Chapter 13 Vitamins and minerals
Reference Values
• Recommended daily allowance (RDA): the average daily intake that is recommended as being sufficient to meet the nutrient requirements of 97% of healthy people; it differs for children, adults, and different sexes.
• Estimated average requirement (EAR): the average daily dietary intake estimated to meet the requirements of 50% of healthy individuals; it differs for children, adults and different sexes.
• Adequate intakes (AI): the recommended average daily intake when there is insufficient scientific data available to determine an RDA. AIs meet or exceed the amount needed to maintain an adequate nutritional state in nearly all members of a specific age and sex.
• Tolerable upper intake levels (UL): the maximum daily intake likely to result in no adverse health effects. If intake above this level is reached the potential risk of adverse effects might increase.
There is now a move to change all these, amalgamating them into the dietary reference intake (DRI), which takes into account the above values.
People at Risk from Deficiencies
Any condition of the gastrointestinal tract that reduces absorption can put the individual at risk of a vitamin deficiency (see Chapter 15 ‘Methods of administration’, p. 119). Other causes include:
Metals
• Usually occur in the form of trace elements, which occur in the body in very small or ‘trace’ amounts. They generally constitute less than 0.001% of the body mass and are incorporated into structures such as haemoglobin.
Two Important Elements
Magnesium
• Structural role: found in bone and in cell membranes. Magnesium is usually combined with calcium in supplements to improve absorption.
• Involved in almost all metabolic process in the body (e.g. carbohydrate and lipid metabolism and detoxification) as a cofactor (a helper molecule required by an enzyme; see Chapter 19 ‘Pharmacodynamics: how drugs elicit a physiological effect’, p. 139).
Thus adequate levels of magnesium in the body are important.
• Facts about Magnesium
Calcium
• Necessary for muscle contraction (see Chapter 31 ‘The nervous system’, p. 237). Low levels of calcium can lead to tetany.
• Effective in reducing blood pressure in some types of hypertension, e.g. high blood pressure during pregnancy.
• Interaction of Food Substances with Calcium
• Phytates: found in whole grains; can combine with calcium (see Chapter 15 ‘Methods of administration’, p. 120).
• Oxalates: interfere with the uptake of calcium in the same food source. Spinach contains calcium but this is chemically bound to the oxalates that are also present in the spinach and is therefore not available for absorption. Similar foods are rhubarb and sweet potatoes.
• Phosphorus: ingested largely in soft drinks. Phosphorus-rich foods tend to increase the calcium content of digestive secretions. This results in a higher than usual calcium loss through removal by the faeces. Although the effect of drinking fizzy drinks is still unclear, the large quantities consumed by some children are currently causing concern.
• Caffeine: the extent to which caffeine affects the loss of calcium through the urine is contested, as there are so many factors affecting bone metabolism. A genetic factor might be important in determining the amount of loss (Rapuri et al 2001).
• Sodium: increased sodium intake results in an increased loss of calcium in the urine. This is possibly due to competition between sodium and calcium by active transport mechanisms in the kidney or the effect of sodium on the parathyroid hormone (PTH) secretion (see Chapter 37 ‘Metabolic disorders’, p. 289). Dietary sodium is thought to have considerable potential for influencing bone loss.
Trace Elements
Iron
• Myoglobin (the primary oxygen-carrying pigment of muscle): responsible for carrying oxygen in the muscle.
• Cytochromes: which form part of the electron transfer mechanism in oxidative phosphorylation (see Figure 2.5, p. 10).
• Cofactors: required by enzymes (e.g. cytochrome p450, which is used in detoxification in the liver and has a central haem core) (see Chapter 17 ‘Metabolism’, p. 129 and Chapter 19 ‘Pharmacodynamics: how drugs elicit a physiological effect’, p. 139).
• When can Iron Deficiency Occur?
• Women and teenagers with heavy menstrual loss (postmenopausal women do not lose much iron and therefore have a low risk of iron deficiency).
• Patients with kidney failure, especially those on dialysis. Due to kidneys not being able to create enough erythropoietin (see Chapter 28 ‘Blood disorders’, p. 209).
• Conditions that lead to chronic malabsorption: Crohn’s disease; chronic inflammation of the small intestine will affect absorption of not only iron but also other nutrients.
• Deficiency of vitamin A: vitamin A helps mobilize iron from its storage site; a deficiency of vitamin A will therefore reduce the body’s ability to use stored iron. This leads to a strange situation in which haemoglobin levels are low, giving the impression of iron deficiency anaemia, even though there is an adequate store of iron.
For symptoms of iron deficiency anaemia, see Chapter 28 ‘Blood disorders’ (p. 209).
• Sources of Iron
• Excessive consumption of high-fibre foods that contain phytates (see Chapter 15 ‘Methods of Administration’, p. 120): phytates can inhibit absorption.
• Large of amounts of highly processed foods with simple carbohydrates (see Chapter 9 ‘Carbohydrates’, p. 64) (Kant 2003).
There are, however, ways of increasing non-haem absorption:
• By eating foods that contain vitamin C in the form of ascorbic acid, e.g. oranges, grapefruits, tomatoes, broccoli.
Copper
Copper is a cofactor in various metabolic processes:
• Scavenging free radicals: copper is a cofactor of superoxide dismutase (SOD; see Chapter 7 ‘Free radicals’, p. 46 and Chapter 19 Pharmacodynamics: how drugs elicit a physiological effect’, p. 139).
• Neurotransmitter synthesis: conversion of dopamine to norepinephrine (noradrenaline) (see Figure 31.6, p. 242).
• Neurotransmitter degradation: degradation of the monoamine neurotransmitters (see Chapter 31 ‘The nervous system’, p. 242).
