Nutrition is a feature implemented somewhere during the 3.13 patches. Nutrition is quite vital to your survival, since it affects how much hunger you gain. It ranges from Abundant to Starved by reading the status under the thirst bar. There's 9 level of nutrition--Abundant, Well-fed, Nourished, Good, Temperate, Scarce, Insufficient, Malnourished and Staving. If your current nutrition level is abundant, your hunger bar increases at a relatively slow rate. If you are malnourished or starved, your hunger bar will increase drastically.
Keeping a close eye on this level is vital in order to stay alive. When you are in Starving state for a long time, the staving bar will increase and affect your physical performance.
Fish provides the most nutrition of all in the game. Salmon or Trouts are best. Among meat, the bear cuts are best. Drinking milk won't decrease the hunger bar, only decreasing the thirst bar, but it will improve the nutrition level of characters. Some tonic herbs, like milkweed and nettle, will increase the nutrition level as well.