There are many theories and much research into the link between diet and disease. Any of the following, may indicate poor nutrition in a human.

• The hair may become dull and brittle, or it may fall out or change colour.
• Headaches may be related to vitamin deficiency.
• Night blindness may arise from lack of vitamin A.
• The tongue may become inflamed as a result of a number of vitamin deficiencies.
• Bleeding gums may be a sign of scurvy (vitamin C deficiency).
• Enlargement of the thyroid gland (goitre) may be linked to iodine deficiency.
• Rashes, itching, soreness, scaliness and cracking of the skin may be a sign of a number of vitamin deficiencies.
• Obese people often experience breathing difficulties.
• Backache may occur as a result of obesity.
• Too much food, particularly fat and carbohydrates, will lead to obesity. Too little food will cause wasting of the tissues and ultimately starvation. Failure to thrive in children may be a sign of marasmus (too little energy intake) or kwashiorkor (too little protein and energy intake).
• Heart disease can occur as a result of obesity, and heart failure may be a result of extreme anorexia nervosa as the balance of electrolytes is disturbed.
• Softening of the bones may be a sign of rickets (lack of vitamin D).
• Loss of motor function in the legs may be a sign of beriberi (lack of vitamin B₁, or thiamine).
• Lesions in the spinal cord may be a sign of vitamin B₁₂ deficiency.
• There are a number of conditions affecting the stomach and digestive system as a result of diet. Symptoms may include diarrhoea, nausea, vomiting, pain and cramps.
• Stones may form in the kidneys as a result of insufficient fluid; obese people may be prone to kidney failure.
• Adrenal glands may enlarge as a result of pantothenic salt deficiency (related to the B vitamins).
• The formation of gallstones is associated with a fatty diet.
• Too much alcohol may cause cirrhosis of the liver.
• Insufficient iron will cause anaemia.
• Constipation can be caused by lack of fibre in the diet.
• Piles (haemorrhoids) may also be a result of lack of fibre.
• Swelling and painful feet may be a sign of vitamin B₁₂ deficiency.
• Numbness in the toes may be a sign of vitamin deficiency.

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