Clinical Pharmacology and Therapeutics

CHAPTER 35
VITAMINS AND TRACE ELEMENTS
● Introduction 265
● General physiology of vitamins 265
● Vitamin A (retinoic acid) and its derivatives 265
● Vitamin B 1 (thiamine) 266
● Vitamin B 3 (niacin and nicotinic acid) 266
● Vitamin B 6 (pyridoxine) 267
● Vitamin C (ascorbic acid) 267
● Vitamin E (tocopherol) 268
● Essential fatty acids 268
● Trace elements 268
INTRODUCTION
Vitamins were discovered during investigations of clinical
syndromes that proved to be a consequence of deficiency
states (e.g. scurvy, beriberi). They are nutrients that are essential
for normal cellular function, but are required in much
smaller quantities than the aliments (carbohydrates, fats and
proteins). Vitamins are essential cofactors to or components of
enzymes that are integral in intermediary metabolism and
many other biochemical processes.
GENERAL PHYSIOLOGY OF VITAMINS
Humans are unable to synthesize adequate amounts of vitamins.
Vitamin deficiency usually results from either inadequate
dietary intake, increased demand (e.g pregnancy or
growth) or impaired absorption (e.g coeliac disease, cystic
fibrosis, pancreatic insufficiency or as a result of certain drugs,
notably orlistat which causes fat malabsorption). Vitamin deficiencies
are rarely diagnosed in the UK, but their true incidence
may be under-recognized, particularly in the elderly, alcoholics,
poor people and certain ethnic groups.
The concept that various vitamin supplements might
decrease the incidence of a variety of diseases, including cancer
and atheroma, has been under investigation. Several large
prospective placebo-controlled intervention trials have investigated
these hypotheses, but to date evidence of clear clinical
benefit is lacking. Not all vitamins are harmless when taken in
excess (especially vitamins A and D). In general vitamins
should only be prescribed for the prevention or treatment of
vitamin deficiency.
Vitamins are divided into two categories:
1. water soluble – vitamin B complex (including vitamin B 12 ,
folate, thiamine, nicotinic acid, pantothenic acid and
biotin), vitamin C;
2. fat soluble – vitamins A, D, E and K.
Vitamin B 12 and folate are discussed in Chapter 49, vitamin
D in Chapter 39, and vitamin K in Chapter 30.
Key points
Major categories of vitamins
• Originally identified by characteristic deficiency states
(now uncommon in most developed countries).
• Water-soluble vitamins include the vitamin B complex
and vitamin C.
• Fat-soluble vitamins include vitamins A, D, E and K.
• The vitamin B complex includes vitamins B 1 (thiamine),
B 6 (pyridoxine), B 12 , folate, plus B 2 (riboflavin), B 3
(nicotinic acid).
VITAMIN A (RETINOIC ACID) AND ITS
DERIVATIVES
Physiology
This vitamin exists in several forms that are interconverted.
Retinol (vitamin A 1 ) is a primary alcohol and is present in the
tissues of animals and marine fishes; 3-dehydroretinol (vitamin
A 2 ) is present in freshwater fish; retinoic acid shares some
but not all of the actions of retinol. Carotene is provitamin A
and is readily converted into retinol in the body. Vitamin A has
many physiological functions (Figure 35.1). Its deficiency
retards growth and development, and causes night blindness,
keratomalacia, dry eyes and keratinization of the skin. Dietary
sources of vitamin A include eggs, fish liver oil, liver, milk and
vegetables.
Use
Vitamin A is used to prevent and treat deficiency states. Dietary
supplementation with halibut liver oil capsules BP (containing
the daily requirement of vitamin A and vitamin D) is used to