Carbohydrate

Carbohydrate ingestion causes blood glucose to increase. In people without diabetes, the normal increase in blood glucose is approximately 0.5-2.8 mmol/l (10-50 mg/dl) above baseline, returning to baseline within 1-3 h. The pancreatic hormonal response to dietary carbohydrate mediates the return to normal. Insulin is the central mediator of energy metabolism. The basics of insulin-dependent energy metabolism in the fed and the fasting states are depicted in Figure 2.

Although carbohydrate intake plays the major role in postprandial blood glucose, there are other factors to consider. The diet is not the only source of glucose in blood; hepatic gluconeogenesis maintains blood glucose in the absence of dietary intake. For example, when a person is ill and dietary intake is curtailed, it would be a mistake to stop insulin administration since hepatic glucose production may in fact be increased. Sick-day instruction is essential for people with diabetes so that they do not simply stop their treatment if they are not eating well. Pharmacologic therapies (insulin or oral agents), of course, also affect blood glucose.

A long-standing debate has surrounded the optimal proportion of intake from carbohydrate, fat, and protein. People with diabetes, especially when insulin is administered, will discover that if they hold back carbohydrate their blood glucose does not increase as much. Holding back carbohydrate, however, unless the diet is hypocaloric, inevitably leads to a high-fat diet, and carbohydrate restriction leaves insulin with no substrate to act on. In our experience, this can cause blood glucose levels to be more unstable, susceptible to swings of hypoglyce-mia and hyperglycemia. We support the recommendation of most professional guidelines that carbohydrate should make up a substantial percentage (50-60%) of total nutrient intake.

Two areas of controversy and of nutrition research deserve special attention: the glycemic response to oral sucrose (concentrated sweets) versus complex carbohydrates and the so-called 'glycemic index.'

Sucrose versus complex carbohydrate Careful metabolic studies suggest that, gram for gram, sucrose does not increase blood glucose more than complex carbohydrates, either acutely or over a matter of weeks. In these studies, sucrose was isoenergetically substituted for other carbohydrates, mostly under carefully defined research ward conditions in which precise substitutions can be made. Since complex carbohydrates and sucrose are both digested to monosac-charides before they are absorbed, it is not unexpected that each should cause the same glycemic excursion if administered in the same number of grams. It does run counter, however, to the traditional advice that people with diabetes should avoid concentrated sweets.

A number of organizations have cited these research studies in support of a recommendation that allows ingestion of concentrated sweets. The caveat, in the words of the American Diabetes Association, is that ''sucrose should be substituted for other carbohydrate sources in the food/meal plan.'' In our view, there is a practical fallacy in this recommendation: People are unlikely to substitute sucrose for complex carbohydrates in equal amounts. Due simply to taste, concentrated sweets are likely to be taken in far greater quantity than the more filling and less sweet starches. Thus, in reality, people who routinely eat concentrated sweets are likely to have greater and less predictable glycemic excursions than those who stick to complex carbohydrates. There is also the significant risk that excess concentrated sweet intake will cause weight gain (as well as dental caries). However, if a person with diabetes can include a fixed amount of concentrated sweet in his or her diet and can demonstrate that his or her diabetes is well controlled and the postmeal glycemia is not excessive, there is no reason to deny the person the sweet.

Glycemic index The glycemic index (GI) is defined as the area under the 2-h curve of blood glucose after the ingestion of a set amount of carbohydrate compared to ingestion of the same amount of carbohydrate from a reference food (white bread or glucose). The GI is expressed as a percentage of the standard food value:

Area under the

Glycemic index . curve °f test food x 100 Area under the curve of standard food

The glycemic load (GL) is an additional measure in which the amount of carbohydrate in a typical

(A)

Figure 2 Influence of insulin on basic energy metabolism. (A) With dietary carbohydrate intake, hyperglycemia induces insulin secretion that acts to enhance glucose entry into cells for utilization as metabolic fuel. Simultaneously, insulin decreases new glucose production in the liver, since dietary glucose is already available, and stores excess caloric intake in adipose tissue as fat. (B) With lack of dietary carbohydrate, as in fasting, the reverse occurs: With lower blood glucose, insulin secretion is suppressed. This minimizes entry of glucose into cells but stimulates enough new glucose production from the liver to provide for obligate glucose using tissues such as the brain. Meanwhile, low insulin concentration promotes fatty acid release from adipose tissue to serve as an alternate fuel for metabolism.

Figure 2 Influence of insulin on basic energy metabolism. (A) With dietary carbohydrate intake, hyperglycemia induces insulin secretion that acts to enhance glucose entry into cells for utilization as metabolic fuel. Simultaneously, insulin decreases new glucose production in the liver, since dietary glucose is already available, and stores excess caloric intake in adipose tissue as fat. (B) With lack of dietary carbohydrate, as in fasting, the reverse occurs: With lower blood glucose, insulin secretion is suppressed. This minimizes entry of glucose into cells but stimulates enough new glucose production from the liver to provide for obligate glucose using tissues such as the brain. Meanwhile, low insulin concentration promotes fatty acid release from adipose tissue to serve as an alternate fuel for metabolism.

portion is taken into account. Table 5 provides examples of foods high and low in GI and GL. These indices have been calculated for more than 500 different carbohydrates, and values are readily available on the Internet. A number of factors in addition to the reported GI and GL actually affect the blood glucose response to meals, however, because mixed meals are ingested in everyday living. Among these are the fat and fiber content of the meal, type of cooking, the patient's absorptive rate, and micronutrient content.

In our opinion, the concept of the GI is valid in a research sense: Certain carbohydrates, gram for gram, do raise blood glucose levels more, or with different glycemic patterns, than others. However, we believe that basing nutrition plans on the GI and the GL of foods is usually too much of a burden for people with diabetes, who have to closely monitor

Table 5 Examples of foods high and low in glycemic index (GI) and glycemic load (GL)a

GI Serving GL

Low GI/low GL

Apple, NS (USA) Oranges (Sunkist, USA) Healthy Choice hearty 7-grain bread (USA) Ice cream, premium, French

Vanilla—16% fat (Australia) Kidney beans (USA) Pizza, Super Supreme, thin and crispy—13.2% fat, Pizza Hut (Australia) Low GI/high GL Barley (Hordeum vulgare) (India)

High GI/low GL

Watermelon, raw (Australia) White wheat flour bread

High GI/high GL

Cornflakes (Kellogg's, USA) Bagel, white, frozen (Lenders,

Canada) White rice, type NS, boiled 13 minutes (Italy)

aHigh GI is considered >70 and low <55. High GL is considered >20 and low <10.

Adapted from Foster-Powell K, Holt SH, and Brand-Miller JC (2002) International table of glycemic index and glycemic load values. American Journal of Clinical Nutrition 76(1): 5-56.

the total amount of carbohydrates. It is more practical to encourage people to learn their own glyce-mic response to different foods from experience. They may learn, for example, that far more insulin is needed before eating pizza or a bagel; they may learn to avoid certain deserts. A general awareness of what preferred foods, in what amounts, raise blood glucose may be more practical than memorizing GI or GL.

Protein

Since the classic experiments by Benedict in the 1910s, it has been known that protein ingestion

Table 6 Nonnutritive sweeteners causes hyperglycemia and glucosuria. The effect of protein ingestion on blood glucose, however, is far less pronounced than the effect of carbohydrate ingestion. A rule of thumb is that a gram of protein raises blood glucose approximately one-third as much as a gram of carbohydrate. In most diets, approximately 50-100 g protein is ingested per day, compared to approximately 200-300 g carbohydrate. Therefore, protein is a calorically less significant part of the diet and far less important in regulating blood glucose. In people with type 2 diabetes, protein does not slow the postprandial absorption of carbohydrate. The same cannot be said about dietary fat.

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