Sugar: The Bitter Truth

Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology, explores the damage caused by sugary foods. He argues that fructose (too much) and fiber (not enough) appear to be cornerstones of the obesity epidemic through their effects on insulin.

This is one of the most popular lectures on YouTube and has been watched approximately 1.5 million times. You can view the complete video right here:


Fructose is a simple sugar (monosaccharide) found in many foods and is one of the three important dietary monosaccharides along with glucose and galactose. Honey, tree fruits, berries, melons, and some root vegetables, such as beets, sweet potatoes, parsnips, and onions, contain fructose, usually in combination with glucose in the form of sucrose. Fructose is also derived from the digestion of granulated table sugar (sucrose), a disaccharide consisting of glucose and fructose, and high-fructose corn syrup (HFCS).

The primary food sources of fructose are fruits, vegetables, and honey. Fructose exists in foods either as a free monosaccharide or bound to glucose as the disaccharide, sucrose. Fructose, glucose, and sucrose can all be present in a food; however, different foods will have varying levels of each of these three sugars.

In general, foods that contain free fructose have equal amount of free glucose. Some fruits have larger proportions of fructose to glucose compared to others. For example, apples and pears contain more than twice as much free fructose as glucose, while apricots contain less than a half of fructose than glucose.


Sucrose (common name: table sugar, also called saccharose) is a disaccharide of glucose and fructose.

Sucrose is the most common food sweetener, although it has been replaced in American industrial food production by other sweeteners such as fructose syrups or combinations of functional ingredients and high intensity sweeteners. This is due to the subsidization of corn in the United States, which has led to a vast surplus. Combined with sugar tariffs, this has driven the price of corn syrup far below that of sugar.

Sucrose is ubiquitous in food preparations due to both its sweetness and its functional properties; it is important to the structure of many foods including biscuits and cookies, cakes and pies, candy canes, ice cream and sorbets, and also assists in the preservation of foods. As such it is common in many processed and so-called “junk foods.” Sucrose is an easily assimilated macronutrient that provides a quick source of energy to the body, provoking a rapid rise in blood glucose upon ingestion.

Overconsumption of sucrose has been linked with some adverse health effects. The most common is dental caries or tooth decay, in which oral bacteria convert sugars (including sucrose) from food into acids that attack tooth enamel. Sucrose, as a pure carbohydrate, has an energy content of 3.94 kilocalories per gram (or 17 kilojoules per gram). When a large amount of foods that contain a high percentage of sucrose is consumed, beneficial nutrients can be displaced from the diet, which can contribute to an increased risk for chronic disease. It has been suggested that sucrose-containing drinks may be linked to the development of obesity and insulin resistance. Although most soft drinks in the USA are now made with high fructose corn syrup, not sucrose, this makes little functional difference, since high fructose corn syrup contains fructose and glucose in a similar ratio to that produced metabolically from sucrose.

The rapidity with which sucrose raises blood glucose can cause problems for people suffering from defects in glucose metabolism, such as persons with hypoglycemia or diabetes mellitus. Sucrose can contribute to development of the metabolic syndrome. In an experiment with rats that were fed a diet one-third of which was sucrose, the sucrose first elevated blood levels of triglycerides, which induced visceral fat and ultimately resulted in insulin resistance. Another study found that rats fed sucrose-rich diets developed high triglycerides, hyperglycemia, and insulin resistance.

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