Can I bring bottles of omega 3 and glucosamine in and out of China?

My flight is from US-> Beijing -> Seoul -> US. I am visiting a friend in beijing for 4days and am flying to seoul with some gifts for the relatives. Can I bring 4bottles of omega3 and a bottle of glucosamine (each bottle 180 pills) from the US, into PRC and then out to Korea? Do I have to declare customs, and does that mean i have to pay duty?