What glucose does in the blood, meal after meal
Every time you eat something with carbohydrate, blood glucose rises and the pancreas releases insulin to direct that glucose into the cells of muscle, liver and adipose tissue. This is normal physiology, not a problem. The issue appears when this cycle repeats in excess, day after day, year after year, in a body that already accumulates visceral fat: the cells start responding less and less to insulin, a state called insulin resistance. The pancreas tries to compensate by producing more insulin, until, in many people, that compensation fails and fasting glucose begins to rise, the classic path to prediabetes and type 2 diabetes.
This does not happen because of one isolated spoonful of sugar. It happens because of the chronic pattern: repeated caloric excess, a sedentary lifestyle and high free sugar intake, a category the WHO defines as sugars added to foods and drinks by the manufacturer, the cook or the consumer, plus the sugars naturally present in honey, syrups and juices, but which explicitly excludes the intact sugar inside whole fruit.
The liver and fructose: a different metabolic route
Sucrose, table sugar, is half glucose and half fructose. Glucose is used by practically any cell in the body. Fructose follows another route: it is taken up almost exclusively by the liver, via the GLUT5 transporter, and there it can become glycogen, lactate or, when the liver's stores are already full, be converted into fat through a process called de novo lipogenesis.
The most cited experiment on this mechanism is the one by Kimber Stanhope and colleagues at the University of California, Davis. In a controlled trial with adults aged 40 to 72 with overweight, participants drank fructose-sweetened or glucose-sweetened beverages for ten weeks, at the same calories. Only the fructose group showed an increase in visceral fat, a drop in insulin sensitivity and greater liver fat production, even with similar weight gain between the two groups. The endocrinologist Robert Lustig, of UCSF, used findings like this one to argue that chronic excess of free fructose works as a risk factor for fatty liver disease associated with metabolic dysfunction.
This does not turn fruit into a villain. The WHO's own definition of free sugars excludes the intact fructose and glucose inside whole fruit: the fiber and cellular matrix slow absorption and increase satiety, and there is no evidence of liver overload from eating an apple, a banana or an orange. The risk described by Stanhope and Lustig comes from free sugar in concentrated, liquid form, such as soda, industrially processed juice and high fructose corn syrup, not from the fructose that reaches the body together with fiber, water and micronutrients.
The mouth: the strongest causal link of all
Among all the effects of sugar on the body, dental caries is the one with the strongest and best established relationship with free sugar, according to the WHO itself. A systematic review commissioned by the organization to support its 2015 guideline found a positive association between sugar consumption and caries in 42 of the 50 studies analyzed in children and in the 5 studies analyzed in adults, with moderate quality evidence showing that caries is consistently lower when free sugar consumption stays below 10% of daily energy, with additional benefit below 5%.
Sugary drinks: the food category with the most consistent association with weight and diabetes
Sugar-sweetened beverages, such as soda, sweetened tea and energy drinks and industrially processed juice, are the food category with the most consistent association with weight gain in large systematic reviews. A central hypothesis is that liquid calories do not activate the same satiety mechanisms as solid food: the body does not fully compensate at the next meal for what it drank.
The meta-analysis by Vasanti Malik and colleagues from the Harvard School of Public Health, published in Diabetes Care in 2010 and pooling more than 310 thousand participants, found that those who consumed the most sugary drinks had a 26% higher risk of developing type 2 diabetes than those who consumed less, and a 20% higher risk of metabolic syndrome. The effect was dose dependent: each additional serving of about 350 ml per day raised the risk of type 2 diabetes by approximately 25%.
Heart: added sugar and cardiovascular mortality
The most direct study on sugar and fatal cardiovascular risk is the one by Quanhe Yang and colleagues, from the CDC and Harvard, published in JAMA Internal Medicine in 2014, with data from 11,733 American adults followed for a median of 14.6 years. Compared with those who got less than 10% of daily calories from added sugar, those who got 25% or more had almost three times the risk of cardiovascular death, even after adjustment for body weight, physical activity and other sociodemographic and clinical factors. The risk rose gradually across each consumption bracket, not only at the extremes, which reinforces a dose response relationship and not just an association between extreme groups.
Glycation and the brain: two less discussed effects
When sugars react chemically with proteins and fats, whether inside the body or during cooking at high temperatures, advanced glycation end products form, known by the English acronym AGEs. These molecules accumulate in tissues over time, activate receptors that amplify oxidative stress and low grade inflammation, and are one of the proposed mechanisms for chronic complications associated with persistent hyperglycemia, such as vascular and kidney damage.
The popular debate that sugar hooks you like a drug, in turn, deserves a fine adjustment. Hyperpalatable foods, typically the combination of sugar with fat and not sugar alone, activate brain reward circuits linked to dopamine, something real and measurable through neuroimaging. But a 2016 review by Margaret Westwater, Paul Fletcher and Hisham Ziauddeen, from the University of Cambridge, concluded that the evidence in humans does not support a picture of dependence comparable to that of psychoactive substances: binge type behaviors appear mostly in contexts of intermittent access to hyperpalatable food in animal models, and not as a direct pharmacological effect of sugar alone. Craving sweets is not synonymous with chemical dependence, at least with the evidence available to date.
How much is too much: the WHO number and sugar's place in training
The official recommendation of the World Health Organization, published in 2015, is clear: free sugars should stay below 10% of daily energy, with additional benefit, especially for caries, below 5%. In a reference 2000 kcal diet, that works out to less than 50 g per day at the 10% limit, and to about 25 g, around 6 teaspoons, at the 5% limit. A single 350 ml can of soda usually already carries between 35 and 40 g of sugar, practically the ceiling for the entire day.
None of this invalidates the role of sugar during exercise. In the intra-workout fueling of long races, combined glucose and fructose increases the intestinal carbohydrate absorption rate, because it uses two different intestinal transporters at the same time, allowing faster energy replenishment during prolonged effort. It is an occasional, functional use, very different from chronic consumption outside this context.
What remains
Going through sugar organ by organ undoes both extremes: panic and complacency. It is not the poison that ruins the body with every spoonful, nor the harmless fuel you can drink at will just because you train. The same sugar that is a legitimate tool in the gel of a three hour race becomes a risk factor when it shows up every day, in liquid form, far above what the body handles well. That is why the useful question was never "is sugar bad for you?", but "how much, how often, and coming from where?". Perhaps the most honest exercise is not policing the occasional dessert, which is almost never the villain, but noticing the sugar that goes unnoticed: the soda at lunch, the boxed juice, the sweetened coffee three times a day, the "fit" bar at snack time. It is in the chronic, invisible dose that the bill adds up, and it is there, not in the Sunday dessert, that a small change pays off more than any heroic prohibition.
