Niacin occurs naturally in various types of plant and animal products and is also added to many foods as supplement.
The best sources are protein rich foods, like lean meats, fish, poultry, peanuts and yeasts. Tuna is an excellent source of niacin.
Although milk and eggs contain only small amounts of niacin, it is quite rich in the precursor of the vitamin, trypthophane. For this reason, these foods can also be counted as significant sources.
Unrefined cereals, such as wholewheat bread, are also relatively good sources.
Add more of these foods to you diet on a daily basis if you want to increase your niacin intake.