Assessing the glycemic impact of starch by in vitro methods

Humans need a dietary source of glucose, which is the preferred substrate for generating metabolic energy in the brain, red blood cells, kidney and reproductive tissues. Starch is the main source of dietary glucose for most humans. The ability to assess the glycemic (i.e., blood glucose-raising) potential of starchy foods is highly relevant to nutrition and health, as the inability to maintain the blood glucose concentration within normal limits is a risk for diet-related illnesses and obesity. In vivo tests assess the glycemic impact of starchy foods in terms of a glycemic index (GI). However, GI testing has inherent limitations and in vitro model systems are used widely as a proxy for assessing glycemic impact; these in vitro methods are important for assessing starch digestibility and the quality of carbohydrates in many applications. This review will focus on in vitro methods for measuring starch digestibility. Starch as the main glycemic carbohydrate in the human diet, how starch is broken down enzymically, and how GI values are determined are first considered briefly, before discussing methods and advantages of in vitro testing. The limitations, which as the literature shows are not always appreciated and can lead to overinterpretation of research data, are also considered. The importance of in vitro methodology for assessing the carbohydrate quality of foods is clear, but there is a need to standardise procedures to facilitate comparability of studies conducted at different times and in different laboratories.