128 The nutrition handbook for food processors Most of the copper in human diets is supplied by vegetable foods, and vege- tarian diets generally provide a higher intake. Plant materials, however, are gen- erally less digestible than animal tissues. A substantial proportion of the copper in whole grains is associated with lectins and glycoproteins. Vegetable tissues fre- quently require more enzymatic attack to digest the copper-binding matrix than do animal proteins, which are generally more easily solubilised, so that percent- age copper absorption may in fact be substantially higher from an animal protein diet than from a plant-protein diet bb Even so, the greater copper content of a ve etarian diet is likely to provide more available copper. 7 Dairy products contain relatively little copper, with cows milk being particularly poor. Absorption is estimated at 24%o for human milk and 18% for cows milk. The quaternary protei structure is thought to exert an effect on the availability of copper in food as cooked meat has been found to supply more available copper than raw. The efficiency of absorption from food is modified by a variety of luminal factors including copper intake levels, other dietary factors and aspects of the intestinal environment. Dietary components known to modify absorption include protein, amino acids, zinc, manganese, iron, tin, molybdenum, sugars, dietary fibre and ascorbate s5 Studies of dietary protein and copper retention in young women have found ighest retention with a diet high in protein. However, copper bioavailability in high-protein foods may be decreased by heat treatments which promote conden sation reaction, such as the maillard Reaction, between sugars and amino acids The formation of products such as lactulosyl-lysine and lysinoalanine depletes the food of free amino acids. This leaves fewer sites available for the formation of organo-metallic complexes, from which copper is highly bioavailable. 7 The bioavailabilities of copper-lysine and copper-methionine complexes, relative to opper sulphate, have been reported as 120%o and 96% respectively Copper uptake by the intestinal mucosa is strongly influenced by chelation of copper ions by amino acids. Chelation may even be a mandatory requirement for copper absorption. Yet, although dietary amino acids can enhance copper absorption, when present in excess they may result in copper malabsorption possibly by competing with binding proteins on the enterocyte membrane. The ratio of chelate to metal may determine whether there is a net inhibition or pro- motion of copper uptake. In one human study, methionine supplementation was found to increase copper absorption. Animal studies have provided less straightforward results. One study of rats found that excess dietary methionine decreased indices of copper status. Jejunal copper uptake has been found to be decreased by high levels of dietary proline or histidine, while excessive cystine and cysteine have been shown to exacerbate the effects of dietary copper deficiency. 7 Cysteine is thought to decrease copper bioavailability by reducing (n) to Cu The high bioavailability of copper in human milk, compared to cow's milk may be a consequence of the two foods' protein and amino acid content. Rumi- nant milk has a higher level of low-molecular-weight ligands, which can inhibit copper absorption. In addition, copper is differently-distributed among the milksMost of the copper in human diets is supplied by vegetable foods, and vege￾tarian diets generally provide a higher intake. Plant materials, however, are gen￾erally less digestible than animal tissues. A substantial proportion of the copper in whole grains is associated with lectins and glycoproteins. Vegetable tissues fre￾quently require more enzymatic attack to digest the copper-binding matrix than do animal proteins, which are generally more easily solubilised, so that percent￾age copper absorption may in fact be substantially higher from an animal protein diet than from a plant-protein diet.66 Even so, the greater copper content of a veg￾etarian diet is likely to provide more available copper.67 Dairy products contain relatively little copper, with cow’s milk being particularly poor. Absorption is estimated at 24% for human milk and 18% for cow’s milk. The quaternary protein structure is thought to exert an effect on the availability of copper in food, as cooked meat has been found to supply more available copper than raw.64 The efficiency of absorption from food is modified by a variety of luminal factors including copper intake levels, other dietary factors and aspects of the intestinal environment. Dietary components known to modify absorption include protein, amino acids, zinc, manganese, iron, tin, molybdenum, sugars, dietary fibre and ascorbate.55 Studies of dietary protein and copper retention in young women have found highest retention with a diet high in protein.68 However, copper bioavailability in high-protein foods may be decreased by heat treatments which promote conden￾sation reaction, such as the Maillard Reaction, between sugars and amino acids.69 The formation of products such as lactulosyl-lysine and lysinoalanine depletes the food of free amino acids. This leaves fewer sites available for the formation of organo-metallic complexes, from which copper is highly bioavailable.70 The bioavailabilities of copper-lysine and copper-methionine complexes, relative to copper sulphate, have been reported as 120% and 96% respectively.71 Copper uptake by the intestinal mucosa is strongly influenced by chelation of copper ions by amino acids. Chelation may even be a mandatory requirement for copper absorption.64 Yet, although dietary amino acids can enhance copper absorption, when present in excess they may result in copper malabsorption, possibly by competing with binding proteins on the enterocyte membrane. The ratio of chelate to metal may determine whether there is a net inhibition or pro￾motion of copper uptake. In one human study, methionine supplementation was found to increase copper absorption.72 Animal studies have provided less straightforward results. One study of rats found that excess dietary methionine decreased indices of copper status.73 Jejunal copper uptake has been found to be decreased by high levels of dietary proline or histidine,74 while excessive cystine and cysteine have been shown to exacerbate the effects of dietary copper deficiency.75 Cysteine is thought to decrease copper bioavailability by reducing Cu (II) to Cu (I).71 The high bioavailability of copper in human milk, compared to cow’s milk, may be a consequence of the two foods’ protein and amino acid content. Rumi￾nant milk has a higher level of low-molecular-weight ligands, which can inhibit copper absorption. In addition, copper is differently-distributed among the milks’ 128 The nutrition handbook for food processors