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Human Biology > Dietary Requirements
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Dietary Requirements

  • Food has two main functions which provide
    • Chemical P.E. (starting point) for respiration
    • Substances used in metabolism to produce/maintain cells and tissues

Role Of Carbohydrates And Lipids In The Body

Starch and Sugar

Provide ≈80% of total chemical P.E.
Breast-fed infants obtain ≈40% of their chemical P.E. from lactose

Non-starch polysaccharides (e.g. glycogen)

Control appetite
Low levels in diet may cause appendicitis, cancer of colon, haemorrhoids, constipation


Source of chemical P.E.
Phospholipids are essential for plasma membranes
Essential fatty acids are precursors of other important substances

Simple Laboratory Techniques For Estimating Energy Content Of Different Foods

  • To determine the energy content of food (using a calorimeter)
    • A known mass of food is fully burnt
    • Under a boiling tube which is filled with a known volume of H2O
    • Increase in temp of the H2O is measured
    • Experimental error due to incomplete combustion / heat loss
  • More accurate results by reduced heat loss
    • Water jacket → less loss of heat
    • Sample burnt in O2 allows more combustion
    • Copper coil transfers more heat to water
    • Stirrer distributes heat evenly to area of thermometer
  • Healthy diet requires right balance between carbohydrates and lipids
    • Food rich in carbohydrates is an instant source of energy (→glucose)
      • Tend to be bulky and contain large amounts of H2O
      • Amount of energy released is low
      • Food/drink rich in pure glucose is absorbed without digestion
  • Lipid-rich food → amount of energy released is high

Role Of Proteins In The Body

Nucleic acid

Genetic info

Cells and tissues

Growth and repair

Enzymes and hormones

Essential role in metabolic pathway

Mineral ions

Synthesis of compounds;
Carrying out functions;
Transmitting nerve impulses

Essential amino acids

Cannot be synthesised and must be present in diet

Non-essential amino acids

Can be synthesised from essential amino acids by transamination in the liver


  • Essential amino acid A + keto acid B → non-essential amino acid B + keto acid A
    • Enzyme: transferase
  • Transfer of amino NH2 group from an essential amino acid to a keto acid
  • Produces a non-essential amino acid and a keto acid of a different sort

Role Of Vitamins With Respect To Vitamin D And Of Inorganic Ions Illustrated By Fe And Ca


Provided in fresh vegetables, meat, fish, eggs, fruit
Decrease appetite / minimises risk of constipation

Vitamin D

Involved in metabolism of calcium
Essential in the diet but required in small amounts


- In bones, teeth, small amounts in tissues, body fluids
- Important for synapses
- During lactation, women tend to eat more → automatic increase in calcium intake


Synthesis of Hb, used by enzymes
≈30% in the body can be stored

The Path Of Iron Through The Body

  • Absorbed from the gut and stored
  • Used for physiological requirements (Hb, enzymes)
  • Lost in blood, urine, rubbed off intestinal epithelial cells by food

People Who Are Vulnerable To A Deficiency Of Iron:

  • People with a high physiological requirement for iron
  • Children with late weaning and inappropriate infant food
  • Growing people \ children and pregnant women
  • Women of reproductive age / menstruation / loss of blood \ iron
  • Old people / absorb iron poorly
  • People who drink too much tea / tannin inhibits absorption of iron from gut

Dietary Requirements Concept Of Basal Metabolic Rate BMR

  • Rate of metabolism when a person is at rest / but awake
  • Energy exhaustion to carry out essential activities that maintain live
    • E.g. diffusion, breathing, pumping of blood
    • NOT movement
  • Measured in kJ kg-1 h-1 or kJ m-2 h-1
  • Measured after meal at constant temp
    • Temp may affect BMR → shivering
    • Energy requirement at different levels of physical activity can be expressed in multiples of BMR
  • BMR depends on number of metabolically active cells in a unit mass of the body
  • Factors affecting BMR
    • LARGER BODY MASS → more cells respire \ release energy
    • LARGER SURFACE AREA → greater heat loss \ higher respiration to maintain core temp
    • SEX → muscle has greater BMR then fat
    • AGE → growth requires high energy amounts; muscle:body fat ratio decreases with age; biochemical reactions slow down, become less efficient
    • FUNCTION OF BASAL METABOLISM → supply heat to maintain body temp above that of the surroundings
  • BMR not helpful for total energy requirement (human is physically active the whole day)
  • Physical activity ratio (PAR) is the energy being used in carrying out the activity concerned
  • to the basal metabolic rate

Protein Requirement

  • Growing people / production of new cells → youngest age group have highest requirement
  • Pregnant woman / growing fetus and organs / lactation (milk is rich in protein)
  • Adults / maintenances / synthesis of Hb, enzymes, hormones → lower requirement with age

Glycogen Loading And The Enhancement Of Athletic Performance

  • High carbohydrate diet (long before exercise) / more glycogen in muscles / more can be broken down to glucose / prolonged rate of respiration / longer but not faster exercise

Concept Of A Balanced Diet And Problems Which Arise From Vegetarian And Weight-Loss Diets

  • Balanced diet: correct amounts and properties of all/essential nutrients
  • Fat in diet: makes foot tasty and → encourages eating / rather stored than used as respiratory substrate / provides essential fatty acids → must be in diet / provides more energy than the same mass of carbohydrates
    • Based on cereal grains, vegetables
    • Total energy yield may be reduced (-)
    • Must contain more protein to provide the supply for essential amino acids
    • Shortage of vitamin B12 and D, iron (→anaemia), zinc possible
    • Obesity → energy imbalance → energy intake with food > energy expenditure in exercise
    • Use of muscle and other tissue proteins for energy production - causing wasting
    • SYMPTOMS: tooth decay / low blood pressure / constipation / menstruation ceasing
    • INCREASED RISK OF: infections / vitamin and mineral deficiency diseases
      • Ca2+ needs can be met by drinking skimmed milk
      • Iron supply may need a supplement (-)

Table 16-5-1: Dietary demands in pregnancy and lactation





Growth of fetus, placenta, uterus, breasts

High amino acid content of milk for growth of baby


For fetal Hb and increase in mother's Hb and blood volume

Synthesise of baby's Hb


Growth of fetal teeth and bones

Growth of baby's bones (and teeth)

Influence Of IUDs And Oral Contraceptives On Menstrual Loss Of Iron

  • Intra-uterine contraceptive (IUD) affects amount of blood lost during menstruation
  • Mean blood loss during menstruation before IUDs were fitted was 41cm�

Table 16-5-2: Sample calculation

Conc of Hb ≈ 13g per 100cm3

13 * 41/100 = 5.3g Hb lost

5.3g Hb = 18.4g Fe over a 28-day cycle

0.7mg day-1 of Fe lost

0.8mg day-1 Fe lost from other causes

Total loss of Fe = 1.56mg day-1

  • Average intake of Fe day-1 for women ≈12.1mg
    • Only ≈15% absorbed → only 1.8mg day-1 taken into body
    • This is a slight surplus compared with 1.56mg day-1 loss
  • Mean blood loss during menstruation after IUDs were fitted was 90cm3!
    • Iron loss and anaemia cause problems for women using IUDs, particularly if blood loss during menstruation was heavy or the diet was low in iron