Deficiency Diseases: KS3 Biology Guide

A deficiency disease occurs when the body lacks a specific vitamin or mineral essential for normal functioning. Unlike infectious diseases, they are not caught — they develop when the diet is consistently missing a nutrient. Scurvy, rickets, anaemia, and night blindness are the key examples at KS3.

What is a deficiency disease?

A deficiency disease is a condition caused by a long-term shortage of a specific essential nutrient in the diet. The nutrient may be a vitamin (an organic compound needed in small amounts) or a mineral (an inorganic element needed in small amounts).

Most deficiency diseases reverse when the missing nutrient is restored to the diet — making diet the both the cause and the cure. This distinguishes them from infectious diseases (caused by pathogens) and inherited diseases (caused by faulty genes).

Deficiency diseases occur when:

• The diet does not contain enough of the nutrient (dietary deficiency)
• The body cannot absorb the nutrient from food (absorption problem, as in some bowel conditions)
• The body's requirement for the nutrient is unusually high (e.g. during pregnancy)

What is scurvy and what causes it?

Scurvy is caused by a deficiency of vitamin C (ascorbic acid).

What vitamin C does: Vitamin C is essential for the synthesis of collagen — the most abundant protein in the body, which forms the structural framework of skin, blood vessel walls, gums, tendons, and cartilage. Without vitamin C, the body cannot make or maintain collagen properly.

Symptoms of scurvy:

• Swollen, bleeding gums; teeth may become loose
• Small skin haemorrhages (bruising, tiny bleeding spots called petechiae)
• Slow healing of wounds
• Joint pain and swelling
• Extreme fatigue and weakness

Historical context: Scurvy was a major killer of sailors on long voyages before the cause was understood. In the 18th century, the Royal Navy discovered that feeding sailors citrus fruit (limes or lemons) prevented scurvy — hence the nickname "limeys" for British sailors. James Lind conducted one of the first controlled clinical trials in 1747, showing that citrus fruit cured scurvy among sailors at sea.

Dietary sources of vitamin C: citrus fruits (oranges, lemons, limes), strawberries, blackcurrants, kiwi fruit, peppers, broccoli, potatoes.

What is rickets and what causes it?

Rickets is caused by a deficiency of vitamin D or calcium (or both).

What vitamin D does: Vitamin D promotes the absorption of calcium and phosphorus from the small intestine and enables their deposition into bone tissue. Without adequate vitamin D, calcium cannot be properly incorporated into bones.

Symptoms of rickets:

• Soft, weakened bones that bend under the body's weight — producing bowed legs, knock knees, or a bent spine
• Pain in bones and muscles
• Delayed tooth development
• Poor growth

Rickets mainly affects growing children whose bones are still developing. In adults, the equivalent condition (vitamin D deficiency causing softening of existing bones) is called osteomalacia.

Sources of vitamin D: Sunlight is the most important source — UVB radiation from sunlight enables the skin to synthesise vitamin D. Dietary sources include oily fish (salmon, mackerel, sardines), eggs, and fortified foods (cereals, spreads). In the UK, the NHS recommends vitamin D supplements for people who have limited sun exposure, including those who stay indoors a lot or cover most of their skin outdoors.

What is anaemia and what causes it?

Iron-deficiency anaemia is caused by insufficient iron in the diet.

What iron does: Iron is a component of haemoglobin — the protein in red blood cells that carries oxygen. The iron atom in each haemoglobin molecule is what actually binds to oxygen. Without enough iron, the body produces fewer red blood cells with less haemoglobin, reducing the blood's oxygen-carrying capacity.

Symptoms of iron-deficiency anaemia:

• Extreme fatigue and lack of energy (muscles and organs receive less oxygen)
• Pale skin and pale gums
• Shortness of breath during mild exercise
• Dizziness and headaches
• Brittle nails; hair loss in severe cases

Dietary sources of iron: red meat, liver, beans and lentils, dark leafy vegetables (spinach), fortified breakfast cereals, tofu. Vitamin C eaten at the same meal improves iron absorption — this is why orange juice with an iron-rich meal is beneficial.

What is night blindness and what causes it?

Night blindness (difficulty seeing in dim light) is caused by a deficiency of vitamin A.

What vitamin A does: Vitamin A is used to make rhodopsin, a light-sensitive pigment found in the rod cells of the retina. Rod cells detect light intensity (not colour) and are responsible for vision in dim conditions. Without enough rhodopsin, rod cells cannot function effectively.

Symptoms: difficulty seeing in dim light or darkness (night blindness); if the deficiency is severe and prolonged, it can lead to xerophthalmia (dry eyes) and eventual blindness.

Vitamin A deficiency is the leading cause of preventable blindness in children worldwide, particularly in parts of South and Southeast Asia and Sub-Saharan Africa.

Dietary sources of vitamin A: liver, dairy products, eggs; beta-carotene (found in carrots, sweet potatoes, mangoes, and leafy greens) is converted to vitamin A in the body.

How can deficiency diseases be prevented?

Most deficiency diseases can be prevented by eating a balanced diet that includes a variety of foods from all food groups. Key strategies include:

• Eating a wide variety of fruit and vegetables (provides vitamins C and A, and minerals)
• Including protein sources — meat, fish, eggs, beans, nuts — for iron and other minerals
• Fortified foods — manufacturers add vitamins/minerals to cereals, bread, and plant-based milk to help populations that might otherwise be at risk
• Sunlight exposure for vitamin D synthesis
• Supplements where dietary intake is insufficient (e.g. folic acid in pregnancy, vitamin D for those with limited sun exposure, iron for those with heavy periods)

Frequently asked questions

Why did sailors historically get scurvy but land-dwellers less often?

Sailors on long voyages ate preserved foods — ship's biscuit, salted meat, dried peas — that contain virtually no vitamin C. Fresh fruit and vegetables were unavailable after the first few days at sea. On land, even a modest diet typically included seasonal fresh produce that provided enough vitamin C to prevent scurvy. The extreme, prolonged vitamin C absence unique to ocean voyages is what made scurvy a disease primarily of sailors before the discovery of citrus fruit as a preventive measure in the 18th century.

Why are children more susceptible to rickets than adults?

Children's skeletons are still growing, so they need calcium and phosphorus to be continuously deposited in new bone tissue. If vitamin D is deficient, this mineralisation process is impaired and the rapidly growing bones remain soft and flexible — causing the visible bending and deformity of rickets. Adults have finished growing; their existing bone matrix is largely already mineralised. In adults, a vitamin D deficiency still causes harm (osteomalacia — gradual softening of existing bones), but the effects are less dramatic and develop more slowly because the skeleton is not undergoing rapid growth.

Can eating too many carrots give you enough vitamin A to prevent night blindness?

Carrots are an excellent source of beta-carotene, which the body converts into vitamin A — but the conversion is regulated and inefficient (roughly 6 µg of beta-carotene is needed to produce 1 µg of vitamin A). Eating large amounts of carrots will not cause vitamin A toxicity (which can occur from eating liver excessively or taking high-dose supplements) because the conversion rate automatically limits how much is produced. For most people in the UK with a balanced diet, vitamin A deficiency is rare — it is predominantly a problem in low-income countries where diets are heavily dominated by a single staple crop with little variety.

Is anaemia always caused by iron deficiency?

No. Anaemia describes any condition in which the blood has a reduced ability to carry oxygen, but several different causes exist. Iron-deficiency anaemia (the most common type) is caused by insufficient iron for haemoglobin production. Vitamin B12 or folic acid deficiency causes megaloblastic anaemia — red blood cells are produced but are abnormally large and few in number. Sickle-cell disease is a genetic form where red blood cells are abnormally shaped and break down prematurely. Blood loss (from injury or heavy periods) can cause anaemia even when the diet is adequate. Treatment depends on the cause — iron supplements will only help if the cause is iron deficiency.

For Socratic KS3 biology with Professor Darwin — connecting the nutrients in your plate to the molecules in your cells — visit aitutors.me.