Sulfur dietary sources

Cysteine inhibits cystathionine 3-synthase and, therefore, regulates its own production to adjust for the dietary supply of cysteine. Because cysteine derives its sulfur from the essential amino acid methionine, cysteine becomes essential if the supply of methionine is inadequate for cysteine synthesis. Conversely, an adequate dietary source of cysteine spares methionine that is, it decreases the amount that must be degraded to produce cysteine. 

Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants (Allaway and Thompson, 1966). Nearly all of the cysteine and methionine is in protein. The typical dominance of protein cysteine and protein methionine in the total organic sulfur is illustrated in Table I by analyses of the sulfur components of a lower plant (Chlorella) and a higher plant (Lemna). Thede novo synthesis of cysteine and methionine is one of the key reactions in biology, comparable in importance to the reduction of carbon in photosynthesis (Allaway, 1970). This is so because all nonruminant animals studied require a dietary source of methionine or its precursor, homocysteine. Animals metabolize methionine via cysteine to inorganic sulfate. Plants complete the cycle of sulfur by reduction of inorganic sulfate back to cysteine and methionine, and are thus the ultimate source of the methionine in most animal diets (Siegel, 1975). 

This is so because non-ruminant animals require a dietary source of homocysteine, which is normally provided in the form of methionine. ibiimals metabolize these sulfur amino acids eventually to inorganic sulfate. Plants complete the cycle of sulfur by reductive assimilation of inorganic sulfate to methionine (and cysteine) (Siegel, 1975), and are thus the ultimate source of methionine in most animal diets. 

Since animals tend to concentrate in their own proteins the sulfur amino acids contained in the plants diey eat, such animal products (meat. eggs, and cheese) are valuable sources of the essential sulfur amino acids in human diets. In regions where die diet is composed almost entirely of foods of plant origin, deficiencies of sulfur amino acids may be critical in human nutrition. Frequently, persons in such areas (also voluntary vegetarians) are also likely to suffer from a number of odier dietary insufficiencies unless supplemental sources are used. 

Cereal proteins are only about 70 percent efficient for dietary replacement purposes. The reason is that cereal proteins are deficient in lysine, an essential amino acid for humans (see Amino Acid Synthesis in this section). Thus a diet based on one source of protein (e.g., corn) can lead to malnutrition. A partial solution to the problem has been the breeding of high-lysine corn. Other plant proteins, particularly those from pod seeds (e.g., peas and beans) are deficient in the sulfur-containing amino acids. A successful vegetarian diet will therefore be balanced in cereals and pod seeds. 

Now, the Physicians Desk Reference classifies inorganic ferrous sulfate as being toxic, but it is the iron source most commonly added to dietary supplements. Further, it is doubtful if inorganic iron actually corrects iron-deficiency anemia, and its poisonous nature ensures diarrhea as the body tries to rid itself of the toxin. The ubiquitous use of ferrous sulfate and other ferrous salts in multivitamins and enriched flour remains unfathomable, except maybe that this is an easy and profitable way to get rid of the stuff, a by-product from, say, sulfuric add manufacture based on iron sulfide. 

These processes in humans allow the synthesis of only 10 of the 20 amino acids required for protein synthesis. These 10 amino acids are classified as nonessential. The 10 which cannot be synthesized in humans are classified as essential, and must be obtained through the diet (see Table 20-2). Dietary problems can arise when the protein derives from a single source such as corn, which is deficient in lysine, and beans, which are deficient in sulfur-containing amino acids. 

The chemical and functional characteristics of proteins in Brazil nnt have not yet been fully explored and little information is available on the health effects of Brazil nut by-products. For example, the cake produced from Brazil nut oil extraction industries, which might be used as functional food ingredients, sources of nutraceutical extracts or dietary protein due to its high content of sulfur-containing amino acids, requires further research. 

Which macronutrient is a major source of dietary sulfur ... 

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