For both safety and practical purposes, body-water 2H-enrichment levels in humans is limited to 0.3-0.5 mol %. For attainment of 0.5 mol % body-water enrichment in a 75 kg male, a loading dose of 253 grams of pure 2H2O is required (assuming body water content is 60 % of bodyweight. For females, body water content is assumed to be 50 % of bodyweight). The development of transient vertigo - sometimes accompanied by mild nausea - is a well-documented effect of drinking a bolus of highly enriched deuterated water. To avoid this, the loading dose is divided into two or three portions, which are taken over a 1-2 h period. The deuterated water can be made more palatable by dilution with around two volumes of bottled spring water to give a ~33 %-enriched solution. The additional water does not significantly dilute the body water enrichment. Body water enrichment is maintained over the duration of the study by providing drinking water with the same 2H-enrichment as the target enrichment level. Steady state enrichment of body water is obtained within four hours of ingesting the last of the loading dose portions. For studies of fasting glucose production from glycogenolysis and gluconeogenesis, which rely on the assumption that endogenous glycogen stores are not enriched with 2H, the 2H2O must be given after meal absorption is complete, typically no earlier than 5-6 hr after the meal. In healthy fasted subjects, steady state enrichment of plasma glucose requires an additional 2-3 hr for the plasma glucose pool to be completely turned over. In diabetic patients, this may take more time if the plasma glucose pool is bigger and clearance is impaired. These factors impose a minimum fasting interval to accommodate absorption and attainment of isotopic steady state, as shown in Figure 8.1. Urinary glucuronide can be sampled slightly earlier because of the relatively small hepatic metabolite pool sizes and the fact that 'old' glucuronide can be disposed of by simply voiding the bladder. As a result, after allowing for 30-60 minutes of transit time from liver to bladder, urinary glucuronide can be harvested around one hour after
body water has reached steady state enrichment. Non-steady state measurements (such as determining the fractional rate of gluconeogenesis from the glucose 5:2 2H-enrichment ratio) can be performed as soon as there is sufficient 2H-enrichment in these positions for accurate quantification. Studies of postprandial fluxes with 2H2O utilise a similar overnight dosing procedure to establish steady-state body water enrichment ahead of the feeding protocol (Jones et al. 2006b).
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