to form the α-hydroxycarboxylic esters RR'COHCOOR On the downside, organozincs are much less nucleophilic than Grignards, and they are expensive and difficult to handle. Commercially available diorganozinc compounds are dimethylzinc, diethylzinc and diphenylzinc. In one study, the active organozinc compound is obtained from much cheaper organobromine precursors. Zinc has found many uses as a catalyst in organic synthesis including asymmetric synthesis, being cheap and easily available alternative to precious metal complexes. The results (yield and enantiomeric excess) obtained with chiral zinc catalysts are comparable to those achieved with palladium, ruthenium, iridium and others, and zinc becomes a metal catalyst of choice. Dietary supplement In most single-tablet, over-the-counter, daily vitamin and mineral supplements, zinc is included in such forms as zinc oxide, zinc acetate, zinc gluconate, or zinc amino acid chelate. Generally, zinc supplement is recommended where there is high risk of zinc deficiency (such as low and middle income countries) as a preventive measure. Although zinc sulfate is a commonly used zinc form, zinc citrate, gluconate and picolinate may be valid options as well. These forms are better absorbed than zinc oxide. Gastroenteritis Zinc is an inexpensive and effective part of treatment of diarrhea among children in the developing world. Zinc becomes depleted in the body during diarrhea and replenishing zinc with a 10- to 14-day course of treatment can reduce the duration and severity of diarrheal episodes and may also prevent future episodes for as long as three months. Gastroenteritis is strongly attenuated by ingestion of zinc, possibly by direct antimicrobial action of the ions in the gastrointestinal tract, or by the absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc), or both. Common cold Weight gain Zinc deficiency may lead to loss of appetite. The use of zinc in the treatment of anorexia has been advocated since 1979. At least 15 clinical trials have shown that zinc improved weight gain in anorexia. A 1994 trial showed that zinc doubled the rate of body mass increase in the treatment of anorexia nervosa. Deficiency of other nutrients such as tyrosine, tryptophan and thiamine could contribute to this phenomenon of "malnutrition-induced malnutrition". A meta-analysis of 33 prospective intervention trials regarding zinc supplementation and its effects on the growth of children in many countries showed that zinc supplementation alone had a statistically significant effect on linear growth and body weight gain, indicating that other deficiencies that may have