Nutrition in Humans: Balanced Diet, Digestion and Absorption

What you'll learn

This revision guide covers the essential principles of human nutrition tested in CXC CSEC Integrated Science examinations. You will master the components of a balanced diet, understand the mechanical and chemical breakdown of food through the digestive system, and learn how nutrients are absorbed and utilized by the body. These concepts form the foundation of Paper 1 (multiple choice) and Paper 2 (structured and extended response) questions on human nutrition.

Key terms and definitions

Balanced diet — a diet containing all six classes of food (carbohydrates, proteins, fats, vitamins, minerals, water) plus dietary fibre in the correct proportions to maintain health and provide energy for daily activities.

Digestion — the mechanical and chemical breakdown of large, complex, insoluble food molecules into small, simple, soluble molecules that can be absorbed into the bloodstream.

Absorption — the process by which soluble food molecules pass through the wall of the small intestine into the bloodstream or lymphatic system.

Peristalsis — rhythmic, wave-like muscular contractions of the alimentary canal that push food along the digestive tract.

Enzyme — a biological catalyst (protein) that speeds up the rate of chemical reactions in digestion without being used up in the process.

Assimilation — the process by which absorbed nutrients are incorporated into cells and tissues of the body where they are used for growth, repair, or energy production.

Egestion — the removal of undigested food material (faeces) from the body through the anus.

Malnutrition — a condition resulting from an unbalanced diet where certain nutrients are lacking (deficiency), in excess, or in wrong proportions.

Core concepts

Components of a balanced diet

A balanced diet must contain six essential classes of food, each serving specific functions:

Carbohydrates provide the primary source of energy for body activities. Found in foods like breadfruit, cassava, rice, green bananas, sweet potatoes, dasheen, yam, and sugar cane products common throughout the Caribbean. Simple carbohydrates (sugars) provide quick energy, while complex carbohydrates (starches) provide sustained energy release.

Proteins are essential for growth, repair of body tissues, and production of enzymes and hormones. Complete proteins containing all essential amino acids are found in fish (snapper, kingfish, tilapia), chicken, eggs, milk, and cheese. Incomplete proteins are found in peas, beans (red beans, black-eye peas), lentils, and nuts. Growing children, pregnant women, and athletes require higher protein intake.

Fats and oils provide concentrated energy, insulate the body, protect organs, and enable absorption of fat-soluble vitamins (A, D, E, K). Sources include coconut oil, vegetable oils, butter, fatty fish, avocado, and nuts. Excess fat consumption leads to obesity and cardiovascular disease.

Vitamins are required in small amounts for specific metabolic functions:

• Vitamin A (found in mango, papaya, pumpkin, carrots, liver) maintains healthy skin and vision; deficiency causes night blindness
• Vitamin C (found in citrus fruits, guava, West Indian cherry/acerola, tomatoes) promotes healing and prevents scurvy
• Vitamin D (produced by skin in sunlight, found in fish oils, fortified milk) promotes calcium absorption for strong bones and teeth; deficiency causes rickets in children
• B vitamins (found in whole grains, meat, fish) are involved in energy metabolism

Minerals are inorganic nutrients needed for various functions:

• Calcium (found in milk, cheese, sardines with bones, callaloo) builds strong bones and teeth; deficiency causes osteoporosis and rickets
• Iron (found in liver, red meat, dark green vegetables, peas and beans) forms haemoglobin in red blood cells; deficiency causes anaemia
• Iodine (found in seafood, iodized salt) prevents goitre and is essential for thyroid hormone production

Water comprises 60-70% of body mass and is essential for all metabolic reactions, temperature regulation, transport of nutrients and waste, and as a solvent. Humans require approximately 2 litres daily from drinks and food.

Dietary fibre (roughage) is indigestible plant material from fruits, vegetables, and whole grains that adds bulk to faeces, prevents constipation, and reduces risk of colon cancer. Common Caribbean sources include breadfruit, plantain, okra, and whole wheat products.

Factors affecting dietary requirements

Dietary needs vary according to several factors:

Age: Infants and children require more protein for growth and higher energy per kilogram of body mass. Elderly people often need less energy but adequate calcium and vitamin D to prevent bone deterioration.

Sex: Males typically require more energy than females due to larger body mass and higher muscle-to-fat ratio. Menstruating females require more iron to replace losses.

Activity level: Manual labourers (construction workers, farmers, fishermen) and athletes require significantly more energy than sedentary office workers. Caribbean agricultural workers may require 3000-4000 kcal daily compared to 2000-2500 kcal for less active individuals.

Pregnancy and lactation: Pregnant and breastfeeding women require additional protein, calcium, iron, and vitamins to support foetal development and milk production.

Climate: People in hot climates lose more water and salts through sweating, requiring increased fluid and mineral intake.

The human digestive system

The alimentary canal is a continuous tube approximately 7-9 metres long extending from mouth to anus, consisting of specialized regions:

Mouth: Mechanical digestion begins through chewing (mastication). Teeth cut, tear, and grind food into smaller pieces, increasing surface area. The tongue mixes food with saliva from salivary glands. Saliva contains the enzyme salivary amylase (ptyalin) which begins chemical digestion of starch into maltose. Food is shaped into a bolus and swallowed.

Oesophagus: A muscular tube approximately 25 cm long connecting the mouth to the stomach. Peristaltic contractions push the bolus downward. No digestion occurs here. The epiglottis prevents food entering the trachea during swallowing.

Stomach: A J-shaped muscular bag that churns food, mixing it with gastric juice secreted by gastric glands in the stomach wall. Gastric juice contains:

• Hydrochloric acid (pH 1.5-2.0) which kills bacteria, provides optimum pH for pepsin, and denatures proteins
• Pepsin (a protease enzyme) which breaks down proteins into polypeptides
• Mucus which protects the stomach lining from acid damage

Food remains in the stomach for 2-4 hours, forming a semi-liquid mixture called chyme. The pyloric sphincter controls release of chyme into the small intestine.

Small intestine: The longest section (approximately 6 metres) consisting of the duodenum, jejunum, and ileum. Most chemical digestion and all absorption occur here.

In the duodenum:

• Bile from the liver (stored in the gall bladder) emulsifies fats into tiny droplets, increasing surface area for lipase action. Bile also neutralizes stomach acid
• Pancreatic juice from the pancreas contains three key enzymes: pancreatic amylase (continues starch digestion), trypsin (continues protein digestion), and pancreatic lipase (digests fats into fatty acids and glycerol)
• Intestinal juice from intestinal glands contains maltase, lactase, and sucrase (digest disaccharides into monosaccharides) and peptidases (complete protein digestion to amino acids)

The small intestine is highly adapted for absorption through:

• Extensive length providing large surface area
• Inner wall folded into millions of tiny finger-like projections called villi (singular: villus)
• Each villus covered in microscopic extensions called microvilli forming a brush border
• Rich blood capillary network in each villus
• Thin epithelial lining (one cell thick) for short diffusion distance
• Lacteal (lymph vessel) in each villus for fat absorption

Glucose and amino acids are absorbed into blood capillaries by diffusion and active transport, then transported via the hepatic portal vein to the liver. Fatty acids and glycerol are absorbed into lacteals, reformed into fats, and transported via the lymphatic system.

Large intestine (colon): Water and mineral salts are reabsorbed from undigested material. Bacteria synthesize vitamin K and some B vitamins. Faeces (composed of undigested fibre, dead cells, bacteria, bile pigments) are stored in the rectum and egested through the anus.

Enzyme action in digestion

Digestive enzymes catalyze the hydrolysis (breakdown using water) of large molecules:

Enzymes are specific to their substrate (lock-and-key mechanism) and function optimally at specific temperatures (37°C for human enzymes) and pH values.

Malnutrition and dietary deficiency diseases

Protein-energy malnutrition occurs when insufficient protein and energy are consumed. Two conditions affect Caribbean children:

• Kwashiorkor: caused by severe protein deficiency despite adequate energy intake; symptoms include swollen abdomen, thin limbs, irritability, and flaky skin
• Marasmus: caused by severe deficiency of both protein and energy; symptoms include extreme weight loss, muscle wasting, and weakness

Obesity results from excessive energy intake relative to energy expenditure, leading to excess fat storage. Complications include type 2 diabetes, hypertension, heart disease, and joint problems. Increasing obesity rates in Caribbean nations correlate with adoption of high-fat, high-sugar diets and sedentary lifestyles.

Anaemia results from iron deficiency, causing reduced haemoglobin production. Symptoms include fatigue, pale skin, and shortness of breath. Common in menstruating women and children with poor dietary iron intake.

Scurvy results from vitamin C deficiency, causing poor wound healing, bleeding gums, and loose teeth. Rare in the Caribbean due to abundant citrus fruits and fresh produce.

Worked examples

Example 1: A student conducted an investigation to identify the food groups present in various Caribbean foods. She used the following tests:

• Benedict's test for reducing sugars
• Iodine test for starch
• Biuret test for protein
• Ethanol emulsion test for fats

(a) Describe the positive result for the iodine test. [2 marks]

(b) She tested boiled dasheen and obtained a positive result for starch. Explain why dasheen must be digested before the glucose can be absorbed. [3 marks]

Mark scheme answer:

(a)

• The iodine solution changes from brown/yellow/orange [1 mark]
• to blue-black/black colour [1 mark]

(b)

• Starch molecules are large/complex/insoluble [1 mark]
• Starch molecules cannot pass through the intestinal wall/are too large to be absorbed [1 mark]
• Starch must be broken down/digested into glucose which is small/simple/soluble [1 mark]

Example 2: The diagram below shows a villus from the small intestine.

(a) State TWO structural adaptations of the villus that make absorption efficient. [2 marks]

(b) Name the blood vessel that transports absorbed glucose from the small intestine to the liver. [1 mark]

(c) Explain why fats are not transported directly to the liver like glucose and amino acids. [2 marks]

Mark scheme answer:

(a) Any TWO of:

• Large surface area/millions of villi [1 mark each]
• Thin epithelium/one cell thick/short diffusion distance
• Rich blood supply/network of capillaries
• Presence of lacteal for fat absorption

(b) Hepatic portal vein [1 mark]

(c)

• Fats/fatty acids and glycerol are absorbed into the lacteal [1 mark]
• They are transported via the lymphatic system (not directly via blood to the liver) [1 mark]

Example 3: A nutrition study compared the daily diets of two teenage boys in Jamaica.

(a) State which boy's diet is most balanced. Justify your answer. [2 marks]

(b) Suggest ONE health problem Boy B may develop if he continues this diet. Explain your answer. [2 marks]

Mark scheme answer:

(a)

• Boy A [1 mark]
• All nutrients are at/close to recommended levels OR Boy B's diet is deficient in all nutrients shown [1 mark]

(b) Any ONE of:

• Anaemia [1 mark] because iron intake is too low to produce sufficient haemoglobin/red blood cells [1 mark]
• Stunted growth/poor muscle development [1 mark] because protein intake is insufficient for growth and repair [1 mark]
• Weak bones/rickets/osteoporosis [1 mark] because calcium intake is too low for bone development [1 mark]
• Underweight/lack of energy [1 mark] because energy intake is below recommended level [1 mark]

Common mistakes and how to avoid them

• Confusing digestion with absorption: Digestion is the breakdown of food molecules; absorption is the uptake of these molecules into the bloodstream. Use precise terminology in answers.

• Stating that enzymes "digest food": Enzymes catalyze specific reactions. Be specific: "pepsin digests proteins into polypeptides" not "pepsin digests food."

• Forgetting that bile is not an enzyme: Bile emulsifies fats (physical process) but does not chemically break them down. Lipase performs chemical digestion of fats.

• Mixing up the products of digestion: Remember the final products must be simple molecules: proteins → amino acids, starch → glucose, fats → fatty acids and glycerol. Intermediate products (polypeptides, maltose) are not absorbed.

• Listing adaptations without explaining function: When asked how villi are adapted, connect structure to function: "thin epithelium provides a short diffusion distance for rapid absorption."

• Confusing deficiency diseases: Link each disease clearly to its cause: scurvy (vitamin C deficiency), rickets (vitamin D deficiency), anaemia (iron deficiency), kwashiorkor (protein deficiency).

Exam technique for "Nutrition in Humans: Balanced Diet, Digestion and Absorption"

• "State" questions require brief, factual answers without explanation (usually 1 mark each). Example: "State the function of bile" requires only "emulsifies fats" or "neutralizes stomach acid."

• "Explain" questions require reasoning or linking cause and effect (typically 2-3 marks). Use connecting words like "because," "therefore," "so that." Example: "Explain why protein must be digested" requires: proteins are large molecules [1], they cannot pass through the intestinal wall [1], so must be broken down into amino acids which are small enough to be absorbed [1].

• Diagram questions often test knowledge of structure and function. Label accurately using proper terminology and read carefully whether the question asks for labels, annotations, or both.

• Food test questions require specific colour changes: Benedict's test (blue → brick red/orange), iodine test (brown → blue-black), biuret test (blue → purple), ethanol emulsion test (clear → milky white emulsion).

Quick revision summary

A balanced diet contains carbohydrates, proteins, fats, vitamins, minerals, and water in correct proportions. Requirements vary with age, sex, activity level, and physiological state. Digestion involves mechanical breakdown (chewing, churning) and chemical breakdown by enzymes in specific regions: mouth (salivary amylase on starch), stomach (pepsin on proteins), small intestine (pancreatic and intestinal enzymes completing digestion). Absorption occurs mainly in the small intestine through villi, which have adaptations including large surface area, thin epithelium, and rich blood supply. Malnutrition results from unbalanced diets, causing conditions like obesity, kwashiorkor, anaemia, and vitamin deficiency diseases.