Body Mass Index (BMI) – is used to determine a person’s ideal weight based on height and weight. The more body fat is derived the greater the risk of a person encountering health problems.
To work out someone’s estimated energy (kilojoule/calorie) requirements, you need to first work out the person’s BMI statistics.
To determine your BMI, follow this BMI 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 the BMI, we can then work out which category of BMI the person is in and which weight we should use when working out that person’s requirements.
For athletes and weightlifters, the BMI may not work as they may be heavier due to a higher muscle mass.
The people within the healthy weight range of BMI category (20-25) we use their actual weight.
If their BMI is in the overweight category (BMI=25-30) we work out their requirements based on what their ideal body weight should be (ie. what their weight would be to be 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 that their fat mass is not metabolically active and therefore we do not want to feed the fat. For the obese, this is for safe and realistic weight loss.
Obviously, as we get closer to the person’s weight loss goal, we can change the requirements based on their new weight category.
Once we know what weight to use;
Actual weight for BMI 20-25
Ideal weight for BMI 25-30
Adjusted weight for BMI 30+
We 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 worked out using an equation of your choice. Equations such as Schofield or Harris Benedict are commonly used.
This will then give a kilojoule value based on the above for weight maintenance. The general rule for weight loss is then to minus 2100kJ from estimated energy requirements.
In Australia, we use the measure of energy in kilojoules, other places they use calories. It’s just a different way of measuring energy. There are 4.2 kilojoules per 1 calorie..
,Keeping in mind;
1g carb =16.7kJ
1g protein = 16.7kJ
1g fat =37.7 kJ
For someone whose estimated energy requirements for weight loss are 5500kJ and they want the 50% of their diet from carbs.
5500kJ x 50% = 2750kJ from carbs (2750 divided by 16.7) = 164g carbs per day
The total carbohydrate recommendation could then be split up over the day to keep carbohyrdrate intake consistent. So you could divide 164g by 3 meals per day to aim for 55g CHO per meal. This assists with better blood glucose/sugar control.
With protein and fat, consume them how you would like throughout the day.
At the end of the day, the above is all estimates worked out 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 our closest estimate.