO advises cutting free sugars to under 10% of daily calories, and the Dietary Guidelines advise avoiding added sugars ^[5][6].