How many calories do you need? - Determining Energy Requirements
hen developing a nutrition plan the first step is understanding how much energy an individual requires. Establishing energy requirements forms the foundation of personalized nutritional strategies. It is difficult to determine how to address and adjust a diet for weight loss, performance, etc. if there is no clear starting point. When you determine current energy needs, you are able to see where a person is starting and then work from there. It is important then that accurate, yet practical, methods are used to find the energy, or calorie, needs of a person.
One of the most precise ways of determining energy needs is using direct calorimetry. This method involves measuring the heat released from a person in a metabolic chamber. The data can be translated into how many calories the individual is burning. Although accurate, direct calorimetry is time consuming, expensive, and not really viable for most people.
Indirect calorimetry is another accurate technique used to calculate energy needs. Oxygen consumption and carbon dioxide production are measured to assess how much energy, or calories, a person is expending. This method is also often utilized to determine VO2 max, or the maximum volume of oxygen an athlete can consume while exercising, a measure of physical fitness. Indirect calorimetry is a little more accessible as it only requires a metabolic cart to take the measurements instead of a large metabolic chamber. However, most athletes still do not have access to this technology.
The doubly labeled water technique is also a popular laboratory-based method of determining energy expenditure. This is a type of indirect calorimetry that measures carbon dioxide production from the ratio of urinary elimination of certain hydrogen and oxygen isotopes. This process can be expensive and unavailable for many people, and as such may not be practical for most people in determining energy requirements.
Because bringing people into a lab whenever energy requirement measures are needed is largely unrealistic, several equations have been developed to quickly, and somewhat accurately, determine estimated energy needs. The equations calculate resting metabolic rate (RMR), which is the amount of energy a person would require to maintain current weight if they just laid in bed all day with little activity. After the RMR is determined, an activity factor must be applied to the RMR. The activity factor takes into account the amount of daily activity and exercise a person performs, increasing energy needs. Different groups of people were used to create each equation, and so it is necessary to consider which equation is most applicable to the individual at hand. For athletes there are 2 equations that are considered to provide the most accurate energy requirement estimation.
RMR = 500 + 22 (lean body mass in kg)
The Cunningham equation is considered the best equation for athletes because it takes into account lean body mass. Lean mass is a large influencer on energy expenditure because muscle is much more metabolically active than fat, resulting in more calories expended with greater muscle mass. Athletes generally have more muscle than the average person, so accounting for this energy consuming muscle when finding calorie needs leads to a more precise prediction. While this equation is the most accurate in estimating RMR, it does require the lean body mass of the individual to be known, something many athletes may not be aware of. There are many ways to assess lean mass such as the BodPod, iDEXA, skin fold measurements, and underwater weighing. Determining lean body mass can be very useful for athletes not only so they can use the Cunningham equation to find RMR, but also as a tool in their training to set body goals against.
Males: RMR = 66.47 + 13.75 (weight in kg) + 5 (height in cm) – 6.76 (age)
Females: RMR = 655.1 + 9.56 (weight in kg) + 1.85 (height in cm) – 4.68 (age)
The second best equation for athletes to find their RMR is the Harris-Benedict equation. This equation does not require lean mass, but only simple measurements like height and weight that are much easier to obtain. The RMR estimation from this equation has been found to be slightly below the actual RMR by around 150-200 calories when tested on endurance athletes.
Example: a male athlete weighs 180 lbs (81.8 kg), is 6’1” (185 cm), is 27 years old, and has 12% fat mass (72 kg lean mass)
Cunningham: RMR = 500 + 22 (72 kg)
RMR = 2,084 calories/day
Harris-Benedict: RMR = 66.47 + 13.75 (81.8 kg) + 5 (185 cm) – 6.76 (27)
RMR = 1,934 calories/day
Once the RMR has been estimated, it is necessary to apply an activity factor (AF) to find the total energy needed for an individual to perform exercise and daily living activities. The activity factor ranges from about 1.2 for a sedentary person to 1.9 for a highly active individual:
Sedentary = RMR x 1.2 (little or no exercise, desk job)
Lightly active = RMR x 1.375 (light exercise/ sports 1-3 days/week)
Moderately active = RMR x 1.55 (moderate exercise/ sports 6-7 days/week)
Very active = RMR x 1.725 (hard exercise every day, or exercising 2 x/day)
Extra active = RMR x 1.9 (hard exercise 2 or more times per day, or training for
marathon, or triathlon, etc.)
(adapted from KSU)
After determining the appropriate AF for an individual, multiply the factor by the RMR to find the total energy requirements. This number is the amount of calories needed for the person to maintain current body weight and activity levels.
Example: the above athlete performs 1-2 hard exercise sessions per day
Cunningham RMR 2,084 calories x 1.7 AF = 3,543 calories/day total energy needs
rom this calorie level, it is possible to make informed adjustments to diet based on body or performance goals. Weight loss will likely require decreased calories and weight gain will require an increase in calorie consumption. However, the amount of energy to add or subtract to the diet, or how to manipulate energy consumption for the athlete’s goals is best determined by a Registered Dietitian. A RD is a professional with education and experience in nutrition and dietetics that is able to create individual diet plans based on research and the needs of the athlete/client. Any athletes who are looking for a personalized nutrition plan for their body and performance goals will find the greatest success by consulting with a trusted dietetics professional as there are other factors that effect nutritional needs not discussed here.
If you are like most people who would like to know their calorie needs, but do not have access to laboratory technology for assessing energy requirements, the above equation and activity factor technique is likely a good method for you. The resulting estimated daily energy needs number can serve as a starting point and guide for your personal nutritional needs for health and performance.