How Eating and Drinking Works
You can only eat or drink through the inventory screen. To consume an item, double click on its icon.
Items will not be wasted by being only partially consumed. If you begin consuming an item but are satiated before you've finished it, the remainder will be saved in your inventory. The weight and calorie amount is decreased accordingly.
You may cancel eating or drinking by pressing [Esc] on PC. The item will be only partially consumed.
Eating food with low condition increases the chance of getting sick. The type of food determines how bad the risk is. For instance, dog food has a higher risk at low condition than most other food.
Some food items also have an affect on thirst. Keep an eye one which ones increase or decrease your thirst meter.
For a complete list of food items and their calorie-to-weight efficiency, see this page.