Tens of dietary trials have been conducted to investigate the metabolic effects of sugar (sucrose) and its impact on human health. All of those studies took into account only the quantity of ingested sugar. By contrast, not a single study attempted to assess whether the form in which sugar is consumed plays a role in producing its metabolic effects. The failure of cohorts of researchers to specify how they administered sugar in their dietary trials may well explain why the results of those studies are extremely contradictory. These discrepant findings, understandably, resulted in conflicting opinions about sugar and in divergent guidelines about its recommended consumption. The evolutionary line of reasoning expounded in this article leads to conclude that the form in which sugar is ingested, not its quantity, constitutes the most important factor in producing the metabolic effects of sugar and its impact on human health. As a consequence, for example, the consumption of 100 g of sugar per day can be either detrimental or innocuous, depending on the form in which sugar is ingested. Specifically, the evolutionary hypothesis advanced in this paper implies that sugar can predispose to type 2 diabetes and can cause unhealthy changes in blood lipids if it is consumed in solid forms or in dense solutions containing more than 250 g/L, whereas sugar is harmless if it is consumed in more dilute concentrations. This evolutionary hypothesis, in view of its intuitively far-reaching clinical implications, should be tested by at least one dietary trial.