Energy & raw diets for cats
This article is part of the series in which we are exploring the science of nutrition. After water (that we addressed in the first article of the series), the second most important component in a diet is energy. In today’s article, we are going to speak about what energy is, how to compute the energy in a diet and the energy need of your cat, its link with the nutrient requirements, and its implications for a raw diet.
What is energy and why is it important?
Energy is not itself a nutrient, but a property of the diet that comes from the contribution of three macronutrients: lipids, carbohydrates, and proteins (more about these in future articles). Energy is required for daily body functions such as maintaining and synthesizing body tissues, breathing, regulating body temperature, engaging in physical activities, ...
As energy is the most critical component of the diet after water, nutrients in the diet are used first to fulfill the energy needs of the pets before being used for other metabolic functions.
When a cat has an energy intake equal to the energy it uses, it is at an energy balance: it doesn’t lose or gain weight and that’s the state in which a healthy adult cat should be.
If the energy intake is greater than the energy spent, the animal will gain weight and store the extra energy as fat. On the contrary, if the energy intake is lower than the energy expenditure, the cat will lose weight.
Energy is measured in kilocalories, which is generally colloquially abbreviated in calories.
Energy in a diet
In a diet, we are going to consider the metabolizable energy, which is the energy available for the body to use. On a pet food label, you will generally see the energy density, which is the metabolizable energy per quantity of food (such as kcal/100g or kcal/kg). Commonly, the energy density is referred to as the energy of the food.
As mentioned before, the energy is provided by the macronutrients in the diet. The formula to compute the energy in function of the diet composition will depend on the digestibility of the diet. In fact, if a food is less digestible, the body will obtain less energy from its nutrients.
Raw diet and home-cooked diets are highly digestible, and the formula used to compute the energy is the following[1]:
Metabolizable Energy of the food (ME in kcal/100 g) = 8.5 x % of lipids in the diet + 4 x % of proteins in the diet + 4 x % of carbohydrates in the diet
You can see that lipids (commonly called fats) have the highest impact on the energy in food.
For industrial pet food, kibbles, or canned food, the digestibility is lower and the (simplified) formula used to compute the energy is the following:
Metabolizable Energy of the food (ME in kcal/100g) = 8.5 x % of lipids in the diet + 3.5 x % of proteins in the diet + 3.5 x % of carbohydrates in the diet.
Energy requirements for cats
The daily energy requirement of a cat to support normal body function is called maintenance energy. The maintenance energy is the sum of three components: the basal metabolic rate (BMR), the voluntary muscular activity, and the thermogenesis.
The basal metabolic rate is the energy needed to maintain body functions when resting in a fasting state at a neutral temperature and it accounts for 60-75% of the maintenance energy. This factor is influenced by age, sex, reproductive status (intact or neutered), body composition (lean or fat)...
Voluntary muscular activity is the energy needed to perform physical activities such as walking, playing, running… It generally represents 30% of the daily energy need and varies in function of the duration and intensity of the physical activities.
Thermogenesis is the energy needed during the digestion and absorption of the nutrient in the food eaten. It represents 10% of the diet, and it varies with the food composition and the number of meals during the day.
Finally, the ambient temperature also plays a role in the daily energy requirement, as in colder temperatures a cat will need to spend more energy to maintain its own body temperature.
Now that we know what influences the daily energy requirement, let’s talk about how to actually estimate it. As the energy requirements depend mainly on the basal metabolic rate which is related to body functions, the energy requirement equation depends on the body weight of the animal. To be more accurate, the metabolic body weight is often going to be used as it represents the body surface area and yields a more precise estimation of the energy needs than the body weight. For cats, the Metabolic Weight (MW) is the body weight (BW) in kg at the power of 0.67:
The equation to calculate the daily metabolizable energy needs for a lean adult cat defined by the NRC is the following[1]:
However, the NRC also indicates that this equation is at best an “educated guess” and can underestimate or overestimate the daily calorie need by 50%. In practice, this equation generally overestimates largely the calorie needs.
An alternative equation for a less active or neutered cat is:
Other existing simplified equations are ME = 60 x BW for active cats or ME = 45 x BW for older or inactive cats[2].
When you know the calorie requirements of your cat, you can then determine how much food your cat needs to eat as the calorie provided in its diet should match its energy requirement.
Energy, food density, and nutrient requirements
In theory, animals stop eating by themselves when they have reached their daily energy requirement (even if, as we have seen in the article on meals, it’s not always the case).
This is something very important because it means that the amount of nutrients (such as taurine, omega-3, calcium, vitamins…) present in the food depends on the energy density of the food. In fact, when a cat stops eating because it fulfilled its energy needs, it should have gotten all the nutrients needed in its diet.
This explains why when you look for information on the amount of nutrients your cat needs, you will find the value expressed in function of the metabolizable energy, and generally expressed per 1000kcal of Metabolizable Energy (another way to express the nutrient requirement is per body weight of the cat).
For example, if a cat needs 200kcal/day and food A has a ME of 300kcal/100g and food B has a ME of 400kcal/100g:
If the cat eats food A, it will need to eat 66g per day while for food B he will need to eat 50g per day to reach its daily energy requirement. But both foods should provide the same amount of nutrients to the cat, so food B should be more concentrated in nutrients than food A, as the cat will eat less of food B.
This also explains why it can be misleading to compare the nutrient analyses on the packaging of cat food. For example, if food A contains 40% protein and a ME of 400kcal/100g and food B contains 35% protein and a ME of 330kcal/100g, when looking quickly at the packaging you may think that food A contains more protein than food B. However, you also need to take into account the energy of the diet, as the cat will eat less of food A than food B, because food A is more energy-dense. Food A contains 35% of protein per 1000kcal while food B contains 37% per 1000kcal.
Ratio diet, PMR and BARF
If we use calories in the food and energy requirements to determine how much to feed a cat, what about the 3% of body weight rule? These guidelines that you can find on many raw feeding websites (including this article of mine) are an empirical estimation. This estimation assumes that the diet of the cat will not be too lean, nor too high in fat. It’s also an easy way to start feeding a raw diet, without having to go too deep into the science of nutrition.
However, while this estimation is great to start and works well for many cats, it may not be adapted to all cats, especially if they have special needs (very active or very inactive) or if they are fed a diet that is very lean or very high in fat. In such cases, having a more scientific approach and calculating the calorie requirement of the cat and the calorie in the diet can be helpful.
For example, let’s take a neutered 4kg cat: according to the formula, for a neutered cat, we can estimate its calorie daily requirement at 190kcal/day.
Now, if we take the 3% rule, the cat should eat 120g a day. According to Raw Fed and Nerdy, a diet moderate in fat will have an average of 175kcal/100g. So, the cat will consume 210kcal/day. In this case, the 3% rule gives a reasonable result.
For a 2kg neutered cat, the daily requirement is of 119kcal/day and the estimation with the 3% rule gives 105kcal/day. The rule is still reasonable.
However, for an 8kg neutered cat, the daily requirement is 302kcal/day: with the 3% rule, we get an estimate of 420kcal. In this case, the 3% rule is not working well.
This isn’t surprising, as the 3% rule is a linear function of the weight, while the calorie needs aren’t linear to the weight. Bigger animals tend to have lower requirements proportionally to their weight.
So, the rule works best for an average neutered cat eating a diet moderate in fat, but it’s nevertheless a good start and gives a starting estimate of how much to feed your cat.
Energy is the second most important component of the diet, as it maintains body functions and allows physical activity. The common approach to energy in a raw diet is an estimation of both the cat calorie needs and the diet energy density. If this approach doesn’t fit your cat’s needs, using a scientific approach may be useful. In a future article on feline nutrition, we will talk about the different macronutrients.
Sources
[1] National Research Council. 2006. Nutrient Requirements of Dogs and Cats. Washington, DC: The National Academies Press.
[2] (2015). Canine and Feline Nutrition (Linda P. Case et al.).