Module 4. Human nutrition

Lesson 25


25.1 Fundamentals of Nutrition

The science of nutrition deals with the processes by which components of food are made available to an organism for meeting energy requirements, for building and maintaining tissues, and, in a more general sense, for maintaining the organism in optimal functional health. Thus nutrition is concerned with many issues traditionally considered to be digestion, absorption, transport, metabolism, and biochemical functions performed by individual chemical substances.
  • Nutrients are those chemical substances needed for growth and maintenance of normal cells, both in animals and plants. Clinical nutrition is a medical specialty dealing with the relationship between disease and nutrition. Acute and chronic illnesses are caused by deficiencies of dietary components and others by their excesses.
  • Malnutrition is a condition characterized by inappropriate quality, quantity, digestion, absorption or utilization of ingested nutrients. It includes: undernutrition – low food intake (calorie deficiency) leading to growth suppression or other deficiency signs, and overnutrition to consume too much food and/or single nutrients leading to specific toxicities.
  • Some 45-50 chemical entities are now known to be required by humans, either preformed in food or added as an appropriate chemical substitute. These can be divided into six main categories: carbohydrates, fats, proteins, vitamins, inorganic elements, and are important in maintaining good health. The term essential or dietary essential means that we must obtain the nutrient from our diet either because we lack the biochemical machinery to manufacture it or we cannot make enough of it.
25.2 Recommended Dietary Allowances (RDA)
  • RDA are developed by the Food and Nutrition Board of the National Academy of Sciences.
  • RDAs are defined as the “levels of intake of essential nutrients considered, in the judgment of Committee of Dietary Allowances of the Food and Nutrition Board, on the basis of available scientific knowledge to be adequate to meet the known nutritional needs of practically all healthy persons.”
  • The RDAs are meant to apply only to a healthy population and should be met from the consumption of a wide variety of readily available foods.
  • RDAs should not be confused with nutrient requirements of individuals because these are too variable. Rather, an RDA represents an average level of daily intake of a nutrient which over time approximates the RDA, and thus the nutritional inadequacy will be rare in that population.
  • RDAs do not provide the needs that have been altered as a result of disease states, chronic usage of certain drugs, or other factors that require specific individual attention.
Table 25.1 Summary of RDA for Indians 2010

t 25.1
  • Energy RDA Each individuals food energy intake must equal the energy expended, in order for the person to maintain their body weight.
  • Protein RDA Protein recommendations are mainly based on the individuals body weight. The protein RDA is high, to cover most person's needs. The average requirement for protein is 0.6 grams per kilogram of body weight; the RDA is 0.8 grams this is said to meet 97.5% of the population's needs.
  • No RDA for Carbohydrate and Fat The amount of protein recommended represents a small percentage of a person's energy allowance; with the remainder acquired from carbohydrates and fats. The general guideline for carbohydrate and fat is that more than half of daily energy should come from carbohydrates, with no more than one-third from fat.
25.3 Minimal Daily Requirement (MDR)
  • (MDR) is the minimum amount of a nutrient from exogenous sources required to sustain normality .
  • Individuals consume food more for satiation of energy needs than for individual nutrients.
  • To express the quality of any food in relation to its content of specific nutrient, the term nutrient density is used. It is defined as the concentration of a nutrient per unit of energy (e.g., 1,000 calories) in a specific food.
  • For any nutrient the higher the nutrient density the better the food source; for example, one whole green pepper contains 20 mg of vitamin C and provides 4 calories, while one medium sweet potato also contains 20 mg of vitamin C but provides 100 calories. Therefore, green pepper is a much better source of vitamin C than sweet potato.
25.4 Metabolism

All cells have in common two major general functions: energy generation and energy utilization for growth and/ or maintenance. These may be termed metabolic reactions or simply metabolism.
  • Anabolism broadly refers to processes in which relatively large molecules such as proteins are biosynthesized from small nutrient materials such as amino acids. These reactions require energy which is available in cells in the form of stored chemical energy in high energy phosphate compounds.
  • Catabolism is the degradation of relatively large molecules to smaller ones. Catabolic reactions serve to capture chemical energy (in the form of adenosine triphosphate, ATP) from the degradation of energy-rich molecules. Catabolism also allows nutrients (in the diet or stored in cells) to be converted into the building blocks needed for the synthesis of complex molecules.
  • Intermediary metabolism refers to all changes that occur in a food substance beginning with absorption and ending with excretion.
  • In the adult there is a delicate regulated balance between anabolic (synthetic) and catabolic (degradative) processes. In the growing child, input of nutrients and anabolism exceed catabolism so that the growth of tissues may occur. In the aging process or in wasting diseases, the catabolic processes exceed anabolic ones.
  • Bio-availability: Bio-availability of a given nutrient from a diet, that is, the release of the nutrient from the food, its absorption in the intestine and bioresponse have to be taken into account. It is the level of the nutrient that should be present in the diet to meet the requirement. This bio-availability factor is quite important in case of calcium and protein and trace elements like iron and zinc. In case of iron, the amount to be present in the diet is 20-30 times higher than the actual iron requirement to account for the low bio-availability of iron from a given diet, particularly a cereal-based diet.
Table 25.2 Basic four food groups

t 25.2

25.5 The Need for Energy
  • The human body needs a continuous regulated supply of nutrients. Energy is required for all body processes, growth, and physical activity. Even at rest the body requires energy for muscle contraction, active transport of molecules and ions, and synthesis of macromolecules and other biomolecules from simple precursors.
  • For example, the heart pumps approximately 8,000 L/day of blood in about 80,000 pulsations. The daily energy required for this heart function alone is estimated to be equivalent to lifting a weight of 1,000 kg to a height of 10 meters.
  • In most processes the energy is supplied by adenosine triphosphate (ATP). Energy is liberated when ATP is hydrolysed to adenosine diphosphate (ADP) and inorganic phosphate. A resting human consumes about 40 kg of ATP in 24 hours. The amount of ATP in the body tissues is limited but is generated continuously from the fuel stores to supply the required energy. These fuel stores must be replenished via food intake.
Last modified: Thursday, 25 October 2012, 8:46 AM