Niacin effectively shuts down lipolysis in fat cells. If you were taking it away from meals, that pretty much leaves the rest of the cells in the body living off of internal reserves or liver glycogen, and of course liver glycogen isn't always so easy to come by for somebody on a low carb diet. Add to this a lack of free fatty acids for the liver to produce ketones from and energy for the process of gluconeogenesis. Taken during a low carb meal, dietary fat replaces lipolysis from fat cells. On the old HeartScan blog, I remember Dr. Davis used to say to take niacin with dietary fat, for it to be effective--this makes sense, with lipolysis decreased, dietary triglycerides should be more efficiently removed from circulation.