Dietary acid load and bone turnover during long-duration spaceflight and bed rest

Abstract

Background: Bed rest studies document that a lower dietary acid load is associated with lower bone resorption.

Objective: We tested the effect of dietary acid load on bone metabolism during spaceflight.

Design: Controlled 4-d diets with a high or low animal protein-to-potassium (APro:K) ratio (High and Low diets, respectively) were given to 17 astronauts before and during spaceflight. Each astronaut had 1 High and 1 Low diet session before flight and 2 High and 2 Low sessions during flight, in addition to a 4-d session around flight day 30 (FD30), when crew members were to consume their typical in-flight intake. At the end of each session, blood and urine samples were collected. Calcium, total protein, energy, and sodium were maintained in each crew member's preflight and in-flight controlled diets.

Results: Relative to preflight values, N-telopeptide (NTX) and urinary calcium were higher during flight, and bone-specific alkaline phosphatase (BSAP) was higher toward the end of flight. The High and Low diets did not affect NTX, BSAP, or urinary calcium. Dietary sulfur and age were significantly associated with changes in NTX. Dietary sodium and flight day were significantly associated with urinary calcium during flight. The net endogenous acid production (NEAP) estimated from the typical dietary intake at FD30 was associated with loss of bone mineral content in the lumbar spine after the mission. The results were compared with data from a 70-d bed rest study, in which control (but not exercising) subjects' APro:K was associated with higher NTX during bed rest.

Conclusions: Long-term lowering of NEAP by increasing vegetable and fruit intake may protect against changes in loss of bone mineral content during spaceflight when adequate calcium is consumed, particularly if resistive exercise is not being performed. This trial was registered at clinicaltrials.gov as NCT01713634.

Figures

FIGURE 1

The dietary NEAP at the FD30 session (an average of 4 d) was related by Pearson correlation to the change in lumbar spine BMC after flight (r = 0.66, P < 0.01). Each point represents a single crew member (n = 17). Data from the FD30 time point, estimated from crew members’ diet for 4 d, was used as a proxy for crew members’ typical intake throughout their mission. BMC, bone mineral content; FD, flight day; L, lumbar; NEAP, net endogenous acid production.

FIGURE 2

Relation of NTX to dietary APro:K ratio before (Pre Bed Rest) and during bed rest in sedentary (Control, n = 6) and exercising subjects (n = 5). During bed rest the Pearson correlation between NTX and APro:K was significant for the Control (r = 0.82, P < 0.05) but not for the Exercise group (r = −0.84, P = 0.07). APro:K, animal protein:potassium; NTX, N-telopeptide.