TABLE 3.
Results of 3 linear mixed-effects models investigating the relation between changes in astronaut urinary NTX excretion and HPK and LPK diets relative to a monitored diet1
| Pre- and in-flight model | In-flight model | In-flight model without outlier | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Estimate | SE | t | P value | Estimate | SE | t | P value | Estimate | SE | t | P value | |
| Measure | ||||||||||||
| Intercept | −1231 | 348 | −3.5 | <0.01 | −1106 | 397 | −2.8 | 0.01 | −641 | 374 | −1.7 | 0.09 |
| HPK diet | −147 | 75 | −2.0 | 0.06 | −218 | 80 | −2.7 | 0.01 | −120 | 78 | −1.5 | 0.13 |
| LPK diet | −61 | 69 | −0.9 | 0.39 | −24 | 71 | −0.3 | 0.74 | 32 | 67 | 0.5 | 0.64 |
| Body weight, kg | 0.2 | 2.5 | 0.1 | 0.93 | 3 | 3 | 1.2 | 0.26 | 1 | 3 | 0.5 | 0.61 |
| Female | 80 | 84 | 1.0 | 0.35 | 56 | 95 | 0.6 | 0.56 | −18 | 86 | −0.2 | 0.83 |
| Age, y | 18 | 6 | 3.1 | <0.01 | 17 | 7 | 2.5 | 0.02 | 14 | 6 | 2.4 | 0.02 |
| Energy, kcal/d | 0.00 | 0.00 | 0.3 | 0.80 | 0.04 | 0.07 | 0.6 | 0.56 | −0.01 | 0.06 | −0.1 | 0.89 |
| NEAP, mEq/d | −2 | 2 | −1.2 | 0.25 | −1 | 2 | −0.3 | 0.74 | −1 | 2 | −0.4 | 0.66 |
| Dietary sulfur, mEq/d | 8 | 3 | 2.3 | 0.03 | 6 | 3 | 1.8 | 0.08 | 5 | 3 | 1.5 | 0.15 |
| Dietary sodium, mg/d | 0.02 | 0.03 | 0.9 | 0.38 | 0.04 | 0.03 | 1.3 | 0.19 | 0.03 | 0.02 | 1.2 | 0.25 |
| Dietary fiber, g/d | −2 | 5 | −0.5 | 0.65 | −5 | 4 | −1.0 | 0.31 | −3 | 4 | −0.8 | 0.42 |
| Flight day, d | −0.2 | 0.2 | −1.2 | 0.22 | 0.04 | 0.39 | 0.1 | 0.92 | −0.26 | 0.38 | −0.7 | 0.50 |
| In-flight status | 453 | 67 | 6.7 | <0.0001 | — | — | — | — | — | — | — | — |
| Exercise, lb lifted | — | — | — | — | 0.00 | 0.00 | −0.7 | 0.47 | 0.00 | 0.00 | 0.1 | 0.94 |
| Mean CO2, mm Hg | — | — | — | — | 43 | 29 | 1.5 | 0.15 | 27 | 27 | 1.0 | 0.32 |
| Random effects | ||||||||||||
| σastronaut | 86 | 100 | 87 | |||||||||
| σ | 134 | 115 | 105 | |||||||||
1Changes and percentage changes in urinary NTX excretion (Table 2) were measured between each 4-d controlled-diet session and the Pre (L–10) session, when crew members were not on a controlled diet. The preflight and in-flight model controlled for body weight; sex; age; energy; NEAP; dietary sulfur, sodium, and fiber; flight day; and in-flight status. The in-flight model controlled for exercise (cumulative resistive exercise for 3 wk before data collection) and mean CO2. The data include all subjects (n = 17), but 1 subject (outlier) was removed from the analyses because of an extremely high response during the FD30 session relative to the preflight baseline (as discussed in Results and shown in Supplemental Figure 1). FD, flight day; HPK, high animal protein:potassium ratio; LPK, low animal protein:potassium ratio; NEAP, net endogenous acid production; NTX, N-telopeptide.