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


McCrory MA, Nommsen-Rivers LA, Mole PA, Lonnerdal B, Dewey KG.  Randomized trial of the short-term effects of dieting compared with dieting plus aerobic exercise on lactation performance. American Journal of Clinical Nutrition 1999; 69: 959-967. 

PubMed ID: 10232637
Study Design:
Randomized controlled trial
A - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:

To determine the effects of a relatively large energy deficit (35%) on the lactation performance of healthy, well-nourished women; and to compare the effects of dietary restriction combined with aerobic exercise. 

Inclusion Criteria:
  • Exclusively breastfeeding women between eight to 16 weeks postpartum
  • No chronic illnesses
  • Not taking medications regularly
  • Non-smoking
  • Delivered a single, healthy term infant
  • Willing to exercise three days a week for more than one month before the intervention.
Exclusion Criteria:

Not reported.

Description of Study Protocol:


Local physicians' offices, childbirth classes, letters to new parents. 


Randomized clinical trial, 12 weeks with 11 days intervention.

Blinding used

Not mentioned.


Total energy requirement (TER) at baseline determined by averaging energy expenditure (including breast milk output and exercise) and intake

  • Diet group (35% energy deficit, calculated as 0.65xTER): no additional exercise prescribed, continue exclusively breastfeeding throughout 11-day trial
  • Diet+Exercise group [35% net energy deficit (60% by dietary restriction, calculated as 0.65xTER+additional energy expenditure prescribed via exercise; 40%, calculated as 0.40x0.35xTER, by additional exercise)]: continue exclusively breastfeeding throughout 11-day trial.
  • Control group: No intervention, continue exclusively breastfeeding throughout 11-day trial.

Note: for diet groups, diets were individualized, food provided in pre-weighed amounts; CHO 55%; Fat 30%; Pro 15%; additional 15g protein per day proviided for lactation requirements.

Statistical Analysis

  • One-way ANOVA, chi-square analysis (group characteristics)
  • ANCOVA using change variables as outcomes, controlling for baseline (differences among groups in changes over time)
  • Tukey's significant differences test (multiple pairwise group comparisons)
  • Pearson correlations (associations among variables)
  • P<0.05 as significant. 


Data Collection Summary:

Timing of Measurements

  • Baseline period (10d-12d): dietary intake, resting metabolic rate (RMR), energy expenditure, VO2max, heart rate, body composition, milk volume and composition measured. Four days of weighed food records and detailed activity records kept.
  • Random assignment to one of three groups for 11d
  • During intervention, body composition, milk volume and composition, and plasma prolactin measured. 

Dependent Variables

  • Dietary intake
  • Energy expenditure
  • Anthropometry and body composition (height, weight, BodPod, skinfolds)
  • Plasma prolactin (basal and response to suckling, measured by immunoradiometric assay)
  • Breast milk volume (24-h milk volume, feeding frequency, total time spent breastfeeding - test-weighing; 24h alternate expression)
  • Breast milk composition
    • Lipids: gravimetric, modified Folch extraction
    • Total N (TN) and non-protein N (NPN): micro-Kjeldahl analysis; protein as 6.25X(TN-NPN)
    • Milk energy density=1.464+(0.397Xmilk lipid)
    • Milk energy output=milk energy densityXavg 24-hour milk volume).
  • Exercise frequency and intensity.

Independent Variables

Assignment to Control, Diet only, or Diet+Exercise for 11d.

Control Variables

Duration of study; timing of measurements.

Description of Actual Data Sample:


Initial N:68 F

Attrition (final N): 67 F

Age: 32±5y

Ethnicity: N=53 White; 7 Hispanic; 3 Black; 4 Asian

Other relevant demographics: 12±4wk postpartum (mean)

Anthropometrics: BMI 25.2±4.2 kg/m2 (mean); mean TER 11.4MJ/d-11.9MJ/d

Location: Department  of Nutrition and Exercise Science, University of California, Davis, California, USA.


Summary of Results:




Diet only




Significance Between Groups

Milk volume (g/d)

After Intervention

Change from Baseline











Energy Density (MJ/L)

After Intervention

Change from baseline











Energy output (MJ/d)

After Intervention

Change from Baseline











  •  Dietary energy intake and percent energy from CHO, fat andprotein did not differ significantly among the three groups at baseline, and did not change significantly between baseline and intervention periods for the two experimental groups (average 34% energy deficit acheived; Diet+Exercise group achieved this by exercising 86min/d, nine days of the trial). 
  • Weight loss and decrease in fat mass did not differ between the two experimental groups (1.9kg Diet and 1.6kg Diet+Exercise), and was minimal in the Control group (0.2 kg). Fat-free mass decreased 0.7kg in Diet group (P<0.0001), but increased by 0.1kg in Diet+Exercise and 0.2kg in the Control group (P=0.003).
  • Feeding frequency and total time spent breastfeeding did not differ among the three groups.  Changes in milk volume and milk lipid concentration (thus energy density) did not differ significantly among the groups. 
  • Lactation Performance: individual changes in milk energy output varied widely, ranging from -458-765kj/d in the control; -656-481kj/d Diet; -565-647 MJ/d Diet+Exercise.  Significant interaction between treatment group and baseline body composition (P=0.05).  Diet group: significant positive association with %BF at baseline and change in milk energy output, as well as milk non-protein N.  No association for other two groups. 
  • Change in basal prolactin differed significantly among groups after controlling for parity and basal prolactin at baseline (P=0.02): basal prolactin decreased sig more in control than in the two experimental groups.  Change in peak prolactin response to infant suckling (P=0.09) significantly different between Control and Diet groups after change in feed duraction and peak prolactin at baseline were controlled for (P=0.03).  NS differences among groups in change in area under the curve. 


Author Conclusion:

Short-term weight loss of approximately one kilogram per week through a combination of dieting and aerobic exercise seems safe for breastfeeding mothers and is preferable to weight loss achieved primarily by dieting because the latter reduces maternal lean body mass.  Longer-term studies are needed to confirm these findings. 

Funding Source:
Government: NIH
Reviewer Comments:

Limitations: Short-term randomized controlled trial (11 days of intervention); 135 women eligible for the study inquired about participation. Of these, 67 elected not to participate. Of the 68 subjects enrolled, one withdrew after assignment to the Diet+Exercise group.


Generalizability limited to educated mid-upper socioeconomic status women who were exclusively breastfeeding in the early postpartum period (self-selection bias: those electing to participate may be a different group in and of themselves; e.g. healthy, fewer barriers to diet and/or exercising, breastfeeding, etc).  (DMDV)

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? N/A
  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? No
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) ???
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  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.) N/A
  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? Yes
  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%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  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? ???
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? ???
  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? N/A
  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? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? No
  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? Yes
  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? Yes
  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