Body Mass Index (BMI) is used to determine a person’s ideal weight based on height and weight. The more body fat you have, the greater your risk of encountering health problems.
To work out your estimated energy (kilojoule/calorie) requirements, you need to first work out your BMI statistics.
To determine your BMI, use this equation:
BMI = Weight (kg) divided by Height squared (m2).
Any easy way to do this calculation is to divide your weight in kilograms by your height in metres, and then divide by your height in metres again.
Example: If your weight is 90kg and your height is 1.8m…
90 / 1.8 = 50
50 / 1.8 = 27.8 – This is your BMI!
Once you get your BMI, your can then work out which category of BMI you’re sitting in, and which weight you should use when working out your requirements.
For athletes and weightlifters BMI may not be an accurate representation of health, as they may be heavier due to having a greater muscle mass.
For people within the healthy weight range of BMI category (BMI = 20-25), we use their actual weight.
If your BMI is in the overweight category (BMI = 25-30), we work out their requirements based on what their ideal body weight is (ie. what they’d need to weigh to have a BMI of 25).
If a person is in the obese weight category (BM = 30 or more) we work out their adjusted body weight using the below formula:
Adj BW = [(Actual Body Weight –IBW) x 0.25] + IBW
The reason we don’t use their actual body weight is their fat mass is not metabolically active, and therefore we don’t want to feed the fat. For obese people, this is for safe and realistic weight loss.
Obviously, as you get closer to your weight loss goal, you can adapt your requirements based on their new BMI weight category.
Once you know what weight to use:
Actual weight for BMI 20-25
Ideal weight for BMI 25-30
Adjusted weight for BMI 30+
You can work out someone’s requirements for weight maintenance.
Then other parameters such as age, gender, activity and stress levels can be looked at.
Requirements can then be determined using an equation of your choice. Equations such as Schofield or Harris Benedict are commonly used.
This will then provide a kilojoule value based on the above considerations for weight maintenance. The general rule for weight loss is then to subtract 2100kJ from your estimated energy requirements.
In Australia, energy is measured in kilojoules, while other countries use calories. It’s just a different way of measuring energy. There are 4.2 kilojoules per 1 calorie.
Once you have your total daily kJ requirements, you can start breaking it down into macronutrients.
Keeping in mind;
1g carb =16.7kJ
1g protein = 16.7kJ
1g fat =37.7 kJ
If your estimated energy requirements for weight loss are 5500kJ and you want 50% of your diet to come from carbs:
5500kJ x 50% = 2750kJ from carbs (2750 divided by 16.7) = 164g carbs per day
Your total carbohydrate recommendation could then be split up over the day to help keep your carbohydrate intake consistent. So you might divide 164g by 3 meals per day, to aim for 55g carbs per meal. This assists with better blood glucose/sugar control.
With protein and fat, consume them as you’d like throughout the day.
At the end of the day, the above are all estimates worked out based on estimated requirements, so they need to be monitored regularly and adjusted appropriately, ideally with the help of an Accredited Practising Dietitian.
Unless you have access to an Indirect Calorimeter (an expensive machine that measures energy usage) then equations such as Schofield and Harris Benedict are your closest estimate.