NUTRITIONAL IMPORTANCE OF PROTEINS

Human body composition The average human body, weighing 65 Kgs, contains about 11 kilograms of protein, 40 kilograms of water, and 9 kilograms of fat. The protein provides the "machinery" of the body, including not only the voluntary muscles and the heart muscles, but also the walls of the gut and the blood vessels, as well as the enzymes, the skin, and the hair.

Sources of proteins The body makes its own proteins and obtains amino acids from the digestion of the proteins in the foods that we eat. These large protein molecules cannot be absorbed through the gut walls into the blood, but a series of digestive enzymes (which are proteins themselves, having these special digestive functions) break down the chains of amino acids into the individual amino acids, which are then absorbed. A small proportion of protein may remain undigested, especially with the more plant foods, where the cellulose cell walls are not easily broken down in the tissues,

The final nutrient that we obtain from our foods are thus amino acids rather than proteins, and we build our own protein from the pool of free amino acids circulating in our body. This pool is also derived in part from body proteins that are continually being broken down and resynthesized. Our bodies are in a dynamic state and are constantly turning over.

FUNCTIONS Each protein in our body has a specific function . If a particular amino acid is missing from the site of synthesis, the process stops. Of the different varieties of amino acids, some are termed "essential.

Protein Functions Obviously, a growing body needs dietary protein in order to increase its own body tissues. This is particularly clear in species where the young grow quickly, such as calves and it is interesting that the milk in these species is considerably higher in protein than is human milk, the natural food that meets the needs of much more slowly growing human infants. It is also obvious that older children need protein for their continued growth and that pregnant and lactating women need it to provide for the growing fetus and then for the infant.

The protein needs of other adults are not so obvious, except for the slow but continuous growth of hair and fingernails, the replacement of rubbed-off skin, and other minor losses. However, adults require much greater amounts of protein than would be expected to replace these losses—something like 50 g/day.

It is true that Living tissue is in a dynamic state, with protein being continually broken down to its constituent amino acids and resynthesized. This is not going to increase requirements because the amino acids are fully available for reuse. However, there are enzymes in the liver whose function is to break down an excess of circulating amino acids. These enzymes allow the elimination of nitrogen in the form of urea and the utilization of the carbon-containing side-chains of the amino acids as energy sources for the tissues. It appears that these enzymes sets the requirement for amino acids to replace those lost in this way.

Important Functions Form the building blocks for body tissues. 10­15% of the total energy is derived from proteins.

Only when If, enough carbohydrates are not available in diet, Amino Acid are then used for yielding energy--- protein sparing effect of Carbohydrates. OR In starvation, when the glycogen and fats stores are exhausted amino acids may act as energy sources. Proteins are the only source of essential Amino Acids.

RDA FOR Protein Different for different individuals/groups Infants 2.4 g/kg body wt/day Children up to 10 Yrs 1.75 g/kg body wt/day Adolescent Boys Girls 1.6 g/kg body wt/day 1.4 g/kg body wt/day Adults (Men & Women) 1.0 g/kg body wt/day Pregnancy 2.0 g/kg body wt/day Lactation 2.5 g/kg body wt/day

RDA FOR Proteins WHO / FAO recommends the safe levels of protein intake for an adult as 0.75 g/kg/day. For the synthesis of body proteins, all the essential amino acids should be supplied in adequate quantities at the same time.

Minimum requirements of essential amino acids Requirement (mg/kg/day) Phenylalanine Valine Methionine Leucine Isoleucine Lysine Theronine Tryptophan 14 11 10 9 6 3

Nitrogen Balance

Intake of N2 = Loss of N2 (U+F+S) Nitrogen Balance A normal healthy adult is said to be in nitrogen balance, when the dietary intake (I) equals the daily loss through urine (U) feces (F) and skin (S); or Intake of N2 = Loss of N2 (U+F+S)

Nitrogen Balance Excretion more than intake  Negative nitrogen balance. Intake exceeds excretion  positive nitrogen balance.

MEASUREMENT OF N2 BALANCE Nitrogen balance can be actually measured by calculating the dietary intake of protein nitrogen (16% of the weight of protein) and measuring the daily excretion.

Factors Affecting Nitrogen Balance

Factors Affecting Nitrogen Balance 1. Growth In period of active growth, a state of +ive N2 balance exists. On an average when a person gains 5 kg, about 1 kg proteins are added to the body. For this, about 160 g of nitrogen has to be retained, so he / she has to be in positive nitrogen balance.

Factors Affecting N2 Balance 2. Hormones Growth hormone, insulin and androgens promote +ive N2 balance, while corticosteroids cause a -ive N2 balance. 3. Pregnancy A pregnant woman will be in a state of +ive N2 balance due to the growth of fetus.

Factors Affecting N2 Balance 4. Convalescence A person convalescing after an illness or surgery will be in positive nitrogen balance, due to active regeneration of tissues. 5. Acute illness Negative nitrogen balance is seen in subjects immediately after surgery, trauma and burns.

Factors Affecting N2 Balance 6. Chronic illness Malignancy, uncontrolled diabetes mellitus and other debilitating diseases show negative nitrogen balance. 7. Protein deficiency The deficiency of even a single amino acid can cause negative nitrogen balance. Prolonged starvation is another important cause.

Maintenance of N2 Balance To maintain the N2 balance, one has to satisfy the need for N2 intake, which are: Obligatory N2 loss is 3.5 grams of N/day for a 65 kg person (urinary, fecal and cutaneous loss). This could be equivalent to 22 grams of protein. B. Requirement for protein turnover. The minimum daily requirement to compensate for the above two categories is 0.7 g / kg wt /day of good quality protein.

Maintenance of N2 Balance Protein requirements for growth This is applicable in the case of infants, children, adolescents, pregnancy, lactation and convalescence. As growth stops, protein requirement also decreases.

Current methods Modern, easier way, to assess the nutritional value of a protein, is to give that protein as the only source of nitrogen to an animal, and assess the weight gain

Biological Value (BV) of Protein It is the ratio between the amount of nitrogen retained and nitrogen absorbed during a specific interval. BV = retained nitrogen x 100 absorbed nitrogen

Nitrogen excreted in urine and feces are measured and BV calculated. To calculate the BV of a protein The pure protein is given to an experimental animal as the only dietary source of nitrogen Nitrogen excreted in urine and feces are measured and BV calculated. Example: Suppose 127 mg of protein "A" was consumed by a rat in a day and 4 mg is recovered in feces and 24 mg is seen in urine. Then

How to calculate BV Amount absorbed = 127 - 4 = 123 mg Amount ingested = 127 mg Amount absorbed = 127 - 4 = 123 mg Amount retained = 123 - 24 = 99 mg Therefore BV = 99 /123 x 100 = 81%

Case of another protein "B“ Suppose amount ingested is 100 mg Absorbed is 5 mg, and Retained is 4.5 mg, then BV = 4.5 / 5 x 100 = 90%. The biological value is better in protein "B". What common sense tells?

PROTEIN-ENERGY MALNUTRITION (PEM) It is the most widespread nutritional problem in developing countries. It is predominantly affecting children. The prevalence rate varies from 20-50% in different areas depending on socio-economic status and level of education and awareness.

Marasmus At one end of the spectrum of malnutrition is Marasmus ( "to waste"), which results from a continued severe deficiency of both dietary energy and proteins (primary calorie inadequacy and secondary protein deficiency).

If food is not properly absorbed, certain forms of mal-nutrition are seen. (Marasmus)

Kwashiorkor At the other end of the spectrum is kwashiorkor, where isolated deficiency of proteins along with adequate calorie intake is seen. Kwashiorkor means "sickness the older child gets, when the next child is born", a term from the local language of Ga tribe of Ghana. A classification by WHO is based on body weight as a percentage of standard body weight

WHO Classification of malnutrition