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Vitamins are necessary for the human body to function. Too much or too little stresses cells and tissues. This can lead to serious, life-threatening diseases.

Overdoses of vitamins are most likely to occur due to supplement abuse, but there are rare reports of cases resulting from excessive dietary intake. For most water soluble vitamins, overdoses cause only minor symptoms because the kidneys excrete the excess in urine. An exception is B6, which can cause severe nerve damage when taken in large doses. Fat soluble vitamins are stored in body fat cells, which makes them more likely to cause a toxic reaction when too much is consumed over time. Examples include liver and bone damage caused by vitamin A and interference in blood clotting resulting from excess vitamin E.

Vitamin deficiencies have become rare in the United States and most other industrialized nations. When they do occur, they are often in conjunction with other factors. These risk factors include:

• Alcohol abuse. Alcohol can affect the body’s ability to use vitamins in many ways. It can limit absorption, increase excretion and make vitamins unusable. Alcoholics also tend to replace foods with alcohol, leading to general malnutrition.
  
  
• Smoking. Smoking produces free radicals that use up antioxidants, including vitamins A, C and E. Without increasing intake, smokers are more likely to suffer deficiency of one or all of these vitamins.
  
  
• Limited fat absorption. Any condition (e.g., inflammatory bowel disease) that reduces the body’s ability to absorb fat has the potential to also cause a deficiency of fat-soluble vitamins.
  
  
• Genetic deficiencies or absorption disorders. Despite dietary intake, some genetic conditions may block chemical reactions necessary to absorb or use individual vitamins, leading to deficiencies.

Prolonged vitamin deficiencies may cause severe disease. A few examples include:

• Beriberi and Wernicke-Korsakoff syndrome. Deficiencies in vitamin B1 (thiamin) can cause neurological and cardiovascular symptoms, possibly leading to paralysis, heart failure and death.
  
  
• Pellagra. Symptoms of niacin (B3) deficiency include diarrhea, skin conditions and dementia.
  
  
• Scurvy. Deficiencies of vitamin C reflect its role in the production of the connective protein collagen. Symptoms of vitamin C deficiencies include bleeding under the skin, bone pain and loose teeth.
  
  
• Rickets and osteomalacia. Both of these diseases are characterized by soft and weak bones, resulting in skeletal abnormalities. Vitamin D deficiency in children causes rickets and osteomalacia in adults.

The following table provides some signs and symptoms of vitamin overdose and deficiency: