What happens to sugar and sweeteners in the body?

The texts shown during the animation, to read without it.

1. Fructose and fat made by the liver
   Stanhope 2009 trial: 32 adults with overweight got 25% of their calories from drinks sweetened with fructose or glucose for 10 weeks. Both groups gained the same weight, but fat around the organs rose only with fructose, and fat made by the liver after meals went up by 83% (7% with glucose). The dose is high. A meta-analysis of trials (Lee 2022) sees the liver load up with fat when sugary drinks are added on top of meals, not with fruit.
2. Li 2022 meta-analysis: trials comparing high-fructose corn syrup and table sugar at the same dose (767 people) find no difference in weight (−0.29 kg, not significant) or in other measures. Only one marker of inflammation, CRP, is slightly higher with the syrup: +0.27 mg/L. The trials are few, and one of them (Lowndes 2014) was funded by corn syrup producers.
3. Gut bacteria
   Suez 2022 trial: 120 healthy adults who never used them received saccharin, sucralose, aspartame or stevia every day, below the acceptable intake, for 2 weeks. With saccharin and sucralose, blood sugar rose more after a sugary drink; not with aspartame or stevia. All four changed gut bacteria. The effect mostly faded after stopping, and the bacteria given to mice reproduced the rise.
4. Weight and long-term health
   WHO review 2022: in trials, replacing sugar with a sweetener leads to an average loss of 0.71 kg. In cohorts followed for years, those who use the most have more type 2 diabetes (×1.23 for drinks) and more heart events (×1.32), with no proof that this is the cause: people who already have extra weight or diabetes may use more, and cause and effect may be reversed. In 2023 the WHO drew from it a “conditional” recommendation, which excludes polyols such as erythritol.
5. Erythritol and platelets
   Witkowski 2023 and 2024: among 1,157 patients who came in for a heart checkup, those with the most erythritol in their blood later had a risk of heart attack, stroke or death multiplied by 2.95. After 30 g of erythritol, the blood level is about 1,000 times higher and stays up for more than 2 days; in 10 volunteers, platelets then stuck together more, which 30 g of glucose did not do. The authors point out that an association does not prove cause.
6. Aspartame and cancer
   IARC and JECFA, July 2023: IARC classified aspartame as “possibly carcinogenic” (Group 2B), based on limited evidence in humans, for a liver cancer. JECFA kept the acceptable intake at 40 mg per kilogram per day: a 154 lb (70 kg) adult would have to drink more than 9 to 14 cans a day to go over it. In the French NutriNet-Santé cohort (102,865 adults), the heaviest users had 15% more cancers, a link that does not prove cause.
7. Heated sucralose
   BfR 2019 and 2026: above 120 °C (248 °F), sucralose breaks down and loses its chlorine: harmful chlorinated compounds can form. In 2019, the German agency BfR found the data insufficient to conclude; in 2026, it judged that a risk cannot be ruled out when cooking with it at home. Separately: in the lab, sucralose-6-acetate, found at up to 0.67% in batches of sucralose, damages the DNA of cells (Schiffman 2023).

Other texts from the animation

• Table sugar (sucrose) is made of one glucose and one fructose joined together. It latches onto the sweet taste receptor, the receptor closes, and the cell sends the “sweet” signal to the brain. To compare, we take 5 g of sugar, about one cube: 20 kcal.
• The syrup is made from corn starch: the starch is cut into glucose, then an enzyme turns part of the glucose into fructose. The two most common forms have 42% or 55% fructose, separate from the glucose; the rest is glucose and water. Its makeup is close to that of sugar, with the same calories.
• Aspartame is made of two protein building blocks joined together (phenylalanine and aspartic acid). It latches onto the same receptor, but much better: 25 mg tastes like 5 g of sugar. It provides 4 kcal per gram, like a protein, or 0.1 kcal at this dose.
• Sucralose is table sugar in which three groups of atoms are replaced by chlorine (the green beads). The receptor still recognizes it, and much better: 8 mg, an almost invisible grain, sweetens like a sugar cube.
• Stevia leaves contain steviol glycosides: a core, steviol (in green), surrounded by sugars (in white). These glycosides are 200 to 300 times sweeter than sugar. They also hit two bitter taste receptors, hence the aftertaste some people notice.
• Acesulfame K is a small molecule made in the lab, with a potassium atom (K). It is about 200 times sweeter than sugar: 25 mg tastes like 5 g of sugar, with no calories.
• Erythritol is a polyol: a short chain of 4 carbons, each carrying an -OH group (in red), like sugars. It is a little less sweet than sugar, 60 to 80%: it takes 6 to 8 g for the same taste. Europe counts it as 0 kcal per gram.
• Xylitol is a polyol, a chain of 5 carbons. It is as sweet as sugar, but Europe counts it as 2.4 kcal per gram instead of 4: 12 kcal for the taste of 5 g of sugar.
• Sweet taste goes through a single receptor made of two parts, T1R2 and T1R3, which recognizes molecules as different as sucrose, saccharin and acesulfame K (Nelson 2001). The structure of the human receptor shows that sucralose binds to the T1R2 part alone (Wang 2025).
• Sweetness compared with sugar: it changes with the dose and the product, and sources do not all give the same value (stevia: 40 to 300 for EFSA, 200 to 400 for the FDA). Values used here: FDA, EFSA and Magnuson 2016.
• The cubes are to scale by volume, assuming the same density for all pure powders.
• In the small intestine, an enzyme (sucrase) splits sucrose into glucose (blue) and fructose (orange). Glucose passes into the blood and feeds the muscles and brain, with help from insulin. Fructose is first handled by the small intestine itself; when there is a lot of it, the excess goes on to the liver. Nothing leaves in the stool.
• The syrup arrives already as separate glucose and fructose: sucrase has nothing to do. After that, it follows the path of table sugar: glucose into the blood, fructose to the gut and liver. HFCS-55 has a little more fructose (55%) than sugar (50%).
• Aspartame does not pass into the blood as is: the gut splits it into phenylalanine (about 50% of its weight), aspartic acid (40%) and methanol (10%). The liver handles these pieces like those from a protein or a fruit, which also provide them. The label flags phenylalanine for people with phenylketonuria.
• About 85% of sucralose does not cross the gut wall and leaves in the stool. The 15% that passes into the blood leaves in the urine, almost unchanged. The body gets no calories from it.
• Steviol glycosides travel through the small intestine without being absorbed. In the colon, bacteria strip off the sugars and release steviol (purple), which passes into the blood. The liver attaches a glucuronic acid to it, and the kidneys send it out in the urine.
• Acesulfame K passes into the blood without being changed, in humans as in animals. The kidneys send it out as is: more than 99% of what leaves the body goes out in the urine, within 24 hours.
• About 90% of erythritol passes into the blood through the small intestine, but the body can barely use it: it leaves unchanged in the urine. The rest reaches the colon. Hence 0 kcal per gram on European labels.
• At 5 g, about 90% of xylitol passes into the blood and the body uses it; at 30 g at once, only 66%. The rest reaches the colon: there it draws in water and bacteria ferment it. Most people tolerate it without diarrhea up to 10 to 30 g at once, depending on the person.
• Sucralose: in humans, 8.8 to 21.7% of the dose is absorbed and about 85% leaves in the stool, more than 99% unchanged (EFSA 2026, Magnuson 2016).
• Stevia: neither the acid nor the enzymes of the upper digestive tract split steviol glycosides; steviol released in the colon leaves mostly in the urine, attached to a glucuronic acid (Magnuson 2016).
• Aspartame: split into methanol (about 10% of its weight), phenylalanine (50%) and aspartic acid (40%) (Magnuson 2016).
• Xylitol: about 90% absorbed at 5 g in one dose, 76% at 15 g, 66% at 30 g (Mäkinen 2016). Erythritol: about 90% of the dose in the urine, unchanged (Munro 1998).
• Fructose: in mice, the small intestine handles about 90% of a small dose; from 1 g per kilogram, it is overwhelmed and more than 30% goes on to the liver (Jang 2018).
• The dots are a picture: each stands for a share of what is swallowed, not a molecule; the speed of the path is not to scale (from a few minutes to a day or two).
• Glucose is the benchmark: it passes quickly into the blood, and it is what blood sugar measures.
• Table sugar raises it less than pure glucose: only its glucose half counts in blood sugar; the fructose half is mostly handled by the gut and liver.
• Glycemic index tables give no value for high-fructose corn syrup. Its makeup is close to that of table sugar: 42 or 55% fructose instead of 50%.
• Fructose alone raises blood sugar only a little, because little of it reaches the blood as glucose: it is mostly handled by the gut and liver. This says nothing about what it does to the liver (“What is debated” tab).
• Xylitol is absorbed slowly and hardly turns into glucose: blood sugar barely rises.
• Erythritol passes into the blood but is not turned into glucose: blood sugar does not move.
• Aspartame provides no glucose: it is split into protein building blocks and methanol.
• Sucralose provides no glucose, and blood sugar does not move after a single dose. One trial suggests that after two weeks it may change the response to sugar in some people, through the gut microbiome (“What is debated” tab).
• Steviol glycosides provide no glucose: blood sugar does not move.
• Acesulfame K provides no glucose and leaves unchanged: blood sugar does not move.
• Glycemic index compared with glucose: sucrose 65, fructose 15 (Atkinson 2008, where glucose itself measures 103); xylitol 13, erythritol 0 (Livesey 2003, cited by Msomi 2021).
• Sweeteners that are not carbohydrates (aspartame, sucralose, stevia, acesulfame K) have no glycemic index: we wrote 0 because they provide no glucose.
• Corn syrup or table sugar: any difference?