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Duration, Intensity, and Exclusivity of Breastfeeding: Recent Research Confirms the Importance of these Variables

Cindy Harmon-Jones, CPM

from Breastfeeding Abstracts, May 2006, Vol. 25, No. 3, pp. 17-20.

Historically, research on breastfeeding has defined breastfeeding arbitrarily. Definitions of breastfeeding have varied between studies and have often been based on convenience, depending on the data the researchers have available. Breastfeeding has often been treated as a categorical rather than a continuous variable, and the effects of the duration, intensity, and exclusivity of breastfeeding on study outcomes have been neglected. The lack of precision in defining breastfeeding has led to conflicting results in studies comparing breastfed and artificiallly fed infants.1

In 1991, the World Health Organization (WHO) established categories for defining breastfeeding status. 2 These categories were created to track the success of breastfeeding promotion programs. The WHO recommends 6 months of exclusive breastfeeding and continued breastfeeding with the addition of appropriate complementary foods until at least 2 years of age. However, breastfeeding rates lag behind public health goals in many parts of the world. In the US, for example, the 2002 National Immunization Survey found that only 13.3% of infants were exclusively breastfed at 6 months and only 16.1% of infants were breastfed to any degree at 12 months. 3

The WHO definition of exclusive breastfeeding is: “the infant has received only breast milk from his/her mother or a wet nurse, or expressed breast milk and no other liquids or solids, with the exception of drops or syrups consisting of vitamins, mineral supplements, or medicines.” An infant is considered exclusively breastfed if fed his or her own mother’s expressed milk or milk from a wet nurse or a milk bank. The WHO definition of predominant breastfeeding is: “The infant’s predominant source of nourishment has been breast milk. However the infant may also have received water or water-based drinks (sweetened or flavored water, teas, infusions, etc.); fruit juice; oral rehydration salts (ORS); drop and syrup forms of vitamins, minerals, and medicines; and folk fluids (in limited quantities). With the exception of fruit juice and sugar-water, no food-based fluid is allowed under this definition.” The WHO definition of partial breastfeeding is: “giving a baby some breastfeeds, and some artificial feeds, either milk or cereal, or other food.” 2 The WHO breastfeeding categories do not distinguish between feeding at the breast and feeding expressed milk, and their partial breastfeeding category does not quantify the proportion of an infant’s diet provided by breast milk.

The Index of Breastfeeding Status was created to assist researchers in quantifying the amount of breast milk received by an infant. 4 The Index of Breastfeeding Status is a 7-point ordinal scale that measures the percentage of breast milk an infant receives: 1) 100% breast milk, 2) >80% breast milk, with <20% artificial milk or solids, 3) 50%-80% breast milk, 4) 50% breast milk, 5) 20%-50% breast milk, 6) <20% breast milk, 7) 100% artificial milk and/or solids (includes weaned). The levels are mutually exclusive, with exclusive breastfeeding defined as level 1 and exclusive artificial feeding defined as level 7. This scale can be administered as a repeated measure to track the course of feeding for an individual child over time. Recently, researchers have frequently used explicit definitions of breastfeeding, often modeled after the WHO definitions. Research employing specific breastfeeding definitions demonstrates that the duration, intensity, and exclusivity of breastfeeding are important determinants of the effects of breastfeeding on health outcomes.

Duration of breastfeeding. Several recent studies have found greater protective effects of breastfeeding with longer durations of breastfeeding. A meta-analysis of the research on breastfeeding and obesity appeared in Pediatrics in 2005. 1 It examined data from 61 studies that had compared the risks of obesity for breastfed and formula-fed participants. Definitions of breastfeeding varied among studies, and in some studies the criteria for classifying an individual as breastfed were not specified. In spite of breastfeeding being poorly defined in many studies, the authors found consistent evidence of a protective effect of breastfeeding against later obesity, even after adjustment for known confounds including socioeconomic status, parental body mass index (BMI), and maternal smoking. In the 14 studies that included information about duration, the protective effect was stronger with breastfeeding durations >2 months. The protective effect of breastfeeding over later obesity was also stronger in the 4 studies that examined exclusive breastfeeding.

A recent study conducted in the Philippines found that longer durations of breastfeeding are associated with improved cognitive performance at approximately 8.5 years of age. 5 This study was unique in that it examined the association of breastfeeding and cognition in an environment where longer breastfeeding durations are negatively correlated with other factors associated with better cognitive development, such as parental education and socioeconomic status. For normal birth weight children at 8.5 years, raw scores of cognitive development were negatively associated with breastfeeding duration. However, after adjustment for confounds such as maternal education and socioeconomic status, longer durations of breastfeeding were associated with better cognitive development. In contrast, for children who had been born with low birth weights, longer durations of breastfeeding were associated with better cognitive development even in raw scores. For low birth weight children, the positive association between breastfeeding duration and cognitive development became stronger after adjustment for confounds.

Research has also found effects of the duration of lactation on maternal health. Stuebe and colleagues6 used data on parous women from the Nurses Health Study (NHS) and Nurses Health Study II (NHS II) to assess the relationship of length of lactation and the later development of Type 2 diabetes. They found that each year of breastfeeding is associated with a reduction in the likelihood of the later development of Type 2 diabetes. For women with a birth in the past 15 years, after adjustment for covariates including BMI and age, each year of breastfeeding was associated with a 15% reduction in the risk of Type 2 diabetes.

Also using data from the NHS, Karlson and colleagues7 found that longer durations of breastfeeding are associated with a reduced incidence of rheumatoid arthritis. In age-adjusted models using parous women, total lifetime breastfeeding of 12-23 months was associated with a 30% reduction in the risk of rheumatoid arthritis, while breastfeeding for 24 months or more was associated with a 50% reduction in risk.

Intensity of human milk feeding. A study by Singhal and colleagues8 examined the proportion of an infant’s diet provided by human milk in relationship to early markers of cardiovascular disease in adolescence. This study followed up a cohort of preterm infants who had been randomly assigned to receive either donated banked breast milk or formula during early infancy. Mothers had elected either to breastfeed, so that the assigned diet was received as a supplement, or not to breastfeed, so that the assigned diet was received as the infant’s sole nutrition. When participants’ blood was assessed for markers of cardiovascular disease at age 13-16, the LDL-to-HDL ratio was significantly lower for those who had received banked human milk. A greater proportion of the diet provided by human milk was associated with larger reductions in the LDL-to-HDL ratio. The level of C-reactive protein (an indicator of the inflammatory process associated with atherosclerosis) was inversely correlated with the percentage of human milk intake.

Exclusivity of breastfeeding. Perhaps the most striking findings regarding the differences between exclusive breastfeeding and partial breastfeeding have been those examining the risk of mother-to-child transmission of human immunodeficiency virus (HIV). In 2001, Coutsoudis and colleagues9 found that the cumulative probability of HIV infection was similar among exclusively breastfed and never breastfed infants through 6 months of age, while the probability of HIV infection was significantly higher for partially breastfed infants. According to WHO, “Available data and presumed biological mechanisms suggest that even small or infrequent deviations from exclusive breastfeeding may increase the risk of HIV transmission through damage to the integrity of the mucous membranes of the gut (caused by inflammation, allergic reaction, or introduction of infectious pathogens).” 2

Other research has confirmed that both the risk of mother-to-child transmission of HIV and the rate of child mortality are lower for exclusively breastfed infants than for partially breastfed infants. In a study conducted in Zimbabwe that examined infants (HIV-negative at birth) born to HIV-positive mothers, infant mortality was 1.96% for exclusively breastfed infants, 3.57% for predominantly breastfed infants, and 4.17% for mixed-fed infants. 10 (This study did not include outcomes from exclusively artificially fed infants.)

Chantry and colleagues11 conducted a study examining the relationship between the duration of full breastfeeding and the risk of respiratory infection in children 6-24 months old. They defined “full breastfeeding” (exclusive or nearly exclusive breastfeeding) as the infant receiving nothing other than breast milk on a daily basis. They found that discontinuing full breastfeeding between 4 and 6 months significantly increased the likelihood of pneumonia and recurrent otitis media, compared to continuing to fully breastfeed for at least 6 months. After adjusting for demographic and lifestyle variables associated with respiratory outcomes, the risk of recurrent otitis media doubled if full breastfeeding stopped between 4 and 6 months. The adjusted odds of contracting pneumonia in the past year were 4 times higher for children fully breastfed 4-6 months than for those fully breastfed 6 months or more.

Recent research has also found that exclusive breastfeeding contributes to maternal health. In the study that found that lactation protects women against the later development of Type 2 diabetes, the protective effects were stronger for exclusive breastfeeding. In age-adjusted models, each year of exclusive breastfeeding reduced the likelihood of later development of Type 2 diabetes by 37%, while each year of any breastfeeding reduced the likelihood of later Type 2 diabetes by 24%.6

Benefits of short-term breastfeeding. The previously reviewed data might lead one to conclude that intense, long-term breastfeeding is necessary in order for mothers and infants to derive health benefits. However, even a short duration of breastfeeding may be important to long-term health outcomes. Sorenson and colleagues12 examined data from the Copenhagen Perinatal Cohort, a large group of participants from whom health data has been collected, beginning prenatally, since 1959. They found that the risk of development of schizophrenia doubled for persons who had never been breastfed or who had been breastfed for less than 2 weeks. Longer durations of breastfeeding were not associated with a greater reduction in the risk of schizophrenia.

Conclusion. Past research has shown superior health outcomes for breastfed individuals and breastfeeding mothers, even when breastfeeding was poorly defined. However, recent research using more precise definitions of breastfeeding is providing a clearer picture of the effects of breastfeeding and artificial feeding. The evidence strongly suggests that greater duration and intensity of breastfeeding, as well as exclusivity, have important, beneficial effects on maternal and infant health, including long-term outcomes.

Cindy Harmon-Jones conducts research on the psychology of emotion and motivation, using psychophysiological measures. She has been a La Leche League Leader since 1999, a certified professional midwife since 2003, and the editor of Breastfeeding Abstracts since August, 2004.


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