Food Nutrients and Diet
Senior 1 · Biology
Food Nutrients and Diet
Lesson 5 of 5
25 min
On long sea voyages in the 1700s, sailors' gums bled and swelled, their teeth loosened, old wounds reopened, and many died of a disease called scurvy. In 1747 James Lind, a doctor on the British ship HMS Salisbury, took twelve sailors with scurvy, kept them on the same basic diet, and gave pairs of them six different extra treatments: cider, dilute sulfuric acid, vinegar, sea water, a spicy paste, or two oranges and a lemon each day. Within a week the pair eating citrus fruit had recovered enough to nurse the others. It was one of the first planned medical experiments in history.
Definition
Deficiency disorder: an illness caused by too little of a particular nutrient in the diet over a long time.
Scurvy: the disorder caused by too little vitamin C. Rickets: soft, weak bones in children, caused by too little vitamin D or calcium; in adults the same cause gives osteomalacia.
Anaemia: a shortage of haemoglobin in the blood, so the blood carries less oxygen. The most common cause is too little iron.
Vitamin C is needed to make collagen, the protein fibre that holds skin, gums, blood vessels and scars together. Without it, collagen breaks down faster than it is made. Signs: bleeding, swollen gums and loose teeth; bruising easily; slow healing of wounds; tiredness and pain in the joints. Prevention: fresh fruit and vegetables every day, such as oranges, passion fruit, guavas, tomatoes and green leaves, not over-cooked.
Figure 1: How vitamin D and calcium build strong bones, and what happens without them.
Bones harden as calcium phosphate is laid down in them, and vitamin D is needed to absorb calcium from food in the intestine. A child short of either develops soft bones that bend. Signs: bowed legs or knock-knees, swollen wrists and ankles, a delay in walking, and pain in the bones. Prevention: milk and other calcium-rich foods, and sunlight on the skin for some time most days, which lets the skin make vitamin D.
About 99% of the body's calcium is in the bones and teeth, and bones act as a calcium bank. If the diet supplies too little, the body takes calcium out of the bones to keep the level in the blood steady, because nerves, muscles and blood clotting all need it. Signs: in children, poor growth of bones and teeth and rickets; in adults, bones that slowly lose their mass and break easily (osteoporosis), especially in older women; very low blood calcium causes muscle cramps. Prevention: milk, ikivuguto, small fish eaten whole such as isambaza, beans and dark green leaves, especially during the teenage years, when most bone is built.
Figure 2: Normal blood and blood from a person with iron-deficiency anaemia, at the same magnification.
Iron is part of haemoglobin, so with too little iron the red blood cells become fewer, smaller and paler, and the blood carries less oxygen. Signs: tiredness and weakness, pale skin, lips and inner eyelids, breathlessness on exercise, headaches and poor concentration at school. Prevention: beans, dark green leaves, meat, liver and eggs. Vitamin C eaten in the same meal (tomatoes, an orange) helps the body absorb iron from plant foods; tea drunk with meals reduces it.
| Disorder | Cause | Main signs | Prevention |
|---|---|---|---|
| Scurvy | too little vitamin C | bleeding gums, loose teeth, bruises, slow healing | fresh fruit and vegetables daily |
| Rickets (children), osteomalacia (adults) | too little vitamin D or calcium | soft bones; bowed legs; swollen wrists; bone pain | sunlight; milk, fish, eggs |
| Calcium deficiency | too little calcium | weak bones and teeth; osteoporosis later; cramps | milk, isambaza, beans, green leaves |
| Iron-deficiency anaemia | too little iron | tiredness, pale skin and eyelids, breathlessness | beans, leaves, meat, liver, eggs, with vitamin C |
Warning
Not every anaemia comes from the diet, and sunshine alone is not enough. In Rwanda, malaria (which destroys red blood cells) and hookworm (which feeds on blood in the intestine) are also important causes of anaemia, so a pale, tired child may need a malaria test and deworming as well as better food. And although Rwanda is sunny, rickets still occurs in children who are kept indoors or fully covered and who drink little milk.
Worked example 1: was Lind's experiment a fair test?
Use the description at the start of this lesson to judge the design of Lind's experiment.
Conclusion: it was a fair comparison for its time, and its result was right: something in citrus fruit prevents scurvy. That substance, vitamin C, was only identified in the 1930s.
Worked example 2: is this student anaemic?
The World Health Organization (WHO, 2011) uses a haemoglobin level below to identify anaemia in children aged 12 to 14. A health worker measures Ishimwe's haemoglobin as . Is she anaemic, and by what percentage is she below the cut-off?
Answer: yes; her haemoglobin is 12.5% below the cut-off. She should eat more iron-rich food with vitamin C, and be checked for malaria and worms.
The Rwanda Demographic and Health Survey 2019-20 found that 36.6% of children aged 6 to 59 months, and 13.1% of women aged 15 to 49, were anaemic. It also found that 33.1% of children under five were stunted (too short for their age, a sign of long-term undernutrition); the 2025 survey reported 26.8%, a clear improvement. National programmes fight these problems with iron-rich beans and orange sweet potatoes, milk for young children, fortified foods, deworming and malaria control.
Deficiency Diseases, Khan Academy. Note each disease the video names and check which nutrient it links to; compare with the table above.
Tip
Memory hook: "C for Collagen and Citrus; D for Daylight and bones; Iron for In the blood."
Practice: deficiency disorders
Key points
Further reading for this unit
Sources and credits
Figures 1 and 2, the table, the worked examples and the practice questions are written by Ganzaa. Survey figures: Rwanda Demographic and Health Survey 2019-20 and 2025, from the DHS Program STATcompiler data (accessed 2026-09-27). Haemoglobin cut-off: WHO, Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity (2011).
