“Why Do Sugars Taste Good?”, Israel Ramirez1990-06 (biology)⁠:

The preference humans and animals show for sweet solutions has been the subject of hundreds of publications. Nevertheless, the evolutionary origin of sweet preference remains enigmatic because of the relatively low nutritional value of sugars and the absence of specific tastes for other, more essential, nutrients.

Moderate concentrations of sugars are found in most plant foods because sugars play an important role in plant physiology. Widespread occurrence of sugars in plants is paralleled by widespread preference for sugar solutions in mammals.

These observations suggest that preference for sugars evolved because they are common in plants and easy to detect rather than because of any special nutritional merits they offer. Perception of sweetness cannot be used to accurately meter the metabolizable energy or nutritive value of a food.

OF all the major energy sources (fats, proteins, starches, and sugars), sugar is the only one which has its own taste receptor system in humans. Starch is an equally important source of carbohydrate calories, yet starch is tasteless to humans. Similarly, there is no specific taste and no generalized preference for many essential minerals and vitamins. Many other species also act as if sugar solutions tasted good to them, although they may be able to taste other kinds of macronutrients as well⁸².

A Darwinian outlook would lead one to expect that innate preferences should enhance the reproductive success, well-being or survival of organisms. Yet sugars are the least essential of all nutrients. Sugars contain less than or no more energy per gram than starch, protein, and fat⁶⁷. Indeed, osmotic factors make sugars retain water, causing sweet foods to have even fewer calories per gram. Elevated osmotic pressure in the gastrointestinal tract after meals containing sugars⁴⁷, can be stressful³⁸. In the laboratory, ingestion of high-sugar diets has many adverse health effects in animals (impaired glucose tolerance, increased cholesterol, hypertriglyceridemia, hypertension, etc.)³³. Excessive appetite for sweets can result in protein malnutrition⁷¹ since the sweetest foods in nature (fruits, phloem, nectar, honey) are low in protein. There are no known advantages of sugars to offset these disadvantages.

…Objective criteria seem to suggest that human beings show relatively weak responses to sugars. Human beings do not recognize sucrose solutions as sweet unless the concentration is at least 0.2–0.5%; the recognition threshold for glucose may be as high as 1.25%²⁴. In contrast, several species (eg. dog, hamster, Mongolian gerbil, pig, etc.) reliably prefer solutions that are insipid to humans…Most species of mammals can apparently detect sugar levels of a few percent but this ability bears no obvious relationship to the species’ ecological niche. This pattern suggests that the ability to detect and the propensity to consume sugars is adaptive for mammals having a wide variety of feeding habits.