Vitamins and minerals within an egg
The body requires different amounts of each vitamin and mineral because each has a different function. People have different requirements according to their age, sex, level of activity and state of health. Eggs contain some of most of the recognised vitamins and minerals that help to maintain essential bodily functions.
Vitamin D
In particular, eggs and specifically egg yolks, are one of the few naturally occurring, significant dietary sources of vitamin D, alongside oily fish, liver and meat. It is now recognised that vitamin D is not only important for bone health, but has other key roles in modulating immune and neuromuscular function and inflammation [1]. There is a raft of emerging evidence from both animal and epidemiological studies suggesting that vitamin D status may influence cell ageing and that vitamin D is involved in the aetiology of various chronic disorders, including type 1 and type 2 diabetes, cardiovascular disease, some cancers and multiple sclerosis [1, 2], although the evidence is not as yet conclusive [1].
Sub-optimal vitamin D status is evident to some degree in most age groups in the UK [3], but there is currently no consensus on the appropriate range of blood vitamin D levels for maintenance of optimal health [2]. Skin synthesis of vitamin D following exposure to sunlight remains the most important source of vitamin D for most people, but excessive sun exposure is considered to be the cause of most cases of skin cancer, especially of melanoma, the most rapidly increasing type of skin cancer in the UK, and there has been considerable emphasis on skin protection [4]. Consequently there is increased focus on the importance of dietary and supplemental sources of vitamin D, especially for individuals with low sun exposure [2].
Carotenoids
There is also evidence that other substances found in eggs that are not classified as nutrients - the carotenoids lutein and zeaxanthin - might be involved in the prevention of cataracts and age-related macular degeneration (AMD), a major cause of blindness in elderly people, although this is the subject of continuing debate [5,6].
Choline
Eggs are also rich in choline, an essential component of all cells. There is no RNI for choline in the UK, but on the basis of emerging evidence, the US Institute of Medicine has set Adequate Intake (AI) recommendations for choline, with the caveat that it may not be a dietary essential at all stage in the lifecycle [7,8].
Like folate and some other B vitamins, choline provides methyl groups for intermediary metabolism. Choline is essential to the integrity and function of cell membranes and animal studies suggest that it has a role in normal development of the brain and of memory [9,10]. On this basis, it has been suggested that choline may be particularly important for women of child-bearing age [10]. On the other hand, there is evidence that oestrogen may induce the endogenous synthesis of choline, suggesting that men and postmenopausal women may be more sensitive to choline depletion [8]. Therefore the US Institute of Medicine have set an AI of 550mg per day for men and 425mg/day for women [7]. Genetic variations in folate metabolism also appear to influence susceptibility to choline depletion [11].
Recent research suggests that a lack of choline may influence breast cancer risk. In a case-control study of more than 3,000 adult women, risks of developing breast cancer were observed to be lowest among those with the highest choline intakes, most of which was derived from coffee, skimmed milk and eggs [12]. Higher choline intakes were also shown to be associated with lower all cause and breast cancer-specific mortality rates [13].
The three tables below indicate the more recognised nutrients found in eggs and their benefits.
Fat Soluble Vitamins
| Vitamins | Main Functions | Sources | Deficiency | Excess |
|---|---|---|---|---|
| A | Essential for vision in dim light; necessary for maintenance of mucous membranes; skin and growth. | As retinol in milk, fortified margarine, butter, cheese, egg yolk, liver and fatty fish. As carotenes in milk, carrots, tomatoes, dark green vegetables. | Reduced night vision; loss of sight through gradual damage to the cornea. Lowered resistance to infection. | Vitamin A is stored in the liver and toxicity can occur. |
| D | Promotes calcium and phosphate absorption from food and is thus essential for bones and teeth. | Sunshine, fortified margarine, oily fish, egg yolk, fortified breakfast cereals. | Failure of bones to grow and calcify leading to rickets in children and osteomalacia in adults. | Vitamin D can be toxic when consumed in large doses in supplement form. |
| E | Protects cell membranes from damage by oxidation. | Eggs, vegetable oils, nuts, vegetables and cereals. | Deficiency may occur in premature infants or due to malabsorption. | Not Known. |
Water Soluble Vitamins
| Vitamins | Main Functions | Sources | Deficiency | Excess |
|---|---|---|---|---|
| Thiamin (B1) | Involved in the release of energy from carbohydrate. It is important for the brain and nerves, which use glucose for their energy needs. | Cereals, nuts and pulses are rich sources. Green vegetables, pork, fruits and fortified cereals contain thiamin. | Deficiency leads to beriberi. Alcoholics sometimes develop deficiency. | The body excretes excess thiamin. |
| Riboflavin (B2) | Involved in energy release, especially from fat and protein. | Rich sources are liver, milk, cheese, yoghurt, eggs, green vegetables and yeast extract, and fortified cereals. | Deficiency includes changes to the mucous membrane and skin around the mouth and nose. | The body excretes excess riboflavin. No known adverse effect. |
| Vitamin B6 (Pyridoxine) | Involved in the metabolism of protein. | Found in a variety of foods: beef, fish and poultry are rich sources. | Deficiency may occur as a complication of disease and drug effects. | Very high intakes (from vitamin supplements) may be harmful. |
| B12 | Is necessary for the proper formation of blood cells and functioning of the nervous system. |
Rich sources are offal, meat and eggs. Dairy products such as milk also contain B12. Almost no plant foods contain B12. Fortified breakfast cereals are a useful source. | Deficiency leads to pernicious anaemia. | No toxic effects known. |
| Folate | Essential to cell division and in the formation of blood cells. Also involved in homocysteine metabolism. Adequate folate intake prior to conception and in the early stages of pregnancy reduces the risk of neural tube defects in babies. |
Liver, orange juice, dark green vegetables are rich sources. Nuts, wholemeal bread, and fortified breakfast cereals and eggs are also sources. | Deficiency leads to megaloblastic anaemia. | No toxic effects known. |
Minerals
| Mineral | Main Functions | Sources | Deficiency |
Excess |
|---|---|---|---|---|
| Phosphorus | 80% of the phosphorus in the body is present as calcium salts in the skeleton. | Phosphorus is present in all plant and animal cells and is essential to metabolism and membrane function as well as to the structure of bones and teeth. | Affects calcium balance. | |
| Calcium | Calcium is the main constituent of hydroxyapatite, the principal mineral in bones and teeth. An adequate calcium intake is vital to health, particularly in times of growth, e.g. childhood, adolescence, pregnancy, and also during lactation. | In milk and dairy products calcium is readily available, but it is usually less available from plant foods. Calcium is sometimes bound by phytates (found in wholegrain cereals and pulses) and oxalates (found in spinach and rhubarb) that reduce its availability. | Deficiency of calcium in bones can result from an inadequate supply of vitamin D, which is essential for its absorption. This condition is known as rickets in children and osteomalacia in adults. | No known toxic effects. |
| Iron | Iron is required for the formation of haemoglobin in red blood cells, which transport oxygen around the body. | Iron is found in plant and animal sources. Bread and many breakfast cereals are fortified with iron in the UK. Iron is particularly well-absorbed from haem sources such as red meat. The absorption of iron from non-haem sources such as eggs is enhanced by the presence of vitamin C. | A lack of iron leads to lower iron stores in the body and eventually to iron deficiency anaemia. Low iron status is associated with poor physical and mental performance. | No known toxic effects. |
| Iodine | Iodine is used to make thyroid hormones, which control many metabolic activities. | Seafood, salt and bread, dairy products and eggs. | Lethargy and swelling of the thyroid gland in the neck to form a goitre. Nowadays this is rare in the UK. Infants born of severely iodine deficient mothers may be mentally retarded (cretinism). | No known toxic effects. |
| Selenium |
It protects DNA, protein and lipid in cells against oxidation. |
Cereals, meat, fish, offal, cheese and eggs. Dietary selenium levels in the UK have declined in recent years, largely due to shifting wheat supplies from North American to European sources. | Keshan disease (type of heart disease). | Excess selenium is toxic. |
| Zinc | Essential for growth, sexual maturation and immunity. Involved in enzyme activity and taste perception. |
Milk, cheese, meat, eggs and fish, wholegrain cereals and pulses. | Dietary deficiency is rare; may cause delayed puberty and retarded growth. | Interferes with copper metabolism. |
References
1) NIH (US National Institutes of Health) Dietary supplement fact sheet: Vitamin D. 2009
2) Ruxton, CHS, Derbyshire E. Health impacts of vitamin D: are we getting enough? Nutrition Bulletin, 341,185-97, 2009
3) SACN (Scientific Advisory Committee on Nutrition) Update on Vitamin D. London. TSO, 2007
4) CRUK (Cancer Research UK) Sunsmart campaign.
5) Moeller, SM, Jacques PF, Blumberg JB, The Potential of Dietary Xanthophylls in Cataract and Age-Related Macular Degeneration, Journal of the American College of Nutrition, 19, 522S, 2000
6) Chong, E W-T, Wong TY, Kreis AJ, Simpson JA, Guymer RH, Dietary antioxidants and primary prevention of age related macular degeneration: systematic review and meta-analysis. British Medical Journal, 335, 755, 2007
7) Dietary Reference Intakes, Institute of Medicine of the National Academies, National Academies Press, Washington, DC, 2006.
8) Dietary Reference Intakes Research Synthesis: Workshop Summary (2006)
9) Zeisel SH, Choline: Needed for Normal Development of Memory, Journal of the American College of Nutrition, 19, 528S, 2000
10) Zeisel SH, Nutritional importance of choline for brain development. Journal of the American College of Nutrition, 23, 621S-626S, 2004
11) Kohlmeier M, da Costa K-A, Fischer L, Zeisel SH, Genetic variation of folate-mediated one-carbon transfer pathway predicts susceptibility to choline deficiency in humans. Proceedings of the National Academy of Sciences, 102, 16025-16030, 2005.
12) Xu X, Gammon, M. D, Zeisel, SH, Lee YL, Wetmur JG, Teitelbaum SL., Bradshaw PT, Neugut, AI, Santella RM, Chen J, Choline metabolism and risk of breast cancer in a population-based study. The FASEB Journal, 22, 2045-2052, 2008.
13) Xu X, Gammon MD, Zeisel, SH, Lee YL, Wetmur JG, Teitelbaum SL., Bradshaw PT, Neugut, AI, Santella RM, Chen J, High intakes of choline and betaine reduce breast cancer mortality in a population-based study. The FASEB Journal, 23, 4022 – 4028, 2009.