Some foods containing carbohydrates cause a rapid increase in blood glucose concentration, followed by a sharp decrease. Others result in a gradual rise and slow decline. This difference in glycemic response (GR) is influenced by the presence of fiber, which reduces the GR. The glycemic index (GI) is a scale from 0 to 100 that ranks carbohydrate-rich foods based on their GR compared to the GR caused by the same amount of carbohydrate in the form of white bread or glucose. A low GI is less than 55, while a high GI is 70 or higher. Studies indicate that a low-GI diet helps regulate blood glucose levels in people with diabetes. Low-GI foods also promote a longer-lasting feeling of fullness and can aid in controlling calorie intake. The glycemic load (GL) refers to how much a typical serving of a food raises blood glucose levels. It is important to note that a food can have a high GI but a low GL.

Carbohydrates are not essential nutrients since the carbon skeletons of most amino acids can be converted into glucose. However, a lack of dietary carbohydrates can lead to ketogenesis and the breakdown of body protein, which provides carbon skeletons for gluconeogenesis. The recommended daily allowance (RDA) for carbohydrates is set at 130 grams for adults and children based on the glucose requirements of carbohydrate-dependent tissues like the brain and erythrocytes. However, most people consume more than this amount. It is recommended that adults obtain 45% to 65% of their total calorie intake from carbohydrates. To promote a balanced diet, added sugars should not exceed 10% of total energy intake to avoid displacing nutrient-rich foods. Added sugars have been associated with weight gain and type 2 diabetes.

There is no direct evidence to suggest that naturally occurring simple sugars in food are harmful. Diets high in sucrose do not cause diabetes or hypoglycemia, despite common beliefs. Carbohydrates themselves are not inherently fattening, as they provide 4 kcal/g (the same as protein and less than half that of fat). Fat synthesis only occurs when carbohydrates are consumed in excess of the body's energy needs. However, consuming sucrose is linked to dental caries, especially without fluoride treatment.

The acceptable macronutrient distribution range (AMDR) for protein is 10% to 35%. Protein in the diet provides essential amino acids necessary for tissue maintenance. Out of the 20 amino acids needed for body protein synthesis, nine are essential and cannot be synthesized by humans.

The quality of dietary protein is determined by its ability to supply the essential amino acids required for tissue maintenance. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is the standard method used by government agencies to evaluate protein quality. PDCAAS takes into account the profile of essential amino acids and adjusts for protein digestibility. The highest possible PDCAAS score is 1.00, allowing for the balancing of intakes of lower-quality proteins with higher-quality dietary proteins.
Some carbohydrate-containing foods produce a rapid rise followed by a steep fall in blood glucose concentration, whereas others result in a gradual rise followed by a slow decline (Fig. 27.18). Thus, they differ in their glycemic response (GR). [Note: Fiber blunts the GR.] The glycemic index (GI) ranks carbohydrate-rich foods on a scale of 0–100 based on the GR they cause relative to the GR caused by the same amount (50 g) of carbohydrate eaten in the form of white bread or glucose. A low GI is <55, whereas a high GI is ≥70. Evidence suggests that a low-GI diet improves glycemic control in diabetic individuals. Food with a low GI tends to create a sense of satiety over a longer period of time and may be helpful in limiting caloric intake. [Note: How much a typical serving size of a food raises blood glucose is referred to as the glycemic load (GL). A food (for example, carrots) can have a high GI and a low GL.]
C. Carbohydrate requirements
Carbohydrates are not essential nutrients, because the carbon skeletons of most amino acids can be converted into glucose (see p. 261). However, the absence of dietary carbohydrate leads to ketogenesis (see p. 195) and degradation of body protein whose constituent amino acids provide carbon skeletons for gluconeogenesis (see p. 118). The RDA for carbohydrate is set at 130 g/day for adults and children, based on the amount of glucose used by carbohydrate-dependent tissues, such as the brain and erythrocytes. However, this level of intake is usually exceeded. Adults should consume 45%–65% of their total calories from carbohydrates. It is now recommended that added sugars represent no more than 10% of total energy intake because of concerns that they may displace nutrient-rich foods from the diet. [Note: Added sugars are associated with increased body weight and type 2 diabetes.]
D. Simple sugars and disease
There is no direct evidence that the consumption of simple sugars naturally present in food is harmful. Contrary to folklore, diets high in sucrose do not lead to diabetes or hypoglycemia. Also contrary to popular belief, carbohydrates are not inherently fattening. They yield 4 kcal/g (the same as protein and less than one half that of fat; see Fig. 27.5) and result in fat synthesis only when consumed in excess of the body’s energy needs. However, there is an association between sucrose consumption and dental caries, particularly in the absence of fluoride treatment (see p. 405).
VII. DIETARY PROTEIN
The AMDR for protein is 10%–35%. Dietary protein provides the essential amino acids (see Fig. 20.2, p. 262). Nine of the 20 amino acids needed for the synthesis of body proteins are essential (that is, they cannot be synthesized in humans).
A. Protein quality
The quality of a dietary protein is a measure of its ability to provide the essential amino acids (EAA) required for tissue maintenance. Most government agencies have adopted the Protein Digestibility–Corrected Amino Acid Score (PDCAAS) as the standard by which to evaluate protein quality. PDCAAS is based on the profile of EAA after correcting for the digestibility of the protein. The highest possible score under these guidelines is 1.00. This amino acid score provides a method to balance intakes of poorer-quality proteins with high-quality dietary proteins.
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