Starvation is measured as FOD under the Hit points (HP) bar. Starvation occurs if the Courier does not eat food after a long enough period of time. It accumulates slowly at all times, and primarily affects Strength, though Perception and Charisma also suffer at higher starvation levels. It takes five days for the Courier to starve without eating anything. Most types of food will reduce starvation by some amount, though certain foods are more effective than others.
There are no items or effects which directly increase the Courier's starvation level.
The Survival skill affects all food items, and increases the amount of starvation they reduce. A Courier with a high Survival skill will not have to carry as much food.
The amount of starvation a given consumable item will reduce may not correlate to the amount of health it provides. For example, iguana bits restore less than a package of Cram, although both items have the same weight, value, radiation, and health restoration properties.
The MREs found in Lonesome Road restore lots of starvation, up to 600 points to be precise, and are plentiful in the DLC.