Many experimental studies on sugar (sucrose) omitted its form of ingestion. Often their findings were mutually
incompatible. A comparison of the results of the few studies that administered sugar in a single specified form suggests that the metabolic effects of sugar depend on its form of ingestion, because even 80% of calories as diluted sugar proved harmless, but only 30% of calories as undiluted sugar proved harmful. These opposite effects of sugar can be explained by the published hypothesis that evolution adapted genetically our ancestors to cope with sugar only in diluted forms, because prehistorically diluted sugar was available abundantly in fresh fruits, but undiluted sugar was inexistent. The purpose of this review, based mainly on the evolutionary interpretation of published data of physiology, is to encourage researchers to perform an unprecedented experimental study to compare the metabolic effects of diluted sugar with the effects of undiluted sugar. The data of physiology analyzed in this review suggest that the absorption of diluted sugar within the caloric range of total sugars diluted in fresh fruits is slow and calorie-constant, thus preserving blood glucose homeostasis, whereas the absorption of concentrated sugar exceeding that caloric range is rapid, which can disrupt blood glucose homeostasis. Dietary salt, which was unknown to prehistoric humans, unnaturally accelerates the absorption of sugars. This can explain the harmful effects attributed to sugar-sweetened beverages per se, because these drinks are generally co-ingested with foods containing salt, which partly yet unavoidably passes into those beverages, thereby unhealthily accelerating their absorption.