方舟子的Blog » 奶是妈妈的好…

虽然缺少Large randomized trials，该文也算是目前的最佳证据（我能找到的）。

Authors’ conclusions
We found no objective evidence of a ‘weanling’s dilemma’. Infants who are exclusively breastfed for six months experience less morbidity from gastrointestinal infection than those who are mixed breastfed as of three or four months, and no deficits have been demonstrated in growth among infants from either developing or developed countries who are exclusively breastfed for six months or longer. Moreover, the mothers of such infants have more prolonged lactational amenorrhea. Although infants should still be managed individually so that insufficient growth or other adverse outcomes are not ignored and appropriate interventions are provided, the available evidence demonstrates no apparent risks in recommending, as a general policy, exclusive breastfeeding for the first six months of life in both developing and developed-country settings. Large randomized trials are recommended in both types of setting to rule out small effects on growth and to confirm the reported health benefits of exclusive breastfeeding for six months or beyond.

Plain language summary
Exclusive breastfeeding for six months (versus three to four months) reduces gastrointestinal infection, does not impair growth, and helps the mother lose weight

The results of two controlled trials and 18 other studies suggest that exclusive breastfeeding (no solids or liquids besides human milk, other than vitamins and medications) for six months has several advantages over exclusive breastfeeding for three to four months followed by mixed breastfeeding. These advantages include a lower risk of gastrointestinal infection, more rapid maternal weight loss after birth, and delayed return of menstrual periods. No reduced risks of other infections or of allergic diseases have been demonstrated. No adverse effects on growth have been documented with exclusive breastfeeding for six months, but a reduced level of iron has been observed in developing-country settings.