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Assays / Trace
Metals/ Boron
The essentiality of boron for plants and animals has long
been established and recent evidence (1) suggests it may also be
essential for humans. Possible roles for boron include the stabilisation
of connective tissue and the mediation of membrane function, both
via the condensation products of boric acid with saccharide moieties.
The neutral molecule boric acid, B(OH)3, diffuses freely across
membranes where it is readily trapped by cis-diols, probably to
specific polysaccharides, to form moderately labile condensation
products which may influence membrane function (2). In humans, dietary
boron has been shown to exert a modest positive effect on erythropoiesis
and haematopoiesis (3) and it is also reported to affect steroid
hormone metabolism. Current clinical interest in boron includes
the use of 10B labelled compounds in boron neutron capture therapy
of malignant brain tumours (4).
Average daily intakes of dietary boron in the UK are
variable, (2.8 + 1.5 mg) and higher than in the USA (1.5 + 0.4 mg).
Dietary boron is efficiently absorbed and also efficiently excreted
into urine with about 85 - 100% of an oral dose of borate appearing
in urine over a 5-7 day period5. The oral toxicity of boron is relatively
low and it has been estimated that safe population mean intakes
are <13 mg/day1 and that individuals are at risk of toxicity
when intakes exceed 100 mg/day6. The richest food sources of boron
are: nuts and dried fruits, 15-30 mg/kg and wine 8.5 mg/l. The use
of boric acid food additives are now prohibited except for caviar
at 4000 mg/kg7. Thus a toxic intake of boron could be provided by
200 g nuts plus 20 g caviar, or 25 g low fat crisps plus 12 L of
wine.
Toxicity
Most reports of boron toxicity are concerned
with the earlier use of borates as weak germicides or with deliberate
self-poisoning5. Nowadays, aqueous solutions of borate are no longer
used as antiseptic agents nor is boric acid used in skin powders
or ointments, because of the toxicity of these preparations. It
is however still possible to purchase boric acid.
Chronic boron toxicity in infants whose 'dummies' were
dipped in a preparation of borax and honey was manifest by scanty
hair, patchy dry erythema, anaemia and seizure disorders. These
features were alleviated when the use of the borax/honey preparation
was stopped1. Skin absorption of boron, via a borax dusting powder
is very high and has, in infants, produced a severe erythema with
weeping skin plus vomiting and diarrhoea with blue/green stools5,6.
Features of acute boron toxicity are profound shock, depressed circulation,
convulsions and coma. The fatal oral toxic doses for infants, adolescents
and adults are respectively: 2-3 g, 5-6 g and 15-20 g (5,6).
Laboratory Indices of Boron Status
Acute and chronically excessive uptakes of
boron are indicated by elevated concentrations in plasma, whole
blood and urine. There are a few reliable data which define the
minimum concentrations associated with toxic effects.
Because of the risk of contamination of specimens e.g.
from borosilicate glassware, one must be wary when assessing published
data.
See references 8,9,10 for the reference concentrations
quoted in the 'yellow pages'.
Extremely high concentrations reported to be associated
with non-fatal poisonings were 3 to 30 mmol/L and 1.9 to 14 mmol/L
in sera of infants and 126-215 mmol/L in serum from an adult. Similarly
high levels, 20-150 mmol/L were seen in five infants who died following
the ingestion of 4.5-14 g boric acid (7).
References:
World Health Organisation (1996). In: Trace
Elements in Human Nutrition and Health. WHO Geneva, Chapter 13,
pp 175-9
Frausto da Silva JJR and Williams RJP (1993. "The biological
Chemistry of The Elements" Oxford University Press, Oxford UK, pp58:
63: 451
Hielsen FH, Mullen LM, Neilsen EJ (1991). Dietary Boron
Affects Blood Cell Counts and Hemoglobin Concentrations in Humans."
J Trace E Exper Med, 4, 211-23.
Hatanaka H, Nakagawa Y (1994). "Clinical results on
long surviving brain tumour patients who underwent boron neutron
capture therapy". Int J Rad Oncol Bio Phys, 28, 1061-6.
Baselt RC, Cravey RH. (1989) Disposition of Toxic Drugs
and Chemicals in Man. Yearbook Medical Publications Inc, Chicago,
USA, pp 91-3.
Mervyn L (1985). The Dictionary of Minerals. Thorsons,
Wellingborough, UK pp 23-4
Commission of European Communities (1995). Food Regulations
EC Directive No XXX.
Iyengar V and Woitties J (1988). 'Trace elements in
human clinical specimens. Evaluation of literature data to identify
reference values", Clin Chem,34, 474-81.
Sabbioni E (1996). In: Trace Elements in Human Nutrition
and Health, Chapter 21 WHO, Geneva, pp 231-644
Delves HT. Unpublished observations 1994-199
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