The bioavailability of minerals is poor in legumes

This paper was published in the British Journal of Nutrition 2002 Dec;88 Suppl 3:S281-5

Study title and author:
Bioavailability of minerals in legumes.

Sandberg AS.
Department of Food Science, Chalmers University of Technology, PO Box 5401, SE 402 29 Göteborg, Sweden. ann-sofie.sandberg@fsc.chalmers.se

This study can be accessed at: http://www.ncbi.nlm.nih.gov/pubmed/12498628

Professor Ann-Sofie Sandberg is head of the Life Science Division at Chalmers University of Technology, Göteborg, Sweden. She has an extensive record in the field of phytate/bioavailability of minerals, food processing/bioavailability of nutrients and in the development of analytical methods for measuring nutrients and bioactive compounds.

Professor Sandberg reviewed the scientific evidence regarding the bioavailability of minerals in legumes.

Professor Sandberg found:
(a) The content of iron and other minerals is generally high in legumes. However, the legumes also contain antinutritional factors, such as proteinase inhibitors, lectin, raffinose oligosaccharides, saponins, polyphenols and phytate. Antinutritional factors lower the nutritional value of a food by lowering the digestibility or bioavailability of nutrients.
(b) Phytate is a well-known inhibitor of absorption of essential dietary minerals such as non-haem iron, zinc and calcium. 
(c) Some polyphenols are potent inhibitors of non-haem iron absorption.
(d) In the modern food industry, the high phytate content of soya-based infant formulas is of concern as low zinc absorption from a soya-protein-based infant formula has been found.
(e) Soya protein per se has an inhibiting effect on iron absorption.
(f) In many developing countries where diet is based on cereal and legume products, deficiency of iron and zinc, is highly prevalent.
(g) In developing countries iron deficiency, due to poor bioavailability, retards normal brain development in infants and affects the success of a pregnancy by increasing premature deliveries, as well as increased disease of mother and child at or around childbirth.
(h) Zinc deficiency prevents normal child growth and greatly weakens the immune system, leading to more infections.
(i) In industrialised countries where diet is based on cereal and legume products, deficiency of iron and zinc, is highly prevalent in certain groups such as women of fertile age, infants, adolescents and vegetarians.

The review finds that the bioavailability of minerals is poor in legumes and may lead to adverse health consequences.

Diets high in bran decrease iron absorption by over 90%

This study was published in the American Journal of Clinical Nutrition 1989 Mar;49(3):542-5

Study title and authors:
Iron absorption: no intestinal adaptation to a high-phytate diet.

Brune M, Rossander L, Hallberg L.
Department of Medicine II, University of Göteborg, Sweden.

This study can be accessed at: http://www.ncbi.nlm.nih.gov/pubmed/2538051

Brune notes that several studies have shown that bran (and its high phytate content) inhibits the absorption of iron in man.

The present study examined the possibility that a high bran and phytate intake over a long period would induce adaption in the intestines or its microflora leading to a reduction of the inhibitory effect of dietary phytates on iron absorption. Such changes might occur in vegetarians and populations in developing countries with a usually very high dietary intake of phytates. The study included 13 dedicated vegetarians or vegans who had had a high phytate intake for around 25 years who were compared with six subjects who had a normal unrestricted diet.

The effect of bran on iron absorption was studied by comparing iron absorption from wheat rolls (i) with bran and (ii) without bran in both the vegetarian and non-vegetarian groups.

The study found:
(a) The bran rolls had a large inhibitory effect on iron absorption in both vegetarians and non-vegetarians.
(b) The effect was almost identical in both groups (93% inhibition in the vegetarian group and 92% in the non-vegetarian group) even though the vegetarians had had a very high phytate intake for a long time and the non-vegetarians had a much lower phytate intake.
(c) It can be concluded that no adaption could be observed among the vegetarians despite their high phytate intake.

The study concludes that the fact their is no intestinal adaptation to a high phytate intake has wide nutritional implications, and that a diet with a high phytate content must always be considered to impair iron absorption even if phytate intake has been high for several years.

Fibre rich diet results in zinc, copper and magnesium deficiencies

This study was published in the Journal of the Trace Elements in Medicine and Biology 1996 Jun;10(2):68-76

Study title and authors:
Zinc, copper and magnesium absorption from a fibre-rich diet.

Knudsen E, Sandström B, Solgaard P.
Research Department of Human Nutrition, Royal Veterinary and Agricultural University, Frederiksberg, Denmark.

This study can be accessed at: http://www.ncbi.nlm.nih.gov/pubmed/8829129?dopt=Abstract

This study investigated the effect of a high-fibre and high-phytate diet on levels of zinc, copper and magnesium. The study lasted for 21 days and included eight healthy subjects who were given a diet of conventional foods that included 1 mmol phytic acid, 29 g dietary fiber, 140 mumol zinc, 13 mumol copper, and 9 mmol magnesium per 2,415 calories.

The study found:
(a) The subjects levels of zinc decreased by 7 mumol.
(b) The subjects levels of copper decreased by 5 mumol.
(c) The subjects levels of magnesium decreased by 3 mmol.

The study reveals that a fibre-rich diet is associated with a net loss of the nutrients zinc, copper and magnesium.