Are we getting enough sulfur in our diet?

Abstract

Sulfur, after calcium and phosphorus, is the most abundant mineral element found in our body. It is available to us in our diets, derived almost exclusively from proteins, and yet only 2 of the 20 amino acids normally present in proteins contains sulfur. One of these amino acids, methionine, cannot be synthesized by our bodies and therefore has to be supplied by the diet. Cysteine, another sulfur containing amino acid, and a large number of key metabolic intermediates essential for life, are synthesized by us, but the process requires a steady supply of sulfur.Proteins contain between 3 and 6% of sulfur amino acids. A very small percentage of sulfur comes in the form of inorganic sulfates and other forms of organic sulfur present in foods such as garlic, onion, broccoli, etc.The minimal requirements (RDA) for all the essential amino acids have always been estimated in terms of their ability to maintain a nitrogen balance. This method asses amino acid requirements for protein synthesis, only one of the pathways that methionine follows after ingestion. To adequately evaluate the RDA for methionine, one should perform, together with a nitrogen balance a sulfur balance, something never done, neither in humans nor animals.With this in mind we decided to evaluate the dietary intake of sulfur (as sulfur amino acids) in a random population and perform sulfur balance studies in a limited number of human volunteers. Initially this was done to try and gain some information on the possible mode of action of a variety of sulfur containing compounds (chondroitin sulfate, glucosamine sulfate, and others, ) used as dietary supplements to treat diseases of the joints. Out of this study came information that suggested that a significant proportion of the population that included disproportionally the aged, may not be receiving sufficient sulfur and that these dietary supplements, were very likely exhibiting their pharmacological actions by supplying inorganic sulfur.

Figure 1

Simplified diagram that depicts the relationships between SAA, GAG synthesis, storage of cysteine as glutathione, protein synthesis and nitrogen metabolism.

Figure 2

Enzymatic conversion of arachidonic acid (AA) to PGs and the sites of inhibition by GSH and GSH peroxidase (GSSPx). (Adapted from Marglit et al. [36]).

Figure 3

Intake of SAA as part of the basic diet (dark bar) are superimposed by a 10 mmole supplement of methionine administered as a single dose on the morning of the experiment. The total height of the bar therefore represents intake of S in mmoles. In an adjacent bar is the amount of free sulfate excreted in the urine over a 24 hour period.

Figure 4

Dietary intake of SAA (methionine plus cysteine) measured in various subgroups of a population. These were compared to suggested requirements: the RDA (1989), 2× the RDA (Rose's safety margin) [4] and Tuttle et al [6] determined in older individuals. A solid bar is included at the right of each group, which represents the SAA intake reduced by 0.9 g/day, to account for the estimated loss of sulfur associated with the consumption of a standard dose of acetaminophen, excreted as a sulfated conjugate.

Figure 5

Urinary excretion of sulfates and creatinine during consumption of a standard diet, over a period of 48 hours.

Figure 6

Urinary excretion of free sulfate following a single oral dose of methyl prednisolone (24 mg) followed by a second dose of (20 mg) the following day, while ingesting a diet that supplied 19 mmoles of SAA/day.

Figure 7

Twenty four hour urinary excretion of sulfate by individuals consuming different amounts of protein combined with 10 mmoles of sulfate from a mineral water source, evenly distributed through out the daytime hours, and compared to control (case 1, basal diet: 17 mM dietary SAA, case 2, basal diet: 26 mM SAA).