BF: Dietary Factors, Breast Milk and Infant Outcomes (2008)


Laskey MA, Prentice A, Hanratty LA, Jarjou LM, Dibba B, Beavan SR, Cole TJ.  Bone changes after 3 mo of lactation: influence of calcium intake, breast-milk output, and vitamin-D receptor genotype.  Am J Clin Nutr. 1998; 67: 685-692. 

PubMed ID: 9537615
Study Design:
Cross-Sectional Study
D - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To investigate the relations between bone mineral changes after three months of lactation and the mother's calcium intake, breast-milk calcium concentration, breast-milk volume, postpartum weight change, contraception pill use and other factors. 

Inclusion Criteria:
  • Healthy white women
  • Age 20y to 40y
  • Living in Cambridge area
  • Recently given birth to healthy full-term infant.
Exclusion Criteria:
  • History of bone disease
  • Taking medications known to affect bone. 
Description of Study Protocol:


Advertisement and word-of-mouth.


Cross-sectional study (initial phase of a longitudinal study, please see reviewer's comment).

Statistical Analysis

  • ANOVA, ANCOVA, multiple regression analyses
  • Repeated measures ANOVA to examine effect of time on calcium intake within-subject; post hoc Scheffe test
  • Regression analyses used for bone mineral content (BMC), adjusted for bone area
  • All continuous variables except age converted to natural log.
Data Collection Summary:

Timing of Measurements

  • Baseline, early postpartum (17d±5d; range,10d to 42d postpartum): Bone mineral measurements as soon as possible, infant growth, infant feeding practices
  • Six weeks to eight weeks postpartum: Breast milk output and food diary completed at home, infant growth, infant feeding practices
  • Three months postpartum (90d±4d; range, 83d to 103d): Bone mineral measurements, infant growth, infant feeding practices.

Dependent Variables

  • Bone mineral measurements: BMC, bone area
  • Breast milk: Volume, calcium concentration
  • Calcium intake (diet, supplements, medications): Two food frequency questionnaires (at six to eight weeks and at three months); seven-day food diary at six to eight weeks
  • Vitamin D-receptor genotype.

Independent Variables

Lactation status/Infant feeding practices (fully breastfeeding); "breastfeeders" defined as mothers who breast fed for three or more months (no formula; introduced solids as they wished).

Control Variables

Timing of measurements.


Description of Actual Data Sample:


Initial N: 47 breastfeeders

Attrition (final N): 42 breastfeeders at six weeks to eight weeks; 37 breastfeeders at three months

Age: 32.0±4.0* in breastfeeders

Ethnicity: All white

Other relevant demographics: Parity range=one to two

Anthropometrics: Height, 1.65m±0.06m; weight, 67.6kg±10.5kg

Location: MRC Dunn Nutrition Centre, Cambridge, UK.


Summary of Results:


NS correlations (P>0.6) between mother's calcium (Ca) intake and breast-milk volume, Ca concentration and total Ca output (N=45).

Variables (at six weeks to eight weeks)

Fully Breastfeeding Mothers

Ca intake, mmol/d 34.8±13.2 (N=45)

Breast-milk volume, L/d

0.890±0.220 (N=37)

Ca concentration, mmol/l

 7.42±1.33 (N=40)

Ca output, mmol/d

 6.61±1.96 (N=37)

Marked decreases in BMC at three months' lactation in breastfeeding mothers, although changes were unrelated to Ca intake, breast milk Ca, vitamin D-receptor genotype, postpartum weight change or use of contraceptive pills.

Author Conclusion:

Marked bone mineral changes that were observed during three months of lactation in breastfeeding moms represented a physiologic response to lactation that was independent of dietary calcium supply. The magnitude of this effect is related to the amount of breast milk consumed by the infant and by several factors related to maternal size (e.g., height).   

Funding Source:
Foundation associated with industry:
Reviewer Comments:

This study was a longitudinal study examining bone changes (the study's primary outcome). For the breastfeeding evidence analysis project, however, bone changes are not of our interest. The key outcome for our review is the relationship between maternal calcium intake and breast-milk volume. Because both maternal calcium intake and breast-milk volume were measured at the same time in this study (six weeks to eight weeks postpartum), for this result it should be classified as a cross-sectional study.

Physical activity level before, during and after three months of lactation not reported (may have effect on BMC changes), intake, etc.

Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) Yes
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? N/A
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? N/A
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? ???
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? N/A
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? Yes
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? Yes
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? ???
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? ???
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes