Association of total lifetime breastfeeding duration with midlife grip strength: results from the Viva project | BMC Women’s Health
We used data from women who participated in Project Viva for this analysis. Project Viva is a prospective cohort study that recruited pregnant women carrying a singleton pregnancy at their first obstetrical care visit at Atrius Harvard Vanguard Medical Associates in eastern Massachusetts between 1999 and 2002 . Study exclusion criteria included multiple gestation, inability to answer questions in English, gestational age ≥ 22 weeks at enrollment, and plans to leave the study area prior to delivery. . There were 2128 births for 2100 mothers (28 women registered with more than one pregnancy). The Viva Project has attempted to follow all participants thereafter, most recently at a mid-life follow-up study visit at a mean (SD) of 18.2 (0.6) years after enrollment under study, conducted from 2017 to 2021. 
For this analysis, we included women from the Viva Project who provided information on lifetime breastfeeding duration and who were tested for grip strength at the midlife visit. A Viva Project research assistant administered the pre-test questions. Whether the participant 1) had hand or wrist surgery on both hands in the last three months or 2) was unable to hold the dynamometer with both hands (e.g. arms, hands or thumbs missing from both hands; paralysis of both hands) then the research assistant did not measure grip strength. If the participant had hand or wrist surgery on one hand within the last 3 months or could not hold the dynamometer with one hand, they performed the grip strength test using only the opposite hand. Of the 2,100 women enrolled in Project Viva, 676 provided data on grip strength at the midlife visit and 631 provided data on lifetime breastfeeding duration. Participants included in this analysis had similar socioeconomic status and race/ethnicity proportions to those without midlife follow-up data, but there was a slightly higher percentage of college graduates among those included versus excluded (75% versus 60%) (Supplementary File 2: Table S1).
We defined the primary exposure variable as the total lifetime breastfeeding duration for all pregnancies, measured continuously in 3-month increments. During the quarantine visit, women provided information on all pregnancies in their lifetime (not limited to the Viva Project Pregnancy Index) via a questionnaire, including year of pregnancy (at from which we calculated the age at first pregnancy) and the duration of breastfeeding for each pregnancy that resulted in a live birth. We calculated the total lifetime breastfeeding duration by adding the total duration of each reported pregnancy. We also calculated the total duration of breastfeeding for life classified into quartiles (with quartile 1 as the reference category), the dichotomous duration never breastfed (yes versus no) and the average duration of breastfeeding per live birth (in increments of 3 months).
The outcome variable was midlife grip strength. Trained research assistants measured grip strength during the quarantine visit in kilograms using a Jamar dynamometer. The Jamar dynamometer is a validated instrument for measuring handgrip force and serves as the gold standard in clinical and epidemiological studies [16, 17]. Using the American Society of Hand Therapists protocol, participants were instructed to squeeze the dynamometer with maximum effort for three separate attempts per hand with thirty seconds of rest in between to avoid muscle fatigue. . We used the average measurement of the three attempts separately for the dominant and non-dominant hand in our analyzes and also calculated the average of the 6 measurements.
We considered a priori covariates that may be associated with both exposure and outcome in our analysis, based on the literature review and using directed acyclic graphs. Women reported annual household income via a self-administered questionnaire . When enrolling in the study, women reported, by interview, the highest level of education and marital status. When enrolling in the study, research assistants also asked mothers, “Which of the following statements best describes your race or ethnicity?” Mothers had a choice of one or more of the following mutually exclusive racial/ethnic groups: Hispanic or Latino, White or Caucasian, Black or African American, Asian or Pacific Islander, Native American or Alaska Native, and other (please to specify). For participants who selected “other” race/ethnicity, we compared the specified responses to the US Census definition for the five other races and ethnicities and reclassified them where appropriate. If a participant chose more than one racial/ethnic group, we coded them as ‘more than one race/ethnicity’. We chose to consider race/ethnicity because we consider it to be a social construct that can affect breastfeeding duration through a variety of mechanisms; there are also differences in body composition proportions by race/ethnicity . We also chose to adjust for age at first pregnancy because it is inversely related to lifetime breastfeeding duration and could potentially confound the association between lifetime breastfeeding duration. of life and the grip strength of quarantine. . At enrollment, pregnant women reported their diet during index pregnancy using a validated food frequency questionnaire [19, 20]. The food frequency questionnaire used in the Viva project was modified for pregnant adults from the Willett FFQ used in the Nurses’ Health Study and other large cohort studies [15, 21, 22]. For our analyses, we calculated the Alternate Healthy Eating Index, slightly modified for pregnancy (AHEI-P)  as a measure of overall diet quality. At enrollment, women also reported having been physically active before pregnancy using a questionnaire modified from the Physical Activity Scale for Older Adults (PASE). . Women were asked to recall their weekly activity in the year before pregnancy and to report the average number of hours of activity per week . Total physical activity was treated as a continuous covariate (hours/week). During the quarantine visit, the women indicated whether they had ever smoked cigarettes. Older age is inversely associated with measures of grip strength . Therefore, age at grip strength assessment was identified as an accuracy covariate for grip strength, as it accounts for variation in grip strength.
We considered prepregnancy weight as a covariate, as previous studies have shown that BMI has a positive association with handgrip strength. [25, 26]. We could not adjust for weight before all pregnancies because we did not collect this variable. We collected the estimated weight status at 10 years reported at study enrollment, but after adjusting it in a sensitivity analysis this did not significantly change the results, we did not therefore not include this variable in our models.
Lifetime parity was found to be moderately associated with increased lifetime breastfeeding duration (Spearman r = 0.33, p
In our main analysis, we estimated associations between duration of breastfeeding in months (continuous, reported in 3-month increments to make estimates meaningful in magnitude) and midlife grip strength in kilograms at l using an unadjusted linear regression (model 1) and adjusted to several variables. We tested the normality of the exposure distribution and decided to use the untransformed lifetime breastfeeding duration for ease of interpretation because both the logarithm2 transformed and untransformed lifelong nursing duration produced models with normally distributed residuals. We adjusted for race/ethnicity, education, marital status, smoking status, household income at enrollment, and mother’s age at first pregnancy (Model 2). Model 3 was also adjusted for age when measuring handgrip strength due to the inverse relationship between age and handgrip strength . Finally, in sensitivity analyses, we further adjusted for diet (AHEI-P units) and pre-pregnancy physical activity (hours/week) as both can impact maternal body composition and therefore breastfeeding outcomes [26,27,28,29]. However, because we only had information on diet and physical activity before pregnancy for the index pregnancy, we limited this sensitivity analysis to women who enrolled in the Viva Project during their first pregnancy (model 4). We then repeated all regression models using quartiles of lifetime breastfeeding duration in months as exposure and compared differences in mean grip strength using the first quartile as a reference.
To assess racial/ethnic differences in exposure-outcome associations, we added an interaction term between lifetime breastfeeding duration and race/ethnicity and also examined outcomes in models stratified by race/ethnicity. We considered evidence of a significant interaction if the interaction pthe value was
We repeated models 1 to 4 using mean breastfeeding duration per live birth as a standardized indicator of breastfeeding duration regardless of parity.
We performed all analyzes using SAS version 9.4 (Cary, NC). Due to the small number of missing covariate values, we did not use multiple imputation and let the sample size decrease slightly in the multivariate models.