15.1 Essential ideas

15.1.1 Human nutrition

  •  A nutrient is a chemical substance that is ingested by a living organism and then used in growth or metabolism.
  • Non-essential nutrients can be synthesised by the body. Examples include carbohydrates, most fatty acids and about half of the 20 amino acids necessary for protein synthesis.
  • Essential nutrients cannot be synthesised by the body. They must be included in the diet. Further information is provided in the table below: 

Essential amino acids

  • 8 amino acids produced by certain plants and animals (leucine, isoleucine, histidine, lysine, methionine, phenylalanine, valine, tryptophan).
  • Arginine is essential during infancy but not in adulthood.
  • Threonine is essential in the absence of phenylalanine.

Essential fatty acids

  • Monounsaturated and polyunsaturated fatty acids, called omega-3 and omega-6 fatty acids.
  • Produced by plants – dietary sources include seeds, nuts and fish oils.


  • 13 chemically diverse carbon compounds, synthesised by plants or animals, e.g. vitamin C, C6H8O6, vitamin B3, C6H5NO2.
  • Water-soluble (C, B complex) or fat-soluble (A, D, E, K).
  • Vitamin D is an essential nutrient depending on the environmental conditions.

Dietary minerals*

  • About 20 chemicals derived from inorganic sources, e.g. runoff from rocks.
  • Ingested as water-soluble ions, e.g. Ca2+, PO43-.


  • Recommended daily water intake is between 1 and 2L/day

* Vitamins and minerals are considered micronutrients, as they are required in very small amounts (less than 100mg/day).

15.1.1aFigure 15.1.1a – Balanced diet
Can you identify sources of essential nutrients in the photograph?

Lack of essential amino acids affects the production of proteins

  • Protein deficiency malnutrition occurs when the body is not able to produce the required proteins due to a shortage of essential amino acids.
  • Abdominal swelling is a common symptom of protein deficiency in children. The swelling occurs due to the accumulation of cellular fluid that would normally be taken into the bloodstream by plasma proteins.

Appetite is controlled by the hypothalamus

The appetite control centre in the hypothalamus induces feelings of satiety in response to three different peptide hormones:

  1. PYY3-36: secreted by cells in the mucosa of the small intestine after a meal.
  2. Insulin: secreted by b–cells of the pancreas as glucose is absorbed into the bloodstream after a meal.
  3. Leptin: a peptide hormone secreted when stores of adipose tissue increase.

Starvation can lead to breakdown of body tissue

  • Glycogen in the muscles is broken down into glucose by the liver and released into the bloodstream when blood sugar is low.
  • Starvation is the most extreme form of malnutrition. Long-term calorie deficiency results in the depletion of glycogen stores.
  • When this occurs, the body begins to use the amino acids from muscle proteins as an energy source. The amino acids are processed into glucose by the liver.
  • The immediate result is a loss of muscle mass. Prolonged starvation can cause permanent organ damage and death.

Application: Cardiac muscle loss resulting from anorexia

  • Anorexia nervosa is an illness characterised by voluntary starvation. It has complex psychological causes.
  • Long-term calorie restriction leads to loss of muscle mass. Loss of cardiac muscle results in poor blood circulation, and low blood pressure.
  • Other effects are hair loss, amenorrhea, infertility and organ failure.

Overweight individuals are more likely to suffer hypertension and type II diabetes

(high blood pressure)

Diabetes mellitus 
(insulin insensitivity)

  • Excess weight requires higher cardiac output to supply oxygen to tissues.
  • Fat accumulation around vessels may increase resistance to blood flow.
  • Obesity is associated with atherosclerosis, which further constricts blood flow.
  • Overweight individuals are more likely to have sedentary lifestyles.
  • Diabetes is associated with diets high in fats and sugar, and low in fibre.
  • High blood lipid levels (a risk factor for diabetes) may be associated with poor fatty acid metabolism.


15.1.1bFigure 15.1.1b – Protein deficiency
Protein deficiency diseases affect thousands of children in Cambodian refugee camps.

Key concepts

  • A balanced diet is essential to human health.
  • Malnutrition may be caused by deficiency, imbalance or excess of nutrients in the diet.

Concept help

  • Chemical energy (measured in calories or joules) is provided by carbohydrates, fats and proteins only. Although carbohydrates are not defined as essential nutrients, carbohydrates must be included in any human diet.
  • Essential fatty acids are unsaturated. The numbers in ‘omega-3’ and ‘omega-6’ refer to the position of the double bond in the carbon chain of the fatty acid.

Language help

Anorexia means ‘no appetite’. People who suffer from anorexia nervosa ignore sensations of hunger.

15.1.1dFigure 15.1.1c – Anorexia nervosa
People with anorexia nervosa have a distorted body image.

International mindedness

The vitamin and mineral nutritional information system (VMNIS) was established in 1991 following a request by the World Health Assembly to strengthen surveillance of micronutrient deficiencies at a global level.

scurvyFigure 15.1.1d – Scurvy
Symptoms of scurvy in a Rhesus monkey

Nature of Science: Falsification of theories

Vitamin C deficiency can lead to scurvy, a disease that weakens the gums and causes spots on the skin, jaundice and, in severe cases, death. For a long time scurvy was thought to be unique to humans, because attempts to induce the disease in other animals were unsuccessful.

Course links

  • Review the structure of fatty acids in 2.1.3 and amino acids in 2.1.4.
  • Review the role of leptin in obesity, and the causes and consequences of late-onset (Type II) diabetes in 6.2.6a.