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,**,3
Department of Psychology, Aras an Phiarsaigh, Trinity College, Dublin, Ireland;
* Department of General Psychiatry, St. George’s Hospital Medical School, University of London, London, UK;
Division of Gastroenterology and Nutrition, The Children’s Hospital of Philadelphia, Philadelphia, PA 19104-4399; and
** Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine, Philadelphia, PA
3To whom correspondence should be addressed. E-mail: nstettle{at}cceb.med.upenn.edu.
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ABSTRACT |
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 TOP ABSTRACT SUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • blacks • growth • Hispanic Americans • infant nutrition • whites
Recently the epidemiological paradigm that chronic diseases are mainly explained by lifestyles in adulthood has been challenged by a large body of research demonstrating the association of early life events with chronic disease in adulthood (1 ). For example, studies of adolescent and adult obesity have indicated that, in addition to genetics, factors such as a sedentary lifestyle and patterns of dietary intake are contributing factors (2 ,3 ). However, research has also indicated the importance of early environmental factors, such as season of birth, on the development of later obesity (4 ,5 ). These studies suggest that adolescent and adult body mass index (BMI) may be associated with the month of birth, and with environmental temperature at birth and during gestation. These findings differ between genders and ethnic groups (5 ). Other studies have demonstrated the importance of season on birth weight (6 –9 ), indicating in general a higher mean birth weight during the winter period and a lower mean birth weight during the summer period. Studies investigating the influence of season on birth weight go as far back as 1941 (10 ) and are as recent as 2000 (4 ).
The importance of seasonal variations in birth weight is shown by findings that high birth weight is a predictor for the development of later obesity (11 ,12 ) and low blood pressure (13 ), whereas a low birth weight predicts hypertension, diabetes (11 ) and risk of coronary heart disease (14 ). Additionally, patterns of weight gain during infancy have been associated with the development, in adolescence and adulthood, of obesity (15 ), type 1 diabetes mellitus (16 ) and cardiovascular disease (14 ), as well as an elevated risk for suicide (17 ). A recent study indicates a negative correlation of birth weight and weight gain with psychological distress in adulthood (18 ). However, to our knowledge, there has been no human research showing whether season of birth is associated with early infancy weight gain, although the animal literature has indicated such an association (19 ).
The postnatal period has been proposed to be a highly sensitive period for the development of overweight status (20 ,21 ), as suggested by the animal literature (22 ,23 ). Seasonal variation in postnatal weight gain may be an important indicator of a possible predisposition to later obesity, as well as for other previously indicated health outcomes in adolescence and adulthood. The present study investigated the association of season of birth with birth weight and weight gain in the first 4 mo of life.
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SUBJECTS AND METHODS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The data for this study were taken from a preexisting database, which was collected for The National Collaborative Perinatal Project (CPP). The CPP was a multicenter cohort study, set up to investigate the risk factors for cerebral palsy at 12 sites in the United States between 1959 and 1965: Boston Hospital for Women and Children, Boston, MA; Children’s Hospital, State University of New York, Buffalo, NY; Charity Hospital, New Orleans, LA; Columbia Presbyterian Hospital, New York, NY; Johns Hopkins Hospital, Baltimore, MD; Medical College of Virginia, Richmond, VA; University of Minnesota Hospitals, Minneapolis, MN; University of Oregon Medical Center, Portland, OR; Metropolitan Hospital, New York, NY; Pennsylvania Hospital Philadelphia, and The Children’s Hospital of Philadelphia, Philadelphia, PA; Child Study Center, Brown University, Providence, RI; and Gailor Hospital, Memphis, TN.
Measurements.
Maternal data were collected from 58,760 women after informed consent was obtained at enrollment starting in 1959, by use of a questionnaire designed specifically for the CPP (24 ). This questionnaire was geared toward the detection of cerebral palsy, and involved an array of infant measurements, as well as maternal information. This questionnaire was slightly modified in 1962, so that not all variables were available in all cases. To eliminate the confounding potential of pre- and postterm differences in patterns of early infant weight gain, the sample was restricted to full term subjects (between 37 and 42 wk of gestation). According to the CPP study protocol, gestational age was determined at delivery by the senior physician, through the use of clinical and historical information. Because this variable was collected routinely in every site only after 1961, this analysis was restricted to the 36,442 subjects born after this date. Among these, 28,175 subjects were documented to be born at full-term gestation, and were therefore the eligible subjects for this analysis. Among these 28,175 eligible subjects, a final sample of 24,325 (86.3%) had complete information on birth weight and weight at age 4 mo. (See Table 1 for characteristics of these 24,325 subjects.)
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Statistical analysis.
The data were analyzed using Stata version 6.0 (25 ). Seasonal variation in birth weight and the association between season of birth and early infancy weight gain were analyzed separately. Descriptive analyses were conducted using the proportion of a variable of interest, or the mean and SD as appropriate. An ANOVA was used to test for the association between ethnicity or season of birth and birth weight. A separate ANOVA was used to test for the association between weight gain in infancy and ethnicity and season of birth. To test for the difference between two groups or two seasons, t tests were used. To detect potential interactions, ANOVA or ANCOVA were used with appropriate interaction terms. Finally, in the multivariate analyses, adjustment for potential confounding variable was carried out by use of multiple linear regression. All statistical tests were two-tailed, and a value of P < 0.05 was considered statistically significant.
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RESULTS |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The mean birth weight was significantly higher in white than in Puerto Rican and black infants (Table 2, F = 241.0, P < 0.0001, t test = 30.4, P < 0.0001, and t test = 9.45, P < 0.0001, respectively). In the unadjusted ANOVA, infants born in the fall had a lower birth weight than those born in the winter (Table 2, t test = 2.15, P = 0.03) and spring (t test = 2.48, P = 0.01).
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Early infancy weight gain.
The mean early infancy weight gain was higher in Puerto Rican and black than in white infants (Table 3, F = 21.4, P < 0.0001, t test = 6.00, P < 0.0001, and t test = 5.28, P < 0.0001, respectively). In the unadjusted ANOVA, infants born in the summer and fall had a lower mean rate of weight gain during the first 4 mo of life than those born in the winter (Table 3, t test = 2.19, P = 0.03, and t test = 6.17, P < 0.0001, respectively) and spring (t test = 2.38, P < 0.02, and t test = 6.27, P < 0.0001, respectively). This difference was significant in all three ethnic groups considered.
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DISCUSSION |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Because we found seasonal variations in the birth weight of full-term infants, a few theories can be advanced. For example, maternal food intake, basal metabolic rate (BMR) and weight all vary with the season. Food intake is higher in energy and carbohydrates during the fall than in the summer (26 ), BMR shows winter highs and summer lows (27 ) and body weight and fat are lower in the summer than in the winter (27 ,28 ), irrespective of indoor temperature. These factors could influence the fatty acid and lipid transfer to the fetus through maternal circulation (29 ), and as a result mediate adipose tissue aggregation prenatally.
Alternative hypotheses could be related to actual sunlight exposure and day length. Studies (30 ) have shown that over a period of 8 wk postnatally infants born in the summer had a higher level of melatonin (6SMT) secretion than those born in the winter. This is attributed to the maternal photoperiodic experience being conveyed to the fetus and the rate of maturation of the pineal gland being modified by prenatal exposure. Given that melatonin has a negative correlation with body weight (31 ), this could be a possible explanation for the influence of seasonality on birth weight as well as on weight gain. In addition to melatonin, vitamin D has recently been indicated to have a negative correlation with BMI (32 ). Low birth weight followed a period of longer maternal sunlight exposure, because such an expected higher production of vitamin D could have an indirect seasonal impact on birth weight. However, other mechanisms could explain the seasonal variations in birth weight, such as seasonal variations in vitamin C intake (33 ) or decreased physical activity during the winter (7 ). These factors need to be measured in future research to assess their possible association with birth weight.
A potential hypothesis for the variation in the described seasonal patterns between ethnic groups could be found in the levels of prenatal care utilization. White mothers have been found to use higher levels of prenatal and postnatal care, to be more likely to receive adequate care and to have more prenatal care contact (34 ,35 ). As such, it could be speculated that white infants might be protected, inadvertently, from seasonal risk factors by obstetric advice. Additionally, with reference to vitamin D absorption, racial variations in skin pigment must be taken into account.
The seasonal variation in birth weight of black infants was relatively small (
45 g for the maximum adjusted difference). However, these variations are within the range of other important determinants of birth weight from this sample and another sample of African American infants (36
): maternal age < 19 y (+25 g), maternal education < 11 y (-82 g), no prenatal care in the first 3 mo (-47 g), first-born status (-29 g) or the history of a previous fetal loss (-55 g).
Seasonal variations in postnatal weight gain could have theoretical explanations somewhat different from those for seasonal variations in birth weight. Furthermore, it is unlikely that these variations arise from seasonal variation in birth weight, given that they remained significant after adjustment for birth weight. Postnatal weight gain could be influenced by infections in early infancy, such as respiratory syncytial virus (RSV), an important respiratory tract pathogen in early childhood (37 ), which is common during the first months of life. Because RSV epidemics tend to occur from October to March (38 ), it could be hypothesized that infants who have their first months of postnatal development during the fall and winter (i.e., born in the summer or fall) have a higher rate of RSV affliction than those going through their first months during the spring and summer. As such, weight could be influenced by poor feeding and weight loss related to the illness. This factor might partially explain the ethnic variations observed in seasonal postnatal weight gain as being the result of differences in utilization of care. The lower level of prenatal and postnatal (35 ) care could increase the chances of viral infections through decreased levels of awareness and protection, thus affecting appetite loss and low weight gain. Additionally, accurate breastfeeding information, per ethnic group, would be an important factor to include in future research. Breastfeeding is a protective factor in early infant infections (such as RSV and other frequent infections), possibly protecting against seasonal variations in early infancy weight gain (39 ).
Although continuing research on seasonal variation in birth weight and weight gain during the postnatal period is important, the present study was limited by several factors, such as the lack of data on breastfeeding, vitamin C and D intake, quantity of maternal physical activity and exposure to daylight. Because much of the speculation around these findings involves climate changes throughout the year, it would be prudent to analyze the actual climatic temperature changes throughout the year as well as hours of daylight. Additionally, season of birth and weight gain in relation to conception and during the three trimesters of fetal development should be investigated to establish a complete understanding of seasonal patterns. Finally, it is unclear whether the present findings, based on a cohort born in the United States in the 1960s, can be generalized to other populations. The present study also has unique strengths. The data support and add to studies ranging from the 1940s to date, indicating that these findings could be consistent over time. It was conducted in a large multicenter and multiethnic sample, allowing comparison among three U.S. ethnic groups. The measurements were collected prospectively as part of a standardized research protocol rather than by use of clinical records.
These findings suggest that future investigations of the association between early life growth and adult chronic disease should take account the season of birth and ethnicity as possible confounding or explanatory factors.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Data were previously presented in an oral session at Nutrition Week, 23–27 February 2002, San Diego, CA. van Hanswijck de Jonge, L., Birketvedt, B. S., Stettler, N., Waller, G. Seasonal variation in birth weight and weight gain during the first four months of life in United States infants: the importance of ethnicity.
4 Abbreviations used: BMR, basal metabolic rate; CPP, Collaborative Perinatal Project; RSV, respiratory syncytial virus.
Manuscript received 2 January 2003. Revision accepted 14 February 2003.
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LITERATURE CITED |
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TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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