Birth Weight, Childhood Size, and Muscle Strength in Adult Life: Evidence from a Birth Cohort Study

Diana Kuh1, Joan Bassey2, Rebecca Hardy1, Avan Aihie Sayer3, Michael Wadsworth1 and Cyrus Cooper3

1 Medical Research Council National Survey of Health and Development, Department of Epidemiology and Public Health, Royal Free and University College Medical School, London, England.
2 School of Biomedical Sciences, Queens Medical Centre, Nottingham University, Nottingham, England.
3 Medical Research Council Environmental Epidemiology Unit, Southampton University, Southampton, England.

Received for publication March 19, 2002; accepted for publication May 7, 2002.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Environmental influences during gestation may have long-term effects on adult muscle strength. It is not known how early in adult life such effects are manifest and whether they are modified by childhood body size. The authors examined the relation between birth weight and hand grip strength in a prospective national birth cohort of 1,371 men and 1,404 women from the Medical Research Council National Survey of Health and Development who were aged 53 years in 1999. A positive relation between birth weight and adult grip strength remained after adjustment first for adult height and weight and then additionally for childhood height and weight (p = 0.006 for men and p = 0.01 for women). The effects of birth weight on grip strength did not vary by childhood or current body size and were not confounded by social class. To the authors’ knowledge, this is the first study to show that birth weight has an important influence on muscle strength in midlife independent of later body size and social class. It suggests that birth weight is related to the number of muscle fibers established by birth and that even in middle age compensating hypertrophy may be inadequate. As the inevitable loss of muscle fibers proceeds in old age, a deficit in the number of fibers could threaten quality of life and independence.

birth weight; childhood development; cohort studies; muscles


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Muscle strength has a major impact on the age-related decline in physical performance, quality of life, and independence (1, 2). Longitudinal studies of elderly people show that muscle weakness is a determinant of functional limitation, disability, falls, fractures, and mortality (38). Muscle weakness in midlife has also been shown to predict later functional limitations, disability, and mortality, most recently in a cohort of 6,000 men followed for 25–30 years (9, 10). All of these studies used hand grip strength as a measure of muscle strength (11).

There is a strong correlation between muscle strength and lean body mass as well as skeletal size (12, 13). The genetic contribution to muscle strength, as assessed by heritability studies, may be up to 65 percent (14, 15), but this still leaves scope for the influence of environmental factors. The major environmental correlate of muscle strength during adult life is physical activity. In addition, adult muscle mass and strength might be modifiable by environmental influences acting at critical periods during intrauterine and early postnatal life. Recent studies have shown that birth weight is positively correlated with adult muscle mass (1618), muscle metabolism (19), and muscle strength (20). These studies have been based on cohorts among whom information on postnatal growth, childhood socioeconomic circumstances, nutrition, and activity are fairly limited. Because the environment during childhood may modify the effects of the intrauterine environment on muscle growth, it is important that such postnatal effect modification is evaluated in cohorts of men and women with prospective information on birth weight and childhood growth and in whom muscle strength is now available in adulthood. We address this question by using prospectively derived data from the Medical Research Council National Survey of Health and Development. This cohort study is larger than the other studies and, to our knowledge, is the first to examine the relation between birth weight and muscle strength in a middle-aged population.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The Medical Research Council National Survey of Health and Development is a socially stratified sample of 2,547 women and 2,815 men followed up since their birth in England, Scotland, and Wales in the first week of March 1946 (21, 22). During a follow-up in 1999 at age 53 years, 3,386 study members were contacted, and 3,035 provided information. No attempt was made to contact the remaining 1,976 (37 percent) members of the original cohort; these study members had previously refused to take part (12 percent), were living abroad (11 percent), were untraced (5 percent), or were known to have died (9 percent). The adult sample remains representative, in socioeconomic respect, of the cohort born in England, Scotland, and Wales (23).

Measurements of muscle strength (in kilograms) were made at the home visit by using electronic handgrip dynamometers according to a standardized protocol in 1,428 men and 1,468 women (95 percent of those interviewed) (11, 24). Two values were recorded for each hand (after a practice), and the higher value was used in this analysis. The dynamometers were calibrated at the start by using a custom-built back-loading rig (25); they are accurate and stable to within 1 percent. The measurements were made by a team of nurses who were trained to use the rigs and who were encouraged to check them manually against their own known grip strength on each occasion. Each interviewer took charge of her own dynamometer. The intrasubject retest variability for maximal voluntary tests of strength for previously untested subjects is approximately plus or minus 9 percent (24). There is also an interviewer effect due to variation in interpersonal dynamics, since strong verbal encouragement was given to subjects to elicit maximal performance. This will contribute to intersubject variability and was controlled for in analyses. Despite these precautions, examination of the data revealed a cluster of rogue results from two of the 59 dynamometer/interviewer pairs. Since 39 percent of these results were either less than 15 or more than 80 kg (outside the two standard deviation limits of the data set), they were all omitted as suspect (n = 121, 4.2 percent of the sample), leaving 1,371 men and 1,404 women for the analysis.

Information on birth weight, recorded to the nearest quarter of a pound, was extracted from the birth records within a few weeks of delivery and converted into kilograms. Health visitors and school doctors measured (in inches) and weighed (in pounds) study members in their underclothes at ages 2, 4, 7, 11, and 15 years (26), and these measurements were also converted to kilograms. Height and weight at age 7 years were selected a priori as markers of prepubertal size. When study members were aged 53 years, trained research nurses weighed and measured them at home using standardized procedures described previously (27). In brief, weight at age 53 years was measured to the nearest 0.1 kg with study members wearing light clothing and no shoes. Height was measured to the nearest 0.5 cm by using a portable stadiometer, with study members standing as tall as possible, without shoes and with heels against the wall and head in the Frankfort plane. Childhood social class (in six categories) was based on the father’s occupation when the study member was age 4 years; at age 53, it was based on their own current or most recent occupation.

Statistical analyses
Mean grip strength at 53 years was first examined by birth weight, weight and height at age 7 years, and adult height and weight grouped into approximately equal fifths of their respective distributions using quintiles as cutpoints. Tests for linear trend across these groups were performed. The unequal distribution of study members in each group of the childhood measures is because the values were rounded to the nearest pound or inch at the time of measurement. Then, for 1,363 men and 1,381 women with information on birth weight and weight and height at age 53 years, the effect of birth weight on adult grip strength was tested in regression models using SPSS (28), before and after adjustment for current body size because birth weight and grip strength are positively associated with both adult height (29, 30) and lean body mass (17, 18). Since this study has no measures of lean mass, adjustments were made for current weight as well as height to control as much as possible for muscle size. All of the heights (in centimeters) and weights (in kilograms) were entered into the regression models as continuous variables centered on their mean values. Adjustments were made for the dynamometer/interviewer pair, since this contributed significantly to the variance in grip strength. Separate analyses for men and women were conducted because the variation in grip strength was greater for men than for women and because some associations showed differential effects (checked using sex-specific standard deviation scores of grip strength and tests for interaction).

We then examined the effect of adding height and weight at age 7 years to these regression models; the sample with these measures was 1,125 men and 1,163 women. The final model tested whether observed associations were independent of childhood and adult social class. Childhood and adult social classes were entered as categorical variables retaining an extra category for those with missing information or no occupation. These models were repeated using height and weight at ages 2, 4, 11, and 15 years instead of age 7. Tests of interaction were conducted to assess whether the effects of birth weight varied by childhood or current body size and whether the effects of childhood body size varied by current body size. A significance level of 5 percent was used throughout the analyses.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Characteristics of the sample are provided in table 1. Mean handgrip strength was 47.9 kg (standard deviation, 12.1) for men and 27.9 kg (standard deviation, 7.8) for women.


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TABLE 1. Characteristics of 1,371 men and 1,404 women in the National Survey of Health and Development, 1946–1999
 
Among both men and women, grip strength increased progressively with increasing weight at birth, age 7, and age 53 years and with increasing height at ages 7 and 53 years (table 2, p < 0.001 for all relations with the exception of grip strength and adult weight among women). Thus, men and women in the highest fifth of the distribution of birth weight had 10 percent greater grip strength than did their counterparts in the lowest fifth of the distribution. Grip strength was greater in men and women in the upper adult social classes than in those in the lower classes. The relation between grip strength and childhood social class was weaker and was not statistically significant.


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TABLE 2. Mean grip strength (kg) at age 53 years by fifths{dagger} of weight and height distributions at various ages and by social class in the National Survey of Health and Development, 1946–1999
 
The results of a linear regression model before adjustment for adult body size show that an extra kilogram of birth weight was associated with a 3.05-kg difference in grip strength for men and a 2.00-kg difference for women. A 1-kg difference in birth weight in this cohort was equivalent to going from the 10th to the 80th percentile of the distribution. These coefficients were considerably attenuated but remained statistically significant after adjustment for adult height (table 3, model b). The addition of adult weight (model c) reduced the effect of birth weight a little more in men. Adult weight was associated with grip strength in men but not in women (p = 0.01 for the test for interaction of current weight and sex). There were no significant interactions between the effects of birth weight and either adult height or weight on grip strength.


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TABLE 3. Mean differences in grip strength (kg) at age 53 years by birth weight and current body size estimated using multiple regression models for 1,363 men and 1,381 women from the National Survey of Health and Development, 1946–1999
 
In the restricted sample with data on childhood height and weight, the effect of birth weight on grip strength in men after controlling for current body size was somewhat stronger than the effect of birth weight in the unrestricted sample (comparing table 3, model c, with table 4, model a). When height and weight at age 7 years were included in these models, birth weight remained positively associated with grip strength in men and women, although its effect was slightly reduced in men (table 4, comparing the birth weight effect in model c with that in model a). An estimated mean increase in grip strength of 1.9 kg (95 percent confidence interval: 0.52, 3.2 kg) for a 1-kg increase in birth weight was observed for men, with the estimate for women being slightly smaller, at 1.2 kg (95 percent confidence interval: 0.29, 2.1 kg). In men, the effect of height at age 7 years was not independently associated with grip strength, but weight at age 7 years had a small additional independent effect after adjustment for current body size and birth weight (table 4, comparing model c with model b). In women, the effect of adult height was slightly strengthened and the effect of height at age 7 years was reversed after mutual adjustment, suggesting that girls who grew most after age 7 years were strongest (table 4, comparing model c with model b). Weight at age 7 years was not associated with adult grip strength in women, independent of height. In these models, there was no evidence that the effect of birth weight varied by childhood size, and neither of these effects varied by adult size. The effects of birth weight and childhood height and weight on grip strength were hardly attenuated by adjustment for social class (not shown).


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TABLE 4. Mean differences in grip strength (kg) at age 53 years by growth in childhood and current body size also adjusted for dynamometer/interviewer pair estimated using multiple regression models for 1,125 men and 1,163 women from the National Survey of Health and Development, 1946–1999
 
In the separate models including height and weight at ages 2, 4, 11, and 15 years in turn, weight at ages 4 and 15 years in men had effects on grip strength similar to weight at age 7 years that were additional to the effects of birth weight and current body size. Heights of men and heights and weights of women at these other childhood ages had no additional effects on grip strength.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In this large, national cohort, the mean values for grip strength at age 53 years were comparable with values observed in an earlier British national survey that used the same make of hand-grip dynamometer (31). The mean grip strength for men in our study (47.9 kg) lay between the value of 49.4 kg for ages 45–54 years and 43.2 kg for ages 55–64 years among men in the National Fitness Survey (31); the mean value for women (27.9 kg) in this study was also between the values for ages 45–54 years (29.9 kg) and those for ages 55–64 years (26.7 kg). The numbers in our study were larger and the response rate was higher than in the earlier survey.

Birth weight was related to adult grip strength. This simple association could be explained by a skeletal size effect, since large persons tend to have larger muscles. However, birth weight remained significantly associated with grip strength even after adjustment for later heights and weights and social class. These results support the importance of prenatal influences on muscle development that have persisting consequences through to later adulthood. This interpretation is reinforced by the absence of an effect of the earliest postnatal values of height or weight at age 2 years on grip strength after taking into account birth weight and current body size. Our findings are consistent with the only other study to examine the relation between early body size and adult grip strength of which we are aware, the British Hertfordshire cohort study (20) of 717 men and women aged 60–74 years. Strong positive associations between birth weight, weight at age 1 year, and adult grip strength were found that remained after adjustment for age, sex, adult socioeconomic status, and height. In comparison with this earlier study, our study was much larger, cohort members were considerably younger, and measures of childhood body size were available.

Our findings are also consistent with published evidence on the relation between birth weight and later muscle mass. In the National Health and Nutrition Examination studies, children who had been small for gestational age at birth were found to have a significantly smaller arm muscle cross-sectional area and more fat at age 4 years than were those who were large for gestational age (32). In a large group of young US men, thigh muscle area was significantly associated with birth weight after controlling for adult height (18). Two British cohort studies (16, 17) have reported associations between birth weight and measures of adult muscle mass: The first (16) estimated muscle mass from urinary creatinine excretion in 217 subjects aged 47–55 years; the second (17) measured lean mass using dual energy x-ray absorptiometry in 143 subjects aged 70–75 years.

There is a potential mechanism for the effect of birth weight on muscle mass. It is generally accepted that the number of fibers in mammalian muscle is determined at or soon after birth (33). Any deficit cannot be explained easily by genetic influences and may be due to undernutrition during critical periods of early development (17). This hypothesis is supported by animal studies that have shown that the number of muscle fibers can be reduced by poor intrauterine nutrition. In large litters of piglets, those positioned less advantageously for placental nutrient delivery in utero have fewer muscle fibers (34). Postnatal growth (and response to activity throughout life) occurs by hypertrophy of each fiber, and this helps to compensate for any deficit in fiber number. Our findings suggest that, even in middle age, compensating hypertrophy may be inadequate. An inevitable loss of muscle fibers contributes to the loss of muscle strength in old age (35), so persons born with fewer muscle fibers would be at considerable disadvantage later.

It may be argued that the association between birth weight and muscle mass or strength, even after adjustment for later body size, could still be due to some genetic factor. We think this is an unlikely explanation. Research findings from human (14, 36, 37) and animal (34, 38) studies suggest that the intrauterine environment has effects in addition to genetic influences. For example, one study (37) has shown that the weights of babies born after ovum donation were strongly associated with the birth weights of the recipient mother but were not associated with the birth weights of the female donors.

Childhood height was positively and indirectly related to adult grip strength through its strong association with adult height. Childhood weight and current weight were more strongly related to grip strength in men than in women. Body weight is a measure that includes contributions from both lean and fat tissue, and men have a higher percentage of lean tissue compared with women, who carry proportionately more fat. This difference might explain our observation.

In summary, this large, prospective study of men and women has shown that birth weight is strongly related to grip strength during the sixth decade of life and that this effect is independent of postnatal, childhood, or adult body size. Thus, environmental influences during gestation have long-term effects on muscle strength in adult life. This observation has not been shown before in a middle-aged population. It raises the possibility that the hypertrophy of individual muscle fibers already fails to compensate for any deficit in the numbers of muscle fibers by this age. Such a deficit would have adverse implications as the inevitable loss of muscle fibers proceeds in old age, threatening physical performance, quality of life, and independence. There are health care implications for an increasingly aged society.


    ACKNOWLEDGMENTS
 
Funded by the Medical Research Council of the United Kingdom.


    NOTES
 
Correspondence to Dr. Diana Kuh, Medical Research Council National Survey of Health and Development, Department of Epidemiology and Public Health, Royal Free and University College Medical School, Gower Street Campus, 1-19 Torrington Place, London WC1E 6BT, England. (e-mail: d.kuh{at}ucl.ac.uk). Back


    REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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