How Many Calories Do You Need?

Every day we face many decisions about our diet. When should I eat? How many times a day should I eat? What types of food? How much food? This article will answer how much food you need, specifically to maintain your current body composition. 

What is a Calorie? 

Energy is released from the carbohydrates, proteins, and fats we eat and store. We quantify energy transfer with kilocalories (kcal), which is raising 1 kg of water 1 degree centigrade. The amount of calories we need depends on many factors such as our activity level and body composition of muscle and fat. 

What is Total Daily Energy Expenditure? (TDEE)

Total energy expenditure is comprised of basal metabolic rate, physical activity, thermoregulation, and the thermic effect of food. (1) We will consider each component to approximate TDEE based on age, body size, sex, and activity level. 

What is Basal Metabolic Rate (BMR)?

BMR is the amount of energy needed for life processes such as breathing and your heart rate at complete rest after a 12-hour fast. (1) Resting metabolic rate (RMR) is a common alternative to BMR because it is easier to measure. RMR normally makes up ⅔ of your TDEE which can vary at the individual level. (4) Unfortunately, RMR is typically 3-10% higher than BMR which we accept when we use these equations. (1,4) 

Harris-Benedict (RMR)

Estimates RMR for normal-weight individuals. (1)

(W) = Kilograms, (H) = Centimeters, (A) = age in years

• Men: kcal/day = 66.5 + (13.7 x W) + (5.0 X H) – (6.8 x A)
• Women: kcal/day = 665 + (9.56 x W) + (1.85 x H) – (4.7 x A)
• Example for a male who is 113.4 kg, 201 cm, and 23 years old
  • 66.5 + (13.7 x 113.4 kg ) + (5 x 201 cm) – (6.8 x 23 years) = 2469 kcal

Mifflin-St. Joer (RMR)

Can estimate RMR within 10% in nonobese and obese people. (1)

(W) = Kilograms, (H) = Centimeters, (A) = age in years

• Men: kcal/day = (9.99 X W) + (6.25 x H) – (4.92 x A) + 5 
• Women: kcal/day = (9.99 X W) + (6.25 X H) – (4.92 X A) – 161
• Example for a male who is 113.4 kg, 201 cm, and 23 years old
  • (9.99 x 113.4 kg) + (6.25 x 201 cm) – (4.92 x 23 years) + 5 = 2281 kcal 

Physical Activity Level Modifiers for RMR Equations

Some practitioners will estimate TDEE by multiplying RMR by the individual’s physical activity level (PAL) category. (1)

• Sedentary
  • Range: 1.00-1.39
  • Average: 1.25
  • Negligible physical activity.
  • Example: Sedentary (1.25) x 2281 kcal = 2851.25

• Low Active
  • Range: 1.40-1.59
  • Average: 1.5
  • Predicts walking 2.2 miles at a 3-4 mph pace for a 154 lb person. A heavier person would burn more calories by walking the same mileage because they are moving more weight. 
  • Example: Low active (1.5) x 2281 kcal = 3422 kcal

• Active
  • Range: 1.60-1.80 
  • Average: 1.75
  • Predicts walking 7.3 miles at a 3-4 mph pace for a 154 lb person
  • Example: Active (1.75) x 2281 kcal = 3991.75 kcal

• Very Active
  • Range: 1.90-2.49
  • Average: 2.20
  • Predicts walking 16.7 miles at a 3-4 mph pace for a 154 lb person
  • Example: Very Active (2.2) x 2281 kcal = 5018.2 kcal

Estimated Energy Requirement Equation (5)

Estimates TDEE or EER, (W) = Kilograms, (H) = Meters, (A) = age in years

• Men: EER = (662 – (9.53 x A)) + PAL x ((15.91 x W) +  (539.6 x H))
• Female: EER = (354 – (6.91 X A)) + PAL x ((9.36 x W) + (726 x H))

Physical Activity Multiplier for Estimated Energy Requirement Equation

Depending on your activity level, you may require more or less calories. The physical activity multipliers address the gap in calories between a desk worker and a nurse who takes 12,000 steps at work every day.  

• Sedentary
  • Negligible activity
  • Men and Women: 1
• Low active 
  • 30-60 min of moderate activity which is defined as walking at 3 – 4.5 mph
  • Men: 1.11
  • Women: 1.12
• Active
  • >60 min of moderate activity
  • Men: 1.25
  • Women: 1.27
• Very Active 
  • >60 min moderate and 60 min vigorous or 120 min moderate activity
  • Men: 1.48
  • Women: 1.45
• Example: [662 – (9.53 x 23)) + 1.25 x ((15.91 x 113.5) +  (539.6 x 2)) = 4049

Physical Activity

Tour de France athletes can expend up to 5x their RMR in physical activity (4). Energy expenditure from activity varies between individuals, contributing 15-50% of TDEE on average. (1,4) 

• Running Modifiers (3)
  • These factors are in units, (kcal x lb^-1 x min^-1). Multiply by pounds and minutes to find kcal. 
  • 5 mph: .061
  • 6 mph: .074
  • 7.5 mph: .094
  • 9 mph: .103
  • 10 mph: .114
  • Example: 250 pounds x 60 minutes at 6mph (.074) = 1,110 kcal
• Walking Modifiers 
  • 3.5 mph: .035
  • 4.5 mph: .048
  • Example: 250 pounds x 60 minutes at 3.5 mph (.035) = 525 kcal
• Weightlifting Modifier 
  • Light/Moderate: .024
  • Vigorous: .048
  • Example: 250 pounds x 60 minutes at moderate (2.4) = 360 kcal
  • Weightlifting increases muscle and lean body mass which will increase RMR.
• Activities of Daily Living 
  • Gardening: .045
  • Vacuuming: .030
  • Studying: .011
  • Example: Studying (.011) x 180 minutes + Vacuuming (.030) 30 minutes + gardening (.045) 10 minutes x 250 pounds = 832.5 kcal

What is the thermic effect of food (TEF)?

Digestion requires 10% of your TDEE, however, we do not usually account for it. Protein increases energy expenditure by 20-30%, carbohydrates by 5-10%, and fat by 0-5%. (1,5)

Thermogenesis

• Heat production in cooler environments or sweating in hotter environments requires energy to maintain your core body temperature. (4)
• Drugs such as nicotine can increase energy expenditure by 10%. (4) Caffeine and capsaicin in food can also contribute to energy expenditure. 

How does body composition impact your caloric needs?

• Muscle burns more calories than fat per pound. Fat-free mass, explains 60-80% of the variation in RMR. (4)
• Predictive equations tend to overestimate RMR in people with excess fat.

Summary

• As you can see, the complexity of nutrition makes finding precise numbers difficult. The equations are designed to get in the ballpark of the true value, but they can only be estimated. There have been many attempts to calculate TDEE, but no equation is 100% accurate. 

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Glossary:

• One kilocalorie is the amount of heat needed to raise 1 kg of water 1 degree centigrade. 
  • 1 Kilocalorie = 1000 calories
• 1 gram of protein = 4kcal
• 1 gram of fat = 9 kcal
• 1 gram of carbohydrate = 4 kcal 
• 1 Pound (lb) = 2.2 Kilograms (kg)
• TDEE=EER

References

1. Gropper, S. S., Smith, J. L., & Carr, T. P. (2018). Advanced nutrition and human metabolism (Seventh edition). Cengage Learning.
2. Gerrior, S., Juan, W., & Peter, B. (2006). An Easy Approach to Calculating Estimated Energy Requirements. Preventing Chronic Disease, 3(4), A129.
3. Rolfes, S. R., Pinna, K., & Whitney, E. N. (2012). Understanding normal and clinical nutrition (9th ed). Wadsworth/Cengage Learning.
4. Gibney, M. J., & Nutrition Society (Great Britain) (Eds.). (2009). Introduction to human nutrition (2nd ed). Wiley-Blackwell ; Nutrition Society.
5. Whitney, E. N., & Rolfes, S. R. (2022). Understanding nutrition (Sixteenth edition). Cengage.