Dietary Sources, Absorption, and Excretion of Magnesium

Reference from the joint report of FAO/WHO expert consultation on Human Vitamins and Minerals verbatim.

17. Koivistoinen, P. 1980. Mineral content of Finnish foods. Acta Agric. Scand. 22: 7-171.

18. Paul, A.A. & Southgate, D.A.T. 1978. The Composition of Foods. London. HMSO.

19. Tan, S.P., Wenlock, R.W. & Buss, D.H. 1985. Immigrant Foods: 2nd Suppt to the Composition of Foods. London. HMSO.

Dietary deficiency of magnesium of a severity sufficient to provoke pathologic changes is rare. Magnesium is widely distributed in plant and animal foods, and geochemical and other environmental variables rarely have a major influence on its content in foods. Most green vegetables, legume seeds, peas, beans, and nuts are rich in magnesium, as are some shellfish, spices, and soya flour, all of which usually contain more than 500 mg/kg fresh weight. 

Although most unrefined cereal grains are reasonable sources, many highly refined flours, tubers, fruits, and fungi and most oils and fats contribute little dietary magnesium (<100 mg/kg fresh weight) (17–19). Corn flour, cassava and sago flour, and polished rice flour have an extremely low magnesium content.

11. Lonnerdal, B. 1995. Magnesium nutrition of infants. Magnesium . 8: 99-105.

20. Lonnerdal, B. 1997. Effects of milk and milk components on calcium, magnesium and trace element absorption during infancy. Physiol. Revs., 77: 643-669.

Table 45 presents representative data for the dietarymagnesium intakes of infants and adults. Stable isotope studies with 25Mg and 26Mg indicate that between 50 percent and 90 percent of the labelled magnesium from maternal milk and infant formula can be absorbed by infants (11, 20).

31. Spencer, H., Lesniak, M. & Gatza, C.A., Osis, D. & Lender, M. 1980. Magnesium absorption and metabolism in patients with chronic renal failure and in patients with normal renal function. Gastroenterol., 79: 26-34.

32. Seelig, M.S. 1982. Magnesium requirements in Human nutrition. J. Med. Soc NJ., 70:

849-854.

33. Schwartz, R., Spencer, H. & Welsh, J.H. 1984. Magnesium absorption in Human subjects. Am. J. Clin. Nutr., 39: 571-576. 

Studies with adults consuming conventional diets show that the efficiency of magnesium absorption can vary greatly depending on magnesium intake (31, 32). In one study 25 percent of magnesium was absorbed when magnesium intake was high compared with 75 percent when intake was low (33).

34. Andon, M.B., Ilich, J.Z., Tzagornnis, & Matkovic, V. 1996. Magnesium balance in adolescent females consuming a low- or high-calcium diet. Am. J. Clin. Nutr., 63:950-953.

35. Abrams, S.A., Grusak, M.A., Stuff, J. & O’Brien, K.O. 1997. Calcium and magnesium balance in 9-14 year old children. Am. J. Clin. Nutr., 66: 1172-1177.

36. Sojka, J., Wastney, M., Abrams, S., Lewis, S.F., Martin, B., Weaver, C. & Peacock, M. 1997. Magnesium kinetics in adolescent girls determined using stable isotopes: effects of high and low calcium intakes. Am. J. Physiol., 273-42: R710-R715.

37. Greger, J.L., Smith, S.A. & Snedeker, S.M. 1981. Effect of dietary calcium and phosphorus magnesium, manganese and selenium in adult males. Nutr. Res., 1: 315- 325. 

During a 14-day balance study a net absorption of 52 ± 8 percent was recorded for 26 adolescent females consuming 176 mg magnesium daily (34). Although this intake is far below the US recommended dietary allowance (RDA) for this age group (280 mg/day), magnesium balance was still positive and averaged 21 mg/day. This provided one of several sets of data illustrating the homeostatic capacity of the body to adapt to a wide variety of ranges in magnesium intake (35, 36). Magnesium absorption appears to be greatest within the duodenum and ileum and occurs by both passive and active processes (37).

38. McCance, R.A. & Widdowson, E.M. 1942. Mineral metabolism on dephytinised bread. J. Physiol., 101: 304-313.

39. McCance, R.A. & Widdowson, E.M. 1942. Mineral metabolism in healthy adults on white and brown bread dietaries. J. Physiol., 101: 44-85.

40. Kelsay, J.L. Bahall, K.M. & Prather, E.S. 1979. Effect of fiber from fruit and vegetables on the metabolic responses of Human subjects. Am. J. Clin. Nutr., 32: 1876- 1880. 

High intakes of dietary fibre (40–50 g/day) lower magnesium absorption. This is probably attributable to the magnesium-binding action of phytate phosphorus associated with the fibre (38-40).

34. Andon, M.B., Ilich, J.Z., Tzagornnis, & Matkovic, V. 1996. Magnesium balance in adolescent females consuming a low- or high-calcium diet. Am. J. Clin. Nutr., 63:950-953.

35. Abrams, S.A., Grusak, M.A., Stuff, J. & O’Brien, K.O. 1997. Calcium and magnesium balance in 9-14 year old children. Am. J. Clin. Nutr., 66: 1172-1177.

36. Sojka, J., Wastney, M., Abrams, S., Lewis, S.F., Martin, B., Weaver, C. & Peacock, M. 1997. Magnesium kinetics in adolescent girls determined using stable isotopes: effects of high and low calcium intakes. Am. J. Physiol., 273-42: R710-R715.

22. Wisker, E., Nagel, R., Tamudjaja, T.K. & Feldheim, W. 1991. Calcium, magnesium, zinc and iron blances in young women. Am. J. Clin. Nutr., 54: 533-559.

However, consumption of phytate- and cellulose-rich products (usually containing high concentrations of magnesium) increases magnesium intake, which often compensates for the decrease in absorption. The effects of dietary components such as phytate on magnesium absorption are probably critically important only at low magnesium intake. There is no consistent evidence that modest increases in the intake of calcium (34-36), iron, or manganese (22) affect magnesium balance.

41. Spencer, H., Norris,C. & Williams, D. 1994. Inhibitory effect of zinc on magnesium balance and absorption in man. J. Am. Coll. Nutr., 13: 479-484.

42. Quarme, G.A. & Disks, J.H. 1986. The physiology of renal magnesium handling. Renal Physiol., 9: 257-269. 

30. Hu, J-F., Zhao, X-H. Parpia, B. & Campbell, T.C. 1993. Dietary intakes and urinary excretion of calcium and acids: a cross-sectional study of women in China. Am. J. Clin. Nutr., 58: 398-406.

37. Greger, J.L., Smith, S.A. & Snedeker, S.M. 1981. Effect of dietary calcium and phosphorus magnesium, manganese and selenium in adult males. Nutr. Res., 1: 315-325.

43. Kesteloot, H. & Joosens, J.V. 1990. The relationship between dietary intake and urinary excretion of sodium, potassium, calcium and magnesium. J. Hum. Hypertens., 4: 527-533.

In contrast, high intakes of zinc (142 mg/day) decrease magnesium absorption and contribute to a shift toward negative balance in adult males (41). The kidney has a very significant role in magnesium homeostasis. Active reabsorption of magnesium takes place in the loop of Henle in the proximal convoluted tubule and is influenced by both the urinary concentration of sodium and probably by acid-base balance (42). 

The latter relationship may well account for the observation from Chinese studies that dietary changes which result in increased urinary pH and decreased titratable acidity also reduce urinary magnesium output by 35 percent despite marked increases in dietary magnesium input for vegetable protein diets (30). 

Several studies have now shown that dietary calcium intakes in excess of 2600 mg/day (37), particularly if associated with high sodium intakes, contribute to a shift toward negative magnesium balance or enhance its urinary output (42, 43).