Carbohydrates are generally categorized into two types:
- Soluble/Digestible: Includes sugars and starches.
- Insoluble/Non-digestible: Includes fiber, which adds bulk to the stool, draws water into the digestive tract, and can be fermented by gut microbes to support GI function and satiety.
Gluconeogenesis: The Cat Connection
It is important to distinguish between the need for glucose and the need for dietary carbohydrates. Through a process called gluconeogenesis, animals can convert proteins (amino acids), lactate, and glycerol into glucose. This is particularly significant for cats; because they can synthesize glucose from amino acids so efficiently, they rely less on dietary carbohydrates than dogs do.
Protein and Amino Acids
Proteins provide 4 kcal/g and serve as the fundamental building blocks for the body. They are essential for body-building, tissue maintenance, and the production of hormones. Protein requirements are highest during periods of rapid growth or lactation.
The quality and digestibility of a protein source are crucial; higher digestibility and quality mean an animal needs to consume less total protein to meet its physiological requirements.
Essential Amino Acids
Amino acids are the individual units that make up proteins. While many can be synthesized by the body, "essential" amino acids must be obtained through the diet. Both dogs and cats require arginine, isoleucine, lysine, phenylalanine, tryptophan, histidine, leucine, methionine, threonine, and valine. However, cats have a unique requirement for taurine, an amino acid that must be provided in their diet.
Fats and Fatty Acids
Fat is the most energy-dense nutrient, providing 9 kcal/g. Beyond energy, fats increase the palatability of food, support the absorption of fat-soluble vitamins, and provide insulation and organ protection.
The building blocks of dietary fat are fatty acids. Essential fatty acids are critical for supporting skin, kidney, heart, and joint health, as well as the central nervous system (especially in puppies) and reproductive functions.
- Omega-6 Fatty Acids: Includes linoleic acid and arachidonic acid. Dogs require linoleic acid, while cats require both linoleic and arachidonic acids.
- Omega-3 Fatty Acids: Includes linolenic acid. These are vital for overall health, and species differences in requirements are important when evaluating diet completeness.
Vitamins and Minerals
Vitamins and minerals are required in much smaller amounts than macronutrients but are essential for normal metabolism.
Vitamins
Vitamins are categorized by their solubility:
- Fat-Soluble (A, D, E, K): These are stored in the body, including the liver. Because they are stored, there is a higher risk of toxicity if consumed in excess.
- Water-Soluble (B Complex and C): These are less extensively stored. While excess amounts are typically excreted in urine, significant losses can occur through diarrhea or GI disease. Note that dogs and cats can synthesize their own Vitamin C.
Clinical Note: Cats have specific vitamin needs, such as requiring pre-formed Vitamin A from animal tissue and a specific dietary requirement for niacin.
Minerals
Minerals act as balance markers; both deficiency and excess can lead to health complications.
- Macrominerals: Required in large amounts (expressed as percentages), such as calcium, phosphorus, potassium, sodium, and magnesium.
- Microminerals (Trace Minerals): Required in small amounts (expressed in ppm), such as iron, copper, zinc, and iodine.
Calculating Nutritional Needs: A Practical Example
To determine how much to feed an animal, clinicians first calculate the Resting Energy Requirement (RER), which is the energy needed just to maintain basic life functions. This is then multiplied by a lifestyle factor to find the Daily Energy Requirement (DER).
The formula for RER is:
RER=(30⋅BWkg)+70
Worked Example: Buster the Cat
Buster is a neutered adult cat weighing 8 kg. His lifestyle factor is 1.2.
Step 1: Calculate RER
\text{RER} &= (30 \cdot 8) + 70 \\
\text{RER} &= 240 + 70 \\
\text{RER} &= 310\text{ kcal/day}
\end{aligned}$$
**Step 2: Calculate DER**
$$\begin{aligned}
\text{DER} &= \text{RER} \cdot \text{Lifestyle Factor} \\
\text{DER} &= 310 \cdot 1.2 \\
\text{DER} &= 372\text{ kcal/day}
\end{aligned}$$
**Step 3: Estimate Water and Food Amount**
* **Water:** Since the rule of thumb is $1\text{ mL}$ per $1\text{ kcal}$, Buster needs approximately $372\text{ mL}$ of water per day.
* **Food:** If Buster's kibble provides $800\text{ kcal/cup}$, his daily amount is:
$$\frac{372\text{ kcal/day}}{800\text{ kcal/cup}} \approx 0.465\text{ cups/day}$$
If fed twice daily, he would receive approximately $0.23\text{ cups}$ (roughly $1/4\text{ cup}$) per meal.
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