Stimulating Fat Tissue Storage With Niacin to Reduce Fat Accumulation in the Liver.

It will be a randomized crossover study with two 12-week treatment phases (niacin vs. placebo) with a 4-week washout period between the two treatment phases. The two 12-week treatment phases will be performed in random order. The treatment will be administered once daily, at the end of the largest meal. There will be a 3-week dose escalation: from 250mg (the first week) to 750mg from week 3 onward. The outcomes will be assessed at the end of each of these two treatment phases in all participants with metabolic visit A and B (i.e., a total of 4 metabolic visits). Each metabolic visit will last 9 hours: it will be a test meal with perfusion of stable tracers, blood sampling, PET acquisitions using radiopharmaceuticals (18FTHA and 11C-palmitate) and MRI acquisitions. The two visits A and B will be performed without and with acute administration of niacin with the test meal, respectively, to determine acute niacin-induced reduction in hepatic fatty acid flux. The two visits will be performed at four to seven-day interval, in random order during the last week of each of the treatment phase.