Title: Association of Total Lifetime Breastfeeding Duration with Midlife Hand Grip Strength:
Findings from Project Viva
Version: 1 Date: 29 Mar 2022
Reviewer:  Imke Janssen
Reviewer's report:
In the paper titled "Association of Total Lifetime Breastfeeding Duration with Midlife Hand
Grip Strength: Findings from Project Viva" the authors find that longer breastfeeding duration
is positively related to midlife hand grip strength, a marker of musculoskeletal function. The
population is predominantly white and well educated and may not be generalizable, as is
mentioned in the discussion.
Major issues:
1. What is the rationale to look at 3 month intervals?  Breastfeeding for at least 1 month has
benefits (see your ref 2). In the methods section, it is actually stated that total duration of
breastfeeding was analyzed in months. In the abstract and results section, however, estimates
per 3 months are reported. This needs to be corrected.
2. Choice of covariates:
(a) Behaviors. Breastfeeding may be part of a healthy lifestyle, often conceptualized as a
combination of PA, diet, and not smoking. Smoking was still quite common about 20 years ago.
What was the relation of breastfeeding to all of these health behaviors? Adjusting for single
factors often results in overadjustment. Instead of taking all the single factors, consider creating
a summary score. A good reference is
Lloyd-Jones DM, Hong Y, Labarthe D, Mozaffarian,D, Appel LJ, Van Horn L, Greenlund
K, Daniels S, Nichol G, Tomaselli GF, et al. Defining and setting national goals for
cardiovascular health promotion and disease reduction: the American Heart Association's
strategic impact goal through 2020 and beyond. Circulation. 2010;121:586-613. doi: 10.1161/
CIRCULATIONAHA.109.192703.
(b) PA measure. What was the rationale to use an instrument designed for the elderly?
3. Figure 1 really begs the question whether it is beneficial to separate analyses by dominant/
nondominant hand instead of taking the average of all grip measurements. This would
minimize the practice effect (always a problem with repeated assessments) and therefore reduce
variability.  What does [CELLRANGE] indicate?
Minor issues:
1. “Asian” includes a wide variety of backgrounds, as do Hispanics. Could this be more detailed
in a footnote to Table 1?
2. Top of p. 7 (line 1): What does “medical” stand for? Medical conditions (examples!)? Where
is this used? Same for “other health related information”.
3. P. 7, 2nd paragraph & Supplementary Figure 1: Much is said about why it might not have
been feasible to get the hand grip measurement. Shouldn’t the first exclusion be for not attending
the mid-teen visit at all?
4. Table 1: In a column to the right, put p-values for differences between the 2 groups.
5. Table 2: All p-values should have 3 decimal places. In footnote, should it be “units” instead
of “unites”? Is it really meaningful to include “Average BFD per live birth”? Models 1, 2, and
3 use the entire cohort of 593, but Model 4 uses only 286 (48%) of the cohort. Estimates for the
different models are only comparable if results were similar in Models 1, 2, 3 for this subset. I
do not see that statement in the paper.
6. Table S1: Title should include what is presented, i.e. N (%). In a column to the right, put p-
values for differences between the 2 groups.
7. P. 16, line 4: Do you mean “modification” instead of “medication”?
8. In general, some sections sound a bit chatty and could be shortened. Unnecessary information
should be eliminated.