their metabolic energy requirements may increase, especially after 12 or 13 years old, but other evidence suggests that metabolic energy needs are not dissimilar at different ages. Furthermore, it has been shown that cats over 12 years old are more likely to be underweight than younger cats, so a high calorie diet may be appropriate to treat weight loss, and thus to maintain an appropriate body condition score. Weight loss can occur when the cat expends more energy but does not increase its food energy intake. It has also been demonstrated that as cats age, they are less able to digest and thus absorb dietary fats and proteins. Pregnancy and lactation are strenuous periods on the female cat. During pregnancy a cat should gain 38% of their body weight by the time they are ready to give birth. It is recommended that a cat's diet should contain 4000 kcal per kg of dry food while pregnant; during lactation it is recommended that the cat consumes 240 - 354 kcal ME per kg of body weight. Studies show that increasing the intake of food for a pregnant animal in order to help it gain weight can have negative effects. It is acknowledged that rather than increasing intake, feeding a highly energy dense food is a way to ensure that the female cat, or queen, receives adequate energy and nutrient requirements are met. It has been found that nutritional support consistent with the resting energy requirement (RER) soon after surgery or the onset of illness decreases the mortality rate and the duration of hospitalization in cats. A recovering cat needs enough energy (calories), as well as more protein and fats. Critical care diets are formulated to be highly palatable and digestible, as well as high energy density. This limits the mass of food required to be consumed to meet the RER. This type of high energy diet has proven to be very important in the nutritional support of post-operative and ill cats. Raw food Raw feeding is providing uncooked ingredients to cats. Most of the diet will consist of animal-based ingredients, though fruits, vegetables and supplements are often added. Commercial raw food is mainly sold in three formats: fresh, frozen and freeze-dried. Thawing and rehydration are necessary before feeding frozen and freeze-dried food respectively. Many available commercial diets are AAFCO certified in meeting the nutrient requirements of the cat. Some diets may