Carbohydrate foods describe those with digestible starch. Bread, pasta, potato and rice are all carbohydrate foods and all contain high amounts of starch. Carbohydrates also include simple sugars such as lactose, glucose and fructose. Carbohydrate foods are interesting nutritionally because they can cause quite different physiological responses postprandially. The category and type of carbohydrate food can have quite different effects on hormone levels and blood sugar following consumption, and this in turn can have implications for health. In particular the rate that they are digested and absorbed can influence the path to disease, with those carbohydrate foods eliciting low insulin and blood sugar responses, generally, but not alway, being considered superior. While it might be thought that simple sugars are more readily digested and absorbed compared to starches, due to their structural simplicity, this is not the case. Large differences also exist between carbohydrate foods of similar starch content.
Comparisons of the hormonal and blood glucose responses to various carbohydrate foods have been performed in various studies. For example, in one study1, the plasma glucose, glucagon and insulin responses to glucose, rise, potato, corn and bread were investigated in human volunteers with glucose intolerance. Each food was administered in a fasting state as a portion that would provide 50 grams of glucose. The data showed that potato and glucose elicited the largest rise in blood glucose levels and were similar in their magnitude. In contrast, rice and bread elicited a much smaller increase in blood sugar, while corn elicited an increase somewhere in between the extremes. In terms of insulin response, glucose and potato elicited the greatest response, with rice the lowest response, and bread and corn a response of intermediate magnitude. As expected all of the carbohydrate foods suppressed glucagon levels, glucagon being the antagonistic hormone to insulin and having opposing physiological effects.
Studies such as this show that predicting the glycaemic response to carbohydrate foods is difficult. It is also not possible to predict the health consequences of foods based solely on their glycaemic responses. The carbohydrate foods in this study were fed at an amount equivalent to 50 grams of glucose, and this is a large intake of carbohydrate in a single serving. In fact many vegetables such as parsnips and potatoes elicit very large insulin and blood glucose responses. However, the total amount of starch that is present in most vegetables is low relative to the amount that we eat, and this is often forgotten when discussing the glycaemic index. Therefore 50 grams of parsnips may elicit a large blood glucose and insulin response, but in reality, we do not consume 50 grams of parsnips. Further, fructose also has very minimal effects on either blood glucose or plasma insulin levels and yet is a metabolic poison with similar effects to alcohol, when it its refined crystalline state.
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