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I. Twenty-five northern Thai mothers, breast-feeding their infants on demand, were studied in their homes for 24 h. All breast-feeds were test-weighed and pre-and post-feed expressed breast-milk samples (0.5 ml) taken at each feed.2. The fat concentration of milk taken during a feed showed significant circadian variation, with maximum values between 16.00 and 20.00 hours and minimum values between 04.00 and 08.00 hours. Fat concentration at the start and at the end of a feed also varied significantly over 24 h.3. Multiple regression analysis showed that the most important predictor of fat concentration at a feed was the length of time elapsed since the previous feed -the longer this interval, the lower the subsequent fat concentration. Other significant predictors were the fat concentration at the end of the previous feed, and the milk intake at the previous and at the current feed.4. Fat concentration declined between feeds in proportion to the length of time between feeds, but the decline was less between sleep feeds than between waking feeds. This would appear to be a reflection of the lower postfeed fat concentration and higher pre-feed fat concentration of sleep feeds compared with waking feeds, when other variables relating to feeding pattern are taken into account.5. The larger the milk intake at a feed, the greater was the increase in fat concentration from the start to the end of the feed. The change in fat concentration was less in feeds taking place during the sleep period than in daytime feeds.Fat is the main source of energy of the breast-fed infant. Fat is also the most variable constituent of breast-milk, fluctuating in concentration over the course of a 24 h period (Hytten, 1 9 5 4~; Prentice et al. 1981 a ; Brown et al. 1982) and also rising during individual breast-feeds (Williams et al. 1986). Information on variation in breast-milk fat concentration is useful for planning breast-milk sampling procedures to obtain representative measures of the infant's energy intake. Descriptions of variations in breast-milk composition may also contribute to an understanding of those aspects of the physiology of human lactation which are not amenable to in vivo experimentation.Most of the information on circadian variation in breast-milk fat concentration was obtained from studies carried out in the West when scheduled feeding of the infant was fashionable. Breast-milk was therefore sampled at regular intervals corresponding to feeds (

Current infant feeding guidelines recommend exclusive breast-feeding until the infant is about 4 months old to reduce the risks of early termination of breast-feeding, undernutrition and infection. In many societies, however, supplementary foods are given well before 4 months of age. The present paper describes weaning practices, factors associated with early supplementation and the effects of supplementation on duration of breast-feeding in a random sample of sixty northern Thai breast-fed infants studied prospectively from birth to 2 years of age. Composition of supplementary foods, energy and protein intake from supplements and changes in the supplementary diet with increasing infant age are also described. Rice-based foods were given from soon after birth; 81 O h of the sample had received supplements by 6 weeks of age. Early supplementary feeding was significantly associated with rural residence, large household size, maternal employment in agriculture and maternal age. Girls and infants with lower birth weights tended to be supplemented earlier. Despite early feeding of supplements, breastfeeding was prolonged, with median duration of 12 months. Early introduction of supplements and quantity of supplements consumed in the first 3 months were not associated with duration of breastfeeding. However, mothers who gave infant formula as the first supplementary food stopped breastfeeding slightly earlier, as did younger mothers living in households with more children. Breast-feeding : Supplementary feeding : Weaning