Agriculture Reference
In-Depth Information
kinds of iron determine the amount of iron absorbed from a meal (Hallberg 1981).
About 40% of the iron in meat is heme iron. Its absorption is not greatly influenced
by other dietary components present in the meal, so that 15 to 35% is absorbed. In
contrast, the absorption of nonheme iron found in plant sources can vary between 2
and 35%, depending on the presence of enhancers and inhibitors in a meal as well as
an individual's iron status (Monsen et al. 1978, Dwyer 1991, Hallberg and Hulthén
2000). The most important inhibitors of iron absorption are phytates, polyphenols,
and calcium. Ascorbic acid is probably the most efficient enhancer of nonheme iron
absorption (Lynch 1997, Hallberg and Hulthén 2000). The absolute amount of ascor-
bic acid in the meal and the ratio between the concentration of ascorbic acid and iron
absorption inhibitors may be more important than the molar ratio of ascorbic acid to
iron (Lynch and Stoltzfus 2003), and sufficient amounts of ascorbic acid in a meal
can counteract the inhibition of iron absorption by phytates (Siegenberg et al. 1991).
In addition to providing considerable amounts of highly bioavailable heme iron,
meat tissue is known to have an enhancing effect on nonheme iron absorption from
other food components in the same meal, even in the presence of dietary inhibitors
(Hallberg and Hulthén 2000). Since this effect was first noted by Layrisse et al.
(1968), there have been numerous studies on the effect of meat, fish, and poultry on
iron absorption; however, the magnitude of the effect and the mechanisms involved
have not yet been conclusively resolved (Hallberg and Hulthén 2000). It had been
suggested that the so-called meat factor could be a protein per se, certain peptides or
amino acids, especially those containing cysteine, or their metabolites or unidenti-
fied components in proteinaceous foods (Taylor et al. 1986). However, more recent
findings indicate that protein and sulfhydryl groups from cysteine residues are not
contributing to iron absorption (Bæch et al. 2003). It has also been suggested that
meat factors stimulate gastric acid secretion and may chelate solubilized iron in the
acid environment of the stomach, thereby maintaining iron solubility during intes-
tinal digestion and absorption (Kim et al. 1993). Meat very effectively counteracts
the inhibition of nonheme iron absorption by phytate and polyphenols (Lynch 1997,
Hallberg and Hulthén 2000), and it seems that the inhibitory effects of phytate on
mineral absorption are not seen in varied diets containing sufficient amounts of ani-
mal protein (Zheng et al. 1993). The bioavailability of nutrients in meat and milk are
not equivalent. If meat and milk are consumed together in one meal, the calcium and
casein in milk can form insoluble complexes with iron and zinc in meat and decrease
their bioavailability (Neumann et al. 2002).
An increase in iron absorption through the addition of meat or ascorbic acid to
meals has been shown in several studies. The addition of 50 and 75 g pork meat to
a phytate-rich meal was found to increase nonheme iron absorption by 44% and
57%, respectively (Bæch et al. 2003). Iron absorption was increased by 85% if 20 g
meat were added to a weaning meal of whole wheat gruel (Hallberg et al. 2003). An
enhancing effect of about 140 and 165% on nonheme iron absorption was observed by
adding 50 mg ascorbic acid or 75 g meat, respectively, to a simple Latin-American-
type meal of maize, rice, and beans (Hallberg and Rossander 1984). In another study,
in which 50 mg ascorbic acid were added to wheat rolls with no detectable phytate,
an increase in iron absorption of 75% was found (Hallberg et al. 1989).
