Nutritional Requirements

Dietary Reference Intakes

The dietary reference intake (DRI) established by the Food and Nutrition Board of the U.S. Institutes of Medicine (IOM) provides guidance on the nutrient needs for individuals and groups across different life stages and by gender (see Tables 55.1 to 55.4 ).

Key concepts on DRI concepts include the estimated average requirement (EAR) , the recommended dietary allowance (RDA), and the tolerable upper limit of intake (UL) ( Fig. 55.1 ). The EAR is the average level of daily nutrient intake that is estimated to meet the requirements for 50% of the population, assuming normal distribution. The RDA is an estimate of the daily average nutrient intake that meets the nutritional needs of >97% of the individuals in a population, and it can be used as a guideline for individuals to avoid deficiency. When an EAR cannot be derived, an RDA cannot be calculated; therefore, an adequate intake (AI) is developed as a guideline for individuals based on the best available data and scientific consensus. The UL denotes the highest average daily intake with no associated adverse health effects for almost all individuals in a particular group. Fig. 55.2 shows the relationships among EAR, RDA, and UL.

Energy

Energy includes both food intake and metabolic expenditure. Deficits and excesses of energy intake yield undesirable health consequences. Inadequate energy intake can lead to growth faltering, catabolism of body tissues, and inability to provide adequate energy substrate. Excess energy intakes can increase the risk for obesity. Adequacy of energy intake in adults is associated with maintenance of a healthy weight. The three components of energy expenditure in adults are the basal metabolic rate (BMR), thermal effect of food (e.g., energy required for digestion and absorption), and energy for physical activity. In children, additional energy intake is required to support growth and development.

Estimated energy requirement (EER) is the average dietary energy intake predicted to maintain energy balance in a healthy individual and takes into account age, gender, weight, stature, and level of physical activity ( Table 55.1 ). The 2015–2020 Dietary Guidelines for Americans refer to the 2008 Physical Activity Guidelines for Americans. These guidelines recommend ≥60 min of moderate- or vigorous-intensity aerobic physical daily for children and adolescents. This activity should include vigorous intensity physical activity at least 3 days per week. In addition, as part of their ≥60 min of daily physical activity, children and adolescents are advised to incorporate muscle- and bone-strengthening activity for ≥3 days a week, to maintain a healthy weight and to prevent or delay progression of chronic noncommunicable diseases such as obesity and CV disease.

The EER was determined based on empirical research in healthy persons at different levels of physical activity, including levels different from recommended levels. They do not necessarily apply to children with acute or chronic diseases. EER is estimated by equations that account for total energy expenditure (TEE) and energy deposition (ED) for healthy growth. EERs for infants, relative to body weight, are approximately twice those for adults because of the increased metabolic rate and requirements for weight maintenance and tissue accretion (growth).

Dietary nutrients that provide energy include fats (approximately 9 kcal/g), carbohydrates (4 kcal/g), and protein (4 kcal/g). These nutrients are called macronutrients . If alcohol is consumed, it also contributes to energy intake (7 kcal/g). The EER does not specify the relative energy contributions of macronutrients. Once the minimal intake of each macronutrient is attained (e.g., sufficient protein intake to meet specific amino acid requirements, sufficient fat intake to meet linoleic acid and α-linolenic acid needs for brain development), the remainder of the intake is used to meet energy requirements, with some degree of freedom and interchangeability among fat, carbohydrate, and protein. This argument forms the basis for the acceptable macronutrient distribution ranges (AMDRs) , expressed as a function of total energy intake ( Table 55.2 ).

Fat

Fat is the most calorically dense macronutrient, providing approximately 9 kcal/g. For infants, human milk and formula are the main dietary sources of fat, whereas older children obtain fat from animal products, vegetable oils, and margarine. The AMDR for fats is 30–40% of total energy intake for children 1-3 yr and 25–35% for children 4-18 yr of age. In addition to being energy dense, fats provide essential fatty acids that have body structural and functional roles (e.g., cholesterol moieties are precursors for cell membranes, hormones, and bile acids). Fat intake facilitates absorption of fat-soluble vitamins (vitamins A, D, E, and K). Both roles are relevant to neurologic and ocular development ( Table 55.3 ).

Triglycerides are the most common form of dietary fat and are composed of 1 glycerol molecule with 3 fatty acids. Triglycerides are found in animal and vegetable fats. Simple sugars (i.e., refined grains and high sugar drinks) are converted to triglycerides in the liver. Elevated serum triglycerides are a risk factor for CV disease and metabolic syndrome. Decreasing simple sugars and increasing complex carbohydrate intake reduces serum triglyceride levels.

Dietary saturated fatty acids (found primarily in animal fat and dairy products), trans fats (found in hydrogenated margarines and oils), and cholesterol increase the low-density lipoprotein (LDL) fraction of serum cholesterol, which is a risk factor for the development of atherosclerosis ( Fig. 55.3 ). Autopsy studies demonstrate that atherosclerosis begins early in childhood, even in infancy. Therefore, dietary advice to optimize CV health should be given starting from age 2 yr, when sufficient fat intake to sustain growth and brain development is less of a concern.

Because saturated and monounsaturated fats can be synthesized endogenously to support adequate structural and physiologic requirements, there is no AI or RDA set for these dietary components. Trans fats, a by-product of the hydrogenation of vegetables oils to form margarine, have no known health benefits in humans. Trans fats do not have an AI or RDA defined. In fact, trans fats behave like saturated fats. An UL has not been set for cholesterol, saturated, or trans fats because there is a continuous positive linear association between intake of these fats and increased risk for CV disease, without a threshold level. Diets low in saturated fats and cholesterol without trans fats are therefore preferred.

Efforts continue to reduce or eliminate trans fats from the diet. For optimal CV health in the general population, rather than limiting fat intake, advice should focus in most cases on changing the type of fat consumed. With respect to preventing obesity, all types of fatty acids have the same energy content and can contribute to increasing the risk for obesity. The current 2015–2020 Dietary Guidelines for Americans no longer restrict how much energy should come from fat intake, but continue to recommend that <10% of total daily calories come from saturated fat, with no trans fat intake. Furthermore, these guidelines do not specify limits on dietary cholesterol intake, because there is no clear strong evidence of the relationship between dietary and blood cholesterol.

Humans are incapable of synthesizing the precursor omega (ω) 3 (α-linolenic acid; ALA) and ω6 (linoleic acid; LA) PUFAs and depend on diet for these 2 essential fatty acids (EFAs). Safflower and sunflower oil are good sources of linoleic acid. Walnut and flaxseed oil are good sources of ALA. Essential fatty acid deficiency with LA is associated with desquamating skin rashes, alopecia, thrombocytopenia, impaired immunity, and growth deficits but is rare in the general population. EFAs are enzymatically elongated and desaturated into longer-chain fatty acids; ALA can be converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) ω3 PUFAs. LA is converted to arachidonic acid (ARA). Long-chain PUFAs such as DHA and ARA have a variety of cellular structural and functional roles; they influence membrane fluidity and function in gene expression and modulate the inflammatory response. ARA and DHA are present in breast milk, often supplemented in infant formulas, and are required for normal growth and development. DHA is present in the retina and is involved in the visual evoked response in infants.

The conversion of ALA to EPA and DHA and of LA to ARA is influenced by many factors, including type and amounts of dietary fats, and by enzymatic substrate affinity among competing ω3, ω6, ω9, saturated, and trans fatty acids. Approximately 0.5% of dietary ALA is converted to DHA, and 5% of ALA intake is converted to EPA; therefore, dietary intake of longer-chain PUFAs is an important determinant of serum and tissue long-chain PUFA status. The biologic activity and health benefits of ALA are thought to be derived from the longer-chain PUFA products EPA and DHA. Consistent with these findings of limited conversion of ALA to EPA and DHA, and that EPA and DHA appear to confer the biologic role and health benefits, the dietary reference intake (DRI) stipulates that up to 10% of the AI for ω3 PUFA (ALA being the major dietary constituent) can be replaced by DHA and EPA to support normal neural development and growth.

The ratio of dietary intake of each type of PUFA influences their relative amounts in different tissue compartments. A dietary ω6:ω3 PUFA ratio of 4-5 : 1 may be beneficial in reducing risk of disease and may be associated with improved health outcomes, compared with the current 15-30 : 1 ratio observed in U.S. diets.

Protein

Protein and amino acids have structural and functional roles in every cell in the body. Dietary protein intake is required to replenish the turnover of protein and to meet amino acid needs for growth. Dietary protein also provides approximately 4 kcal/g as an energy substrate when intake is in excess of needs, or during periods of catabolism. Inadequate energy intake or inadequate protein intake increases catabolism of body protein reservoirs (i.e., lean body mass) for energy and free amino acids required to support normal physiologic function. Nitrogen from protein turnover is excreted in urine, stool, and other bodily excretions. Increased protein intake may be required for rare hypermetabolic states, such as extensive burn injury. Protein-energy malnutrition, although relatively rare in the noninstitutionalized U.S. population, is more common in the developing world. Protein-energy malnutrition impairs brain, immune system, and intestinal mucosal functions (see Chapter 59 ).

The DRI for protein is provided in Table 55.3 . According to the 2015–2020 Dietary Guidelines for Americans, the average intake of protein from poultry, meat, eggs, nuts, seeds, and soy products are close to the recommended amounts for all ages. Protein intake is higher than recommended amounts in adolescent males (mostly from meats, poultry, and eggs). Intake of seafood protein is low across all age and sex groups. An UL for protein has not been set. Some athletes may have increased protein needs, of approximately 2 g/kg/day, to prevent loss of fat-free mass or lean body mass. Certain conditions may require a modest increase in protein intake, including conditions with high protein turnover, inflammatory conditions, or postsurgical states, as well as with cystic fibrosis, critical illnesses, burn injuries, compensated liver disease, and bariatric surgery (e.g., laparoscopic sleeve gastrectomy and Roux-en- Y gastric bypass). Intake of protein or specific amino acids needs to be limited in some health conditions, such as renal disease and decompensated liver disease, and metabolic diseases such as phenylketonuria and maple syrup urine disease, in which specific amino acids can be toxic.

The amino acid content of dietary protein is also important. Certain amino acids are indispensable , and humans depend on dietary sources to meet adequacy and prevent deficiency. Certain amino acids are termed conditional essential/indispensable , meaning they become essential in patients affected by some diseases or during a certain life stage, such as with cysteine, tyrosine, and arginine in newborns because of enzyme immaturity ( Table 55.4 ). Human milk contains both the indispensable and conditionally indispensable amino acids and therefore meets the protein requirements for infants. Breast milk is considered the optimal protein source for infants and is the reference amino acid composition by which biologic quality is determined for infants. If a single amino acid in a food protein source is low or absent but is required to support normal metabolism, that specific amino acid becomes the limiting nutrient in that food. For soy-based infant formula, supplementing the formula with the limiting amino acid (methionine) is necessary. Certain amino acid–like substances, such as creatinine, are used by some athletes and may enhance performance. Such supplementation should be monitored for potential side effects.

To ensure appropriate growth and to promote satiety, children should consume the recommended amount of protein. Specific recommendations for appropriate dietary protein sources to meet indispensable amino acid requirements are available for groups adopting specific diets, such as vegetarians and vegans. Inclusion of legumes and corn, as well as the use of a variety of food sources to provide all of the required amino acids is a strategy advocated for vegetarians and vegans (see Chapter 56 ).