Energy & Water: Nutrition Guide for Dogs and Cats

Overview

All companion animals need a balanced diet to maintain health. A balanced diet supplies nutrients in the right amounts and proportions. Nutrients fall into two broad groups:

• Essential nutrients: must be obtained from the diet because the body cannot synthesize enough of them.

• Nonessential nutrients: can be synthesized by the body from other dietary components.

Along with energy, pets require six major classes of nutrients: water, carbohydrates, proteins, fats, minerals, and vitamins.

Although energy itself is not a nutrient, it powers every bodily function. A large share of the diet is ultimately used to meet energy needs. Once energy requirements are covered, remaining nutrients support other functions such as growth, tissue repair, reproduction, and immune health.

Energy Intake & Feeding Behavior

When offered a balanced, moderately palatable diet, many dogs and cats tend to regulate food intake according to energy needs. Because of this, energy density (calories per unit weight/volume) is a key concept: pets usually eat to meet calorie needs rather than to achieve specific grams of protein, fat, or carbohydrate.

However, unrestricted access, very high palatability, and indoor/lower‑activity lifestyles can disrupt self‑regulation and predispose pets to overconsumption and obesity. Today, the roles of dogs and cats have shifted from working/hunting to primarily companion animals, which reduces daily energy expenditure. Pet parents should therefore be mindful of both portion control and diet composition.

How Energy Is Expressed

The energy content of pet food is typically shown as kilocalories (kcal) or kilojoules (kJ). Labels generally provide energy alongside crude protein, crude fat, and other analyses. The energy value you see on a label is metabolizable energy (ME)—the portion of gross energy available to the animal after losses in feces and urine (and gases, when relevant).

Metabolizable Energy (ME) = Gross Energy of Food − Energy in Feces − Energy in Urine (± gases).

Example: If a food contains 3000 kcal of gross energy and 2000 kcal are lost in feces and urine, then ME = 1000 kcal.

ME can be measured in feeding trials by collecting and analyzing excreta, or estimated analytically using composition data and standard factors.

Modified Atwater Factors

When gross energy is adjusted for digestibility and urinary losses, we can estimate ME using the Modified Atwater Factors (commonly used in North America):

• Protein: 3.5 kcal/g

• Fat: 8.5 kcal/g

• Carbohydrate (NFE): 3.5 kcal/g

These factors underlie the AAFCO method for predicting ME from proximate analysis.

 

Energy Density

Energy density is the number of calories per unit weight or volume (e.g., kcal/kg, kcal/100 g, kcal/cup, or in Europe sometimes kJ/kg). Diets with too low an energy density may limit intake by gut fill, while very dense diets can promote overconsumption if portions aren’t controlled. Palatability, fiber level, and moisture also influence intake.

We often convert the guaranteed analysis (presented on an as‑fed weight basis) to an energy basis to compare foods fairly.

Key insight: Although dry foods usually show higher % protein by weight than wet foods, wet foods contain much more water and less carbohydrate per gram. On an energy basis (per 1000 kcal), a wet food can provide similar protein per calorie as a dry food. Labels can look very different while delivering comparable nutrient density per calorie.

To make sound decisions, evaluate moisture content and caloric density, and also look at Nutrient Density which is expressed nutrients per 1000 kcal ME.

Nutrient Density Example using Table 1‑2 (wet food):
If a wet food provides 98 kcal per 100 g and contains 7% protein (as‑fed), then

Calculating Energy Density from a Label

 

 

Dog food example (as‑fed):

Because proximate components form a mass balance (≈100%), you can estimate carbohydrate by difference:

• Crude protein: 26%

• Crude fat: 15%

• Crude fiber: 5%

• Ash (estimated minerals): 5–8% (use a single value for calculation; e.g., 6%)

• Moisture: 10%

Estimated carbohydrate with 6% ash:

Carbs = 100 − 26 − 15 − 5 − 6 − 10 = 38% (≈ 39% if you assume 5% ash)

You can then estimate ME using Modified Atwater Factors.

How Much Food Is Enough?

Once you know the ME density of the diet (e.g., 348 kcal per 100 g), you can estimate the daily portion.

Example (adult dog): Required energy = 1100 kcal/day; food provides 348 kcal/100 g.

Tip: Round to practical measures (e.g., cups) using the product’s kcal per cup if provided.

Energy Imbalance

Too much or too little energy both cause problems.

• Excess energy (overfeeding): may accelerate growth in juveniles and increase risk of skeletal disorders (e.g., osteochondrosis, hip dysplasia) in predisposed breeds; promotes adiposity and later‑life obesity through fat cell hyperplasia.

• Insufficient energy: leads to poor growth in young animals, and weight/muscle loss in adults. In healthy pets, shortfalls most often occur in high‑demand states (working dogs, late gestation, lactation) or during illness.

Water

Water is the most critical nutrient. It functions as a lubricant, solvent, and transport medium; it supports thermoregulation thanks to its high heat capacity and evaporative loss through panting and (in dogs) limited sweating.

• Dehydration is dangerous: loss of ~10% of body water can be life‑threatening.

• Biochemical roles: water is essential for enzymatic reactions, digestion, and excretion (kidneys filter blood and excrete wastes in urine).

• Daily need: because water is continually lost via respiration, urine, and feces, it must be replenished daily.

Dietary Moisture

• Dry foods: typically ≤10% moisture.

• Wet foods: typically ~80% moisture.

Some studies suggest dogs can adjust water intake when dietary moisture is very high (e.g., >~67%), while cats may still under‑consume water even on wet diets. Regardless of diet type, always provide fresh, clean drinking water freely

Metabolic Water

Oxidation of fat, carbohydrate, and protein generates metabolic water (often roughly 5–10% of daily needs), but this is not sufficient on its own.

Practical Takeaway

Consider both dietary moisture and caloric density, but always ensure free access to drinking water so pets can match intake to daily activity and environmental conditions.