In addition to a food's expiration date, the main considerations for hiking food are water content, caloric density (more energy per pound for a given space), and nutritional density (more nutrition per pound for a given space). Water weighs , so a food container can weigh up to less when it contains dehydrated food. Dehydrating foods can reduce weight and may reduce nutrition while increasing the need for water to reconstitute the food. More weight also expends more energy, so packing lighter foods reduces the need for more calories. Calories equate to energy. Nutrition becomes more important as the number of hiking days increases; for example MREs are not intended to be used beyond ten days. Multi-vitamins may help offset some nutrition deficiencies.