Energy and Nutrients: The Science Behind Every Feeding Plan

Every feeding plan answers two questions: how much energy does this animal need, and does its diet supply the nutrients it needs within that energy? This lesson explains both, step by step, with one worked example you can repeat with any healthy adult dog.

You will learn the difference between energy and nutrients, the standard formula for estimating a daily calorie need, how to turn calories into grams of food, and why cats need to be treated as a separate case.

Energy first, then nutrients

Food supplies two things. Energy (measured in kilocalories, written kcal and often just called calories) comes from protein, fat and carbohydrate. Nutrients are the specific building blocks the body needs: essential amino acids from protein, essential fatty acids from fat, and the vitamins and minerals that carry no energy at all.

The two are linked in a way that matters for every plan. A food designed as "complete and balanced" packs its nutrients into a certain number of calories. If an animal eats the amount of calories the food was designed around, it gets the nutrients too. If it eats far less (a very inactive dog on a strict ration of a high-calorie food, for example), it can end up short of nutrients even though it is not hungry. That is why consultants start with the energy estimate, then check the food suits it.

A few points about nutrients that come up constantly with clients:

• Protein supplies amino acids for muscle, skin, coat and repair. More is not automatically better for a healthy adult; the quality and digestibility of the protein matter as well as the amount.
• Fat is the most concentrated source of energy (roughly twice the calories of protein or carbohydrate per gram) and supplies essential fatty acids. High-fat foods make portion control more important.
• Vitamins and minerals work in balance. Adding a supplement to a complete and balanced diet can do harm rather than good: extra calcium for a large-breed puppy is the classic example, because excess calcium during growth is linked to skeletal problems.

Estimating a daily calorie need

The standard starting point is the resting energy requirement (RER), the energy an animal uses at rest in a comfortable environment:

RER (kcal per day) = 70 × body weight in kg to the power 0.75

The power 0.75 reflects the fact that energy use does not rise in a straight line with weight: a 40 kg dog does not need four times the calories of a 10 kg dog. Any phone calculator with an x^y key will do it.

Resting energy is then multiplied by a factor for life stage and activity. Commonly used starting factors for dogs include about 1.6 for a neutered adult with typical activity, about 1.8 for an intact adult, and lower factors for inactive dogs or those prone to weight gain. Growing puppies need much higher factors.

Worked example. A healthy, neutered, moderately active adult dog weighs 10 kg and is at a good body condition.

1. 10 to the power 0.75 is about 5.62.
2. RER = 70 × 5.62, which is about 394 kcal a day.
3. Daily need = 394 × 1.6, which is about 630 kcal a day.
4. The dog also gets treats. A common guideline is to keep treats to about 10% of daily calories, so about 63 kcal for treats and about 567 kcal from the main food.
5. The food contains 3,500 kcal per kg (the label gives this figure). 567 ÷ 3,500 × 1,000 is about 162 g of food a day.

Two things make this estimate honest. First, it is a starting point for a population, not a measurement of this dog: individual dogs of the same weight can differ widely. Second, it is checked against the animal over the following weeks. If body condition drifts up, the amount comes down, and vice versa. Weighing the food on kitchen scales matters more than any decimal place in the formula, because cups and scoops vary a lot.

The free dog calorie calculator runs the same formula, and the article on how to calculate a dog's calorie needs explains the factors in more depth.

Why cats are a separate case

The RER formula works for cats too, but the factors are lower (commonly about 1.2 for a neutered adult cat) and the nutrient picture is different. Cats are obligate carnivores: they need certain nutrients that come from animal tissue, such as the amino acid taurine and preformed vitamin A, and they are poorly adapted to going without food. A cat that stops eating for even a few days can develop a serious liver condition, which is why weight loss in cats is always planned with a vet, never by cutting food sharply.

In practice that means you never copy a dog plan onto a cat, you check that a cat food is complete and balanced for cats specifically, and you treat any cat that has stopped eating as a vet case. The article on how to calculate a cat's calorie needs covers the cat factors.

Lesson 3 puts this into practice with a real case: a growing puppy whose owner is doing several things with good intentions and some risk.