The values you see are the energy expenditure and not an intake recommendation. You have 300-500 grams of stored glycogen in your body that you can distribute over the competition time you are doing. For even if you consume 180 grams of carbohydrates per hour, it doesn't mean you should consume 180 grams per hour because then your stomach will completely crash. Your body will take from the stored reserves and as glycogen decreases, you'll get more tired, your intensity will drop, and you will increase the amount of fat as fuel, which works well at lower intensities.

Formulas and Calculations

This is undoubtedly one of the more advanced and reliable calculators available on the market today. However, it should be pointed out early on that it is a theoretical calculator. It takes into account known parameters such as age, gender, weight, height, maximum heart rate, and uses these values in the algorithm to provide you with the most accurate value possible. What it does not take into account are the individual variations that exist from person to person where genetics and training habits play a role.

If you want 100% accurate values, you simply need to go to a test lab and conduct an RQ test or VO2max test. But we can guarantee that the equations in this calculator are based on solid, conflict-free studies where lab equipment has been used to obtain values from a larger number of athletes.

Gender

How fats and carbohydrates are burned varies between men and women, where women tend to be able to use fat as fuel at higher intensities compared to men. This means that different formulas are used for your substrate utilization depending on whether you are a woman or a man, and therefore you must specify gender (1)

Your gender also affects the equation for calculating your basal energy expenditure. For this equation, Cunningham's equation is used which is the most validated and reliable formula for athletes currently available. Often, Harris & Benedict's formula is used (2). However, that formula is less accurate for an athletic population and has shown significant error margins of up to 50% (3). Cunningham's formula is much more accurate for athletes with an error margin of 2-8%, which is a very good margin of error for an equation (3, 4, 5, 6). 

Age, Weight & Height

This is the basic data needed to calculate your basal energy expenditure. The energy required to survive and maintain weight stability. Basal metabolism (BMR) or Metabolic Basal Rate (MBR) as it is also called accounts for approximately 50% of your total energy expenditure per day (60-70% on an inactive day (3)). These values are used in Cunningham's equation to calculate your basal expenditure, so be sure they are correct to get the most accurate result possible.

When all of this is entered and completed, your basal energy expenditure is calculated. This is the energy required to keep you alive if you were to lie still in bed all day.

Max Heart Rate

It is best if you have done some form of maximum heart rate test and know your own maximum heart rate. In such cases, you can input it yourself. Otherwise, it is calculated through a scientifically validated formula (HUNT) developed and validated in a large population (3320 individuals) of healthy individuals (7).

For men: Max Heart Rate = 213 − (0.65 × age)
For women: Max Heart Rate = 210 − (0.62 × age)

It should be noted that there is a standard deviation of 11 beats/min (5%). This deviation is better (i.e., lower) than the classic 220 – age, which has a margin of error of up to 20 beats/minute (~10%) in us athletes. But obviously, a proper maximum heart rate test is the best possible.

Note: We also analyzed the formula (208 – 0.7 x age, and the sports-adapted 206 – 0.7 x age) which is quite a common formula. However, after many tests, we found that this formula had a fairly large margin of error for us athletes and therefore was removed from the equation. Feel free to test it yourself and see what number it gives you and provide feedback if you find it more reliable.

Distribution of Fat & Carbohydrate Burning

Energy Expenditure

To calculate your total energy expenditure, MET values are used. MET stands for Metabolic Equivalent and is frequently used to calculate energy expenditure. 1 MET corresponds to 3.5ml of oxygen/kg of body weight per minute. The problem with MET is that it is an unreliable and very unspecific system for calculating energy expenditure during activity. The MET system was developed in the 1970s by measuring oxygen consumption in a (1) 70kg man aged 40 years who had a consumption of 3.5ml of oxygen/kg of body weight and minute. The system was developed as a classification system to rank the energy cost of different activities and works poorly for calculating energy expenditure to more precise values (14).

A corrected table was released in 2000 and generally works a little better as the values are adjusted (15). In this calculator, we have made a correction to the MET values based on a new comprehensive review conducted in 2005 (14), resulting in the calorie expenditure you get here being the most realistic expenditure currently calculable based on the values we have here.

Substrate Distribution

The numbers are derived from studies that examined substrate distribution at different intensities among moderately to highly active athletes (8).

When you investigate the distribution of fats and carbohydrates as energy substrates, VO2max is used as a measurement tool. VO2max is tested in participants in studies, then an intervention is performed where, for example, the diet is changed to a strictly LCHF with a maximum of 5-10% carbohydrate intake of the total energy intake, and after that, post-tests are conducted again. You can introduce any factor during the intervention, change training model, medications, etc. But VO2max is often included as a measurement tool. Participants in studies by Noakes et al have achieved oxidations just over 1 gram of fat per minute at around 65% of VO2max (9,10). 65% of VO2max is considered to be our upper FatMax window where athletes can oxidize the most fat, which corresponds to a heart rate of about 155 beats/minute for a person with a maximum heart rate of 195 beats/minute.

But in our equation, the formulas are based on equations derived from athletic individuals (50/50 between men and women) with a varied diet that includes carbohydrates, fats, and protein as a natural part. FatMax for these individuals lies at about 50-60% of VO2max (3,11). Above 70-75% of VO2max, substrate oxidation is essentially identical between both trained and untrained athletes, as well as “fat-driven” and regular athletes. But up to that point, well-trained individuals are better at utilizing fat and conserving their muscle glycogen compared to untrained individuals. Thus, the equations in this calculator are based on the assumption that you are fairly well-trained and capable of utilizing fat as fuel up to at least 70-75% of VO2max (12).

Now you enter your maximum heart rate or have it calculated. The first thing we need to do is convert your maximum heart rate to a VO2max value, i.e., your maximum oxygen uptake. This is also what, for example, the heart rate monitor on your arm does to calculate which zone you have trained in. For this, we use the equation from Swain et al, which was developed for both men and women (50/50 in the study) (13). An interesting note is that this is also the equation that your heart rate monitor uses if you have a Suunto or Garmin (14).

((Heart Rate / Max Heart Rate) * 100) – 37) / 0.64

This equation is validated up to 92% of your maximum heart rate. For values above that, we have extrapolated the values (assuming that the equation that holds for the points up to 92% of max heart rate (85% of VO2max) also holds for the data points above 85% of VO2max).

Individual Variations and Sources of Error

Individual variations where people after long periods of dietary change and training adaptation have reached values of up to 1g of fat oxidation per minute. This can be compared to the normal population that consumes a varied standard diet that typically reaches a maximum of 0.7g/min.

The equations in the calculator are not based on individual studies aimed at demonstrating that metabolic flexibility exists in us humans. That we can reach values of up to 1g of fat per minute is proven, but it comes at the cost of reduced carbohydrate burning. However, total energy expenditure remains the same.

Common sense applies; these are recommendations and not absolute values. The distribution between fat/carbohydrates at different intensities varies based on several factors such as genetics, training habits, and dietary patterns. These are values extrapolated from studies on trained individuals with varied dietary habits and moderate training backgrounds. But this is as good an estimate as you can get through formulas, and it holds true for the majority of athletes.