WATER HYDROGEN INCORPORATION INTO BODY-FAT IN PIGS - EFFECT ON DOUBLE TRIPLE-LABELED WATER METHOD

A basic assumption of the doubly labeled water (DLW) and triply labeled water (TLW) methods for measuring water flux (r(H2)O), CO2 production (r(CO2)), and fractionated water loss (X) is that the H of body water only leaves the body as water. Any loss of isotopes in other products will introduce an error into these techniques. The body fat represents the largest potential sink for water H. H-2 sequestration into the carcass fatty acids was investigated in eight pigs labeled with (H2O)-H-2 for 21 days. r(CO2) was measured simultaneously in respiration chambers to allow an accurate assessment of the effect of H-2 sequestration on the estimated r(CO2). The fat content of the diet (1.63%), level of intake, and stage of maturity were all designed to give the widest possible range of sequestration effects. Four animals were restricted to their estimated maintenance requirement and four were allowed to feed ad libitum giving a range of weight gain from 100 to 650 g/day. This was reflected in the estimated error on r(H2O) (+0.42% in the restricted group and +2.52% in the fast-growing animals) and on r(CO2) (-1.30 and -7.59%, respectively). The error on the calculation of X using TLW was +0.03 units in the restricted group and +0.20 units in the fast-growing animals. The error of +0.2 on X propagates through to an underestimate of r(CO2) of approximately 4%, and since this is additive with the error on DLW the ultimate error on r(CO2) using TLW would be approximately -12%. Extrapolation of the errors to zero weight gain indicated that there would be very little turnover of the carcass fatty acids and no significant error on either r(H2O), r(CO2), or X in weight-stable animals. However, the errors on the isotopically derived parameters of r(H2O), r(CO2), and X can be substantial during rapid growth. It would therefore be prudent to consider the likely extent of water H sequestration when interpreting isotope flux data from DLW and TLW studies when there is growth or export of tissue, particularly when this involves fatty acid synthesis.