Starting With the Basics
Mifflin-St. Jeor is a mathematical equation (as seen below) and was designed and validated to calculate energy (calorie) needs for healthy adults of both normal weight and obese. After using the calculation, multiply the answer by an activity factor and an injury factor. If the individual does not have a chronic or acute inflammatory state (i.e. illness, infection, trauma, disease), then omit the injury factor.
The recommended injury factor may evolve with newer research. So if you are practicing as a clinical dietitian (even as an intern), it is a good idea to stay up to date on the latest research for energy needs in particular population groups. Otherwise, calculating calorie needs is just an estimation that often requires clinical judgement.
How to calculate calorie needs for normal nutrition for adults?
You can use any of the following equations. Your university or workplace may vary on which equation is recommended, but this is what I learned at a hospital I worked at. Harris-Benedict was used for most adult patients, especially the elderly. Mifflin-St. Jeor was used in the obese population. I didn't use kcal/kg very much as it was frowned upon by my professors and clinical manager for not being evidenced-based.
Mifflin-St. Jeor
Male: 5 + (10 x wt kg) + (6.25 x ht cm) - (5 x age)
Female: -161 + (10 x wt kg) + (6.25 x ht cm) - (5 x age)
Harris-Benedict
Male: 66.5 + (13.8 x wt kg) + (5 x ht cm) - (6.8 x age)
Female: 655 + (9.6 × wt kg) + (1.7 × ht cm) – (4.7 × age)
Kcal/kg
25 kcal/kg
After you've calculated basic needs, you are not done! Next, add an activity factor and/or an injury factor to multiply your answer with. Remember, your final answer should be a range. For example, "1400-1600 kcal/day".
How to calculate calorie needs for various activity levels
Note: I have found that these numbers vary depending on the textbook or resource you are reading. I've looked at many of them, and these are the average numbers in the most simple way I can present here for you. Keep in mind, calorie needs are an estimation.
Activity Factors in the table are from Krause 14th ed on pages 23-24.
If you use the 25 kcal/kg method, you still need to multiply this by an activity factor. Or you could use a range 25-30 kcal/kg. If a person is very underweight you could even go up to 35 kcal/kg to promote weight gain. But keep in mind, even the the kcal/kg is a common approach to estimating energy needs, it is not a validated method.
How to calculate calorie needs for hospitalized patients
Note: The following injury factors are for acute conditions (i.e. fever, trauma, surgery, illness) require additional calories/protein. But don't forget about chronic disease (i.e. diabetes, heart disease, COPD) which also need additional calories, protein, and micronutrients.
Activity Factor (AF)
Out of bed: 1.2
Confined to bed: 1.1
Average Injury Factors
Surgery: 1.0-1.3
Infection: 1.0-1.4
Skeletal trauma: 1.2-1.4
Head Injury: 1.5
If you need any sort of clarification, leave a comment below!