The debate over the optimal duration of exclusive
breastfeeding has had a long history. Growth faltering
is a commonly observed phenomenon in developing
countries after about 3 months of age.1,2 This growth
faltering has traditionally been attributed to three
factors: (1) the inadequacy of energy intake from breast
milk alone after 3 or 4 months; (2) the poor nutritional
quality (i.e., low energy and micronutrient content) of
the complementary foods commonly introduced in
many developing countries; and (3) the adverse effects
of infection on energy intake and expenditure. The
inadequacy of breast milk for energy requirements
beyond 3 or 4 months was initially based on calculations
made by the Food and Agricultural Organization (FAO)
and World Health Organization (WHO) in 1973.3 More
careful studies since the 1980s4–7 and a later FAO/WHO
report,8 however, have shown that the earlier FAO/
WHO figures substantially overestimate true energy
requirements in infancy.4–7