BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.8. Recommended Nutrient Intake and Reference Values

8.8.3. The Influence of Growth, Sex, and Activity on Dietary Reference Values (DRVs)

Nutritional requirements vary depending on age, sex, and physical activity. Tables 8.8 and 8.9 set out the DRVs for males and females across various age groups. These figures are based on the average body mass for each group and apply to healthy individuals.

Energy

(Table 8.9)

Energy is required to sustain the basal metabolic rate (BMR), that is, to maintain bodily Functions at rest. Since the basal metabolic rate accounts for the major portion of total Energy Expenditure, it must be estimated with a high degree of accuracy. For a man with a body mass of 65 kg, the energy required for basal METABOLISM is 7.56 MJ/day; for a woman with a mass of 55 kg, this value is 5.98 MJ/day.

Any movement incurs additional energy expenditure because movement involves muscular activity, and Muscle contraction is an energy-dependent process. The more active a person is, the higher their Energy Requirements. It is convenient to express the energy required for a specific activity as a multiple of the basal metabolic rate. This factor is known as the physical activity ratio (PAR). For example, the PAR for walking is 4, which means that walking requires 4 times as much energy as maintaining basal metabolism.

Physical activity depends on whether a person is working or resting. Occupational activity is traditionally classified as follows (using the average PAR for the entire working day):

1) sedentary or inactive lifestyle: PAR = 1.7;

2) moderate activity: PAR = 2.2 for women and 2.7 for men;

3) high activity: PAR = 2.3+ for women and 3.0+ for men.

Nowadays, leisure-time activity is just as important as work-related activity and must be taken into account when calculating total energy expenditure. It is generally useful to categorize leisure activity as sedentary (PAR 2), moderately active (PAR 3), and highly active (PAR 4).

Men have a proportionally higher muscle-to-fat ratio and less body fat than women. On average, men weigh more, and maintaining, supporting, and moving a heavier body requires more energy. Consequently, the average energy requirements for men are higher (Table 8.8). During the first six months of Pregnancy, a reduction in physical activity and metabolic rate in women helps compensate for the extra energy needed for fetal growth and the accumulation of fat reserves required for Lactation and milk production. Additional energy intake only becomes necessary during the final three months of pregnancy (Table 8.8). This extra energy fuels fetal growth and builds up the mother's fat reserves (about 2 kg) to support lactation. Breastfeeding requires an excess of energy because breast milk serves as the sole energy source for the infant, who continues to grow rapidly after birth.

Protein

Table 8.9 presents the reference protein intake values, which take AGE AND SEX into account. Compared to adults, the reference protein intake values for adolescents and children are relatively higher per unit of body mass because growing bodies require protein for development and more intensive metabolic processes. For all adults (over 19 years of age), the reference protein intake relative to body mass is uniform.

In practice, actual protein intake among adults in the United Kingdom exceeds the reference values. Men consume an average of 84 g of protein per day, and women consume 64 g.

The higher average muscle-to-fat ratio and greater average body mass in men compared to women mean that males from the age of 11 onwards require a higher protein intake (Table 8.9).

Increased physical activity does not necessarily require a higher protein intake, as protein is rarely used as an energy source. However, if excess body mass is the result of muscle hypertrophy, a corresponding increase in protein intake is entirely justified.

Pregnancy demands an increased intake of protein (Table 8.9) to support the growth of the fetus and maternal Tissues, such as the Uterus, Placenta, and Mammary Glands, as well as the expansion of the Circulatory system. Breast milk contains the protein necessary for infant growth, meaning that nursing mothers also require a higher protein intake.

Minerals

During pregnancy and lactation, special attention must be paid to the mother's diet, ensuring an adequate supply of iron, calcium, Vitamins A, D, and Folic acid (Table 8.9) for The formation of Hemoglobin, bones, Teeth, and Muscles. A well-balanced diet should fully meet the requirements for these nutrients. Pregnant women are sometimes prescribed folic acid supplements to reduce the risk of cleft lip in the infant.