Foods rich in tryptophan (an amino acid), such as dairy products, can compensate for not consuming enough niacin in the diet because the body can convert tryptophan to niacin. Niacin is sometimes used in two ways: as a synonym for nicotinic acid and as a broader term that includes nicotinamide and nicotinic acid, two forms of this B vitamin.
Most babies are able to begin sitting up when they reach the age of 4 months. However, some of them might need more time and start sitting independently when they become 7 months old. Until this time, the baby would definitely have learned how to roll and would be able to hold his head up too.
Susie Alessi's son was also around 6 months when he began to sit up by himself. We did tummy time and 'baby pushups' where I would lay him across my legs and make him lift up or push up off of the floor with his hands, she says. That kind of practice is very important in helping babies reach milestones like sitting.