CVT, VTS (Nutrition)
Ed is the director of technician learning and development for Ethos Veterinary Health and VetBloom. He is also the 2020 president of the Massachusetts Veterinary Technician Association and the treasurer of the New Hampshire Veterinary Technician Association. Ed has served on multiple NAVTA committees and is the 2020 NAVTA president-elect. He obtained his VTS (Nutrition) in 2014 and lectures frequently at local, regional, and national veterinary conferences on a variety of nutrition topics. Ed was also the recipient of the NAVTA 2019 Technician of the Year award.Read Articles Written by Ed Carlson
Optimal veterinary nursing care requires the ability to perform calculations. Veterinary nurses routinely calculate drug doses, fluid rates, and constant-rate infusion rates; however, basic nutrition calculations are sometimes overlooked. This article explains how to easily perform common and important nutrition calculations.
Calculating Energy (Caloric) Requirements
Establishing whether a patient is currently eating enough or too much involves determining its resting energy requirement (RER). However, many other factors also go into calculating a patient’s energy needs, including growth, desired weight loss or gain, and activity level.
Resting Energy Requirement
RER is a function of metabolic body size and represents the energy requirement of the patient while at rest at a controlled temperature.
There are several formulas for calculating RER based on body weight in kilograms (BW kg)1:
- 30 × (BW kg) + 70 = RER
- 70 × (BW kg)0.75 = RER
- √√ (BW kg × BW kg × BW kg) × 70 = RER
The formula 30 × (BW kg) + 70 = RER is commonly used in many veterinary hospitals and may be used for patients weighing more than 2 kg and less than 45 kg. For patients weighing less than 2 kg or more than 45 kg, the formulas 70 × (BW kg)0.75 = RER and √√ (BW kg × BW kg × BW kg) × 70 = RER provide a more accurate estimate of caloric requirements.
The difference in estimates when 30 × (BW kg) + 70 is used can be seen by looking at 2 patients, Max and Tiny. Max weighs 20 kg (44 lb) and Tiny weighs 1.36 kg (3 lb).
Using the formula 30 × (BW kg) + 70 = RER, Max and Tiny have the following RERs:
- Max: 30 × 20 kg = 600 + 70 = 670 kcal/day
- Tiny: 30 × 1.36 kg = 40.8 + 70 = 110.8 kcal/day
Using the formula 70 × (BW kg)0.75 = RER, their RERs are:
- Max: 70 × (20 kg)0.75 = 662 kcal/day
- Tiny: 70 × (1.36 kg)0.75 = 88.2 kcal/day
The difference between the 2 results is only 8 kcal/day for Max, but for Tiny, the first equation overestimates RER by 22.6 kcal per day. For this reason, it is preferable to use the formula 70 × (BW kg)0.75 or √√ (BW kg × BW kg × BW kg) × 70 to calculate RER for all patients.
Daily Energy Requirement
Once the patient’s RER has been determined, the next step is to calculate the patient’s daily energy requirement (DER). DER is calculated by multiplying RER by a coefficient based on the patient’s life stage and body condition. Coefficients for common life stages to determine DER are listed in TABLE 1.
It is important to remember that all caloric calculations are estimates of the patient’s energy needs; actual caloric intake may vary between individuals depending on the patient’s age, lifestyle, activity level, body condition score, and other factors. However, the above RER and DER calculations are an excellent starting point for every nutritional recommendation.
Calculating Amount to Feed
Once the patient’s DER is known, the amount of food needed to meet it can be calculated. The first step is to find the caloric content per can or cup of the food the patient is going to be fed. The caloric content of any food (and some treats) can be easily found on the product packaging or obtained from the manufacturer’s website or product guide. The caloric content of most “people” food that patients eat can be found on the product packaging or manufacturer website.
The number of calories in a can or cup of the selected food is simply divided into the patient’s DER. To continue with Sandy as an example:
The above amounts can be divided by the number of times the patient will be fed per day.
Instructions to the client should be specific and include the name and brand of the food being recommended, the amount to be fed per day and per feeding, how many feedings per day, how many treats, and when the plan may change. To continue with Sandy as an example:
For many healthy pets being fed a good-quality, complete, and balanced diet manufactured by a reputable company, the above calculations may be the only ones required. Online calculators also exist to help with these computations (BOX 1). However, there are many others veterinary nurses can use in their role as a veterinary nutrition advocate.
As-Fed Versus Dry Matter Basis
Ingredients are listed on the label as percentages on an “as-fed” basis; essentially, this means that the moisture content is included. Dry and canned pet food both contain moisture, but in different amounts. To be compared between different foods, especially when comparing canned food to dry kibble, each nutrient must be converted from as-fed to dry matter basis (DMB).
One method of estimating the crude moisture in a pet food is to subtract the percentage of moisture listed in the guaranteed analysis section on the label from 100% and then divide the percentage of each nutrient listed in the guaranteed analysis by this number.3
Note that before conversion from as-fed to DMB, the canned food in this example appeared to contain less protein than the dry food; however, it actually has more.
A simpler, perhaps not quite as accurate, method is to multiply the percentages of nutrients listed in the guaranteed analysis by 1.1 for dry foods and by 4 for canned foods.
Both of these calculations provide an estimated amount of nutrients on a dry matter basis. If the actual amount is required, it can be obtained from the pet food manufacturer.
Comparison on a Caloric Basis
The simple method described above can be useful to help clients understand that comparisons are not as easy as simply reading pet food labels. However, comparing nutrients on a caloric (or energy) basis as grams per 1000 kcal provides a more useful, more accurate comparison and is often preferred.
For this comparison, locate the caloric density (kcal/kg) and the percentage of the nutrient to be compared on the product label. For protein, add 1.5% to the minimum listed on the label, and for fat, add 1%.4
Calculating Estimated Carbohydrate Content
Guaranteed analyses do not list the amount of carbohydrate in a pet food; however, this amount is easily estimated. Start by converting the protein, fat, fiber, and ash contents to DMB (as explained above). Add these percentages together and subtract the total from 100%. This is the estimated carbohydrate content of the food.5
These basic nutrition calculations are important when veterinary nurses are developing nutritional recommendations and plans for patients and educating clients on the importance of nutrition.
1. Ramsey JJ. Determining energy requirements. In: Fascetti AJ, Delaney SJ, eds. Applied Veterinary Clinical Nutrition. Wiley-Blackwell;
2. Gross KL, Yamka RM, Khoo C, et al. Macronutrients. In: Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ, eds. Small Animal Clinical Nutrition. 5th ed. Mark Morris Institute; 2010:49-105.
3. McNamara JP. Determining food nutrient content. In: McNamara JP, ed. Principles of Companion Animal Nutrition. 2nd ed. Pearson;
4. Hill RC, Choate CJ, Scott KC, Molenberghs G. Comparison of the guaranteed analysis with the measured nutrient composition of commercial pet foods. JAVMA. 2009;234(3):347-351. doi:10.2460/javma.234.3.347
5. Case LP, Daristotle L, Hayek MG, Raasch MF. Energy and water. In: Case LP, Daristotle L, Hayek MG, Raasch MF, eds. Canine and Feline Nutrition: A Resource for Companion Animal Professionals. 3rd ed. Elsevier Mosby; 2011:3-12.