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First published online October 27, 2003

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Limits to sustained energy intake VII. Milk energy output in laboratory mice at thermoneutrality

E. Król^1,* and J. R. Speakman^1,2

¹ Aberdeen Centre for Energy Regulation and Obesity (ACERO), School of Biological Sciences, University of Aberdeen, Aberdeen AB24 2TZ, UK
² ACERO, Division of Appetite and Energy Balance, Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, UK

View larger version (14K): [in a new window]   Fig. 1. Daily energy expenditure (DEE) as a function of body mass for lactating (filled circles, N=24) and non-reproductive (open circles, N=17) female mice exposed to 30°C. DEE was measured when the reproductive females were at day 14 of lactation. Body mass is the mean value for days 9–13 of lactation (reproductive females) or the mean value for eight consecutive days before the day of doubly labelled water (DLW) injection (non-reproductive females). The relationship for lactating females is described by y=–28.18+2.58x; the relationship for non-reproductive females failed to reach significance.

View larger version (12K): [in a new window]   Fig. 2. Correlation between daily energy expenditure and metabolizable energy intake in non-reproductive female mice at 30°C (r=0.75, P=0.001, N=17). The line of equality is shown.

View larger version (14K): [in a new window]   Fig. 3. Total water turnover (rH2Ofem) as a function of body mass for lactating (filled circles, y=–19.07+1.09x, N=24) and nonreproductive (open circles, y=–3.34+0.26x, N=24) female mice at 30°C. rH2Ofem was measured when the reproductive females were at day 14 of lactation. Body mass is the mean value for days 9–13 of lactation (reproductive females) or the mean value for eight consecutive days before the day of doubly labelled water (DLW) injection (non-reproductive females).

View larger version (11K): [in a new window]   Fig. 4. Total water turnover of pups (rH2Opup; N=43) at 30°C as a function of body mass. The measurements were taken on day 14 of lactation. The relationship is described by y=–0.36+0.26x.

View larger version (10K): [in a new window]   Fig. 5. Correlation between the sum of faecal (FWL), urinary (UWL) and evaporative water loss (EWL) and total water turnover (rH2Ofem) in non-reproductive female mice at 30°C (r=0.71, P<0.001, N=24). The line of equality is shown.

View larger version (14K): [in a new window]   Fig. 6. Milk energy output (MEO) of 24 female mice at 30°C, determined as the difference between metabolizable energy intake and daily energy expenditure (MEI–DEE), from female water turnover (rH2Ofem), from pup water turnover (rH2Opup) and from litter energy budget (LEB). All estimates of MEO refer to day 14 of lactation. MEO determined from female water turnover was significantly lower than the three other estimates, as denoted by the asterisk (for statistical details, see Results). Bars are means ± 1 S.D.

View larger version (11K): [in a new window]   Fig. 7. Milk energy output (MEO) of 24 female mice at 30°C, evaluated from (A) female water turnover (rH2Ofem), (B) pup water turnover (rH2Opup) and (C) litter energy budget (LEB) as a function of MEO calculated from the difference between metabolizable energy intake (MEI) and daily energy expenditure (DEE). Only correlations B (r=0.90, P<0.001) and C (r=0.95, P<0.001) are significant. The lines of equality are shown.

View larger version (13K): [in a new window]   Fig. 8. Milk energy output (MEO) determined from litter energy budget of female mice at 30°C (N=67) as a function of litter size. Both MEO and litter size are those on day 14 of lactation. Females with six pups exported more energy in milk than those with 1–3 pups. For litter sizes of 6–15, no further increase in MEO was observed (for statistical details, see Results).

View larger version (11K): [in a new window]   Fig. 9. Dry mass (A), fat (B), protein (C) and sugar (D) content of milk produced by mice exposed to hot (30°C), warm (21°C) and cold (8°C) temperatures. Milk composition data refer to day 15 of lactation. Bars are means + 1 S.D. (N as in text). Different letters above bars indicate significant differences between the temperatures (P<0.05), as assessed by ANOVA followed by Tukey pairwise comparisons.

View larger version (13K): [in a new window]   Fig. 10. Gross energy content of milk (A), the rate of milk flow (B) and milk energy output (C) in mice exposed to hot (30°C), warm (21°C) and cold (8°C) temperatures. Milk energy content refers to day 15 of lactation. Milk flow and milk energy output are calculated from female water turnover and refer to day 14 of lactation. Bars are means + 1 S.D. (N as in text). Different letters above bars indicate significant differences between the temperatures (P<0.05), as assessed by ANOVA followed by Tukey pairwise comparisons.