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The Scientific Evidence
The optimal duration of exclusive breastfeeding – and thus the optimal timing of when complementary feeding should begin – is an important public health issue that WHO keeps under continual review. Consistent with available scientific and epidemiological evidence, WHO's current recommendation is:
Infants should be exclusively breastfed for the first 4 to 6 months of life. Thereafter, to meet their evolving nutritional requirements, infants should begin to receive nutritionally adequate and safe complementary foods while breastfeeding continues for up to two years of age or beyond.
In 1995 WHO published the report of the WHO Expert Committee on physical status; this included a meticulous review of available scientific evidence, including the Organisation's own research, on the influence of exclusive and non-exclusive breastfeeding patterns on the growth of infants and children in a number of countries in disparate regions. The evidence showed that, in predominantly breastfed infants, weight and growth velocity begin to fall at 3 months of age when judged against the current NCHS/WHO reference. In contrast, weight and growth velocity begin to fall at about 5 months of age when judged against a trial breastfed growth reference. This suggested that whatever the underlying cause (e.g. inadequate nutrient supply, infection) nutrient intake in predominantly breastfed infants may become insufficient to meet their normal growth requirements from about 5 months of age (or even earlier, depending on the growth reference used). It was also clear that there was considerable variation in the age at which this insufficiency occurred.
On this basis the WHO Expert Committee and its Working Group on Infant Growth:
- reconfirmed the suitability of WHO's current recommended timing of the introduction of complementary foods – that is, 4–6 months of age; and
- urged that a new growth reference be developed based on breastfed infants who are living under conditions that favour achievement of genetic growth potential.
Given the worldwide variation in growth velocity and other health and development outcomes, an age range is an essential element of WHO's population–based infant feeding recommendation. Since the publication of the Expert Committee's findings in 1995, there have been a number of additional studies and reviews, including a WHO–commissioned scientific review on complementary feeding (document WHO/NUT/98.1), a randomized intervention study in Honduras (Cohen et al., The Lancet 1994;343;288-93), and several other recent studies and reviews (e.g. Werk & Alpert, The Lancet 1998;352:1569 and The WHO Collaborative Study Team on the Role of Breastfeeding on the Prevention of Infant Mortality, The Lancet 2000;355:451-55). While contributing in a variety of important ways to the growing body of knowledge on the topic, study and review results have not, by themselves, warranted any change, up or down, in WHO's current infant feeding recommendation. Meanwhile, a systematic review presently under way will help ensure that WHO's infant feeding recommendation continues to reflect the most up-to-date globally applicable scientific and epidemiological evidence. In addition, a multicentre study under way in 6 countries is also expected, among other things, to contribute to improved understanding of the age range during which breast milk alone is sufficient to meet the healthy infant's nutritional requirements.
A Systematic Review of the Optimal Length of Exclusive Breastfeeding
WHO is currently undertaking a systematic review of the relevant scientific literature in accordance with the Cochrane Collaboration's criteria and framework. There are several advantages to using this internationally recognised approach. For example, the use of explicit, systematic methods in reviews limits bias (systematic errors) and reduces random errors (simple mistakes), thus providing more reliable results upon which to draw conclusions and make decisions. Meta-analysis, the use of statistical methods to summarize the results of independent studies, can provide more precise estimates of the effects of interventions than those derived from the individual studies included in a review. Results will be submitted to the Cochrane Library for appraisal and inclusion in its database of systematic reviews.
The aim of the systematic review is to examine and draw conclusions from the published scientific literature on the optimal duration of exclusive breastfeeding. The main outcomes being looked at include infant growth, morbidity and mortality; breast-milk nutrient intake versus requirements; child development outcomes; and influence of environmental contamination. The systematic review will include a separate review of observational studies, which is to say that it will not be limited to randomized clinical trials. Once the systematic review is complete, it will be sent for external peer review by a number of experts. Following the peer review, a WHO scientific working group will be convened in Geneva to review the evidence and, on this basis, formulate its recommendation on the optimal length of exclusive breastfeeding.
The review's findings, including their implications for WHO's current infant feeding recommendation, will be available early in 2001.
The WHO Multicentre Growth Reference Study
Consistent with the recommendation of the Expert Committee on physical status, WHO is currently conducting a multi-country study involving more than 10 000 children from geographically and ethnically diverse sites. The study combines a longitudinal component from birth to 24 months with a cross-sectional study from 18 to 71 months. The aim is to establish a new international growth reference that reflects growth patterns of healthy breastfed infants and children living in environments that do not constrain growth. The new reference will establish the breastfed infant as the normative model against which all alternative-feeding methods must be measured in terms of growth, health and development. The prospective follow-up from birth to 24 months, involving 1800 infants, will include assessments at fortnightly intervals from birth to 2 months, and monthly from 3-12 months of age, of breastfeeding and complementary feeding patterns; anthropometry; and health, morbidity, mortality and other indicators. The study's findings will be available in 2003.
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