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  • Q&A with Australian Health Practitioners

    How is goal weight determined?

    Related Topic
    Weight Management
    My doctor has set me a goal weight of 70kg but the weight watchers goal weight is different. Do I need to provide a doctors certificate in reference to goal weight due to the differences or is this goal weight from my Dr OK?
    • 2 answers
    • Martin Flood
    • Elizabeth Newsham-West
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    Martin Flood

    Pharmacist

    Martin is a Lifestyle Medicine Practitioner who specializes in helping people overcome their physical, mental and emotional struggles with food, eating and weight.His online Breakfree … View Profile

    Is there currently a universally accepted & scientifically validated method to determiny your goal weight?
    Well, actually no there isn't.
    But there are some things you should be aware of.

    A ‘goal weight’ is often a figure that is selected to get you closer to a ‘healthy’ BMI (body mass index). But unfortunately, BMI is not a good instrument to use when it comes to determining an appropriate weight loss goal (I won't bore you with the details for why this is).

    Your fat mass is actually far more important than your body weight. Losing 5Kg of body fat will provide you with far more benefit than simply losing 5Kg of body weight. And your fat mass is something your body tightly controls. We are all born with a particular body fat set-point (determined by our genes & it's different for everyone). Your body has in-built protective mechanisms in place to try & prevent you from losing too much fat at once, & so your fat mass will always tend to hover around this set-point +/- 10-15%.

    However, your set-point is not fixed for life. Unfortunately it tends to increase (many factors involved here) & as it does, so too will your weight. The focus of  any successful long term weight loss program shouldn't be on weight loss per se, but on reducing your body fat set-point to a healthier level.

    So how does this practically work?
    Well, you need to firstly know how much body fat you are currently carrying. There are specific types of body composition scales that can tell you this. So if you are currently 40% body fat & you weigh 80Kg, the you are carrying around 32Kg of fat. Your non-fat mass (lean body mass) is therefore 80-32= 48Kg.

    The next step is to determine your body fat % goal. There are a number of factors to consider here including your weight history, age, health & genetics. But as a rough guide,(and this may not be appropriate for you) let's say your goal is to be 30% body fat.

    The following formula is now used to calculate your goal weight:
    Goal Weight = Lean Body Mass / 100-Body fat % goal
    = 48 / 100-30
    = 48 / 70%
    = 48 / 0.70
    = 68Kg

    So your goal weight in this case is 68Kg with a body fat % of 30% (which is 20Kg fat). If you achieve this you would have lost 12Kg of body fat.

    The other thing to know is that you shouldn't be trying to lose this 12Kg in one hit. If you lose it tooi quickly, your body fat set-point wont fall. You should only lose 10% fat mass at a time. Maintain this for a few months & then lose another 10%. Maintain this again & so on, repeating until your goal weight has been achieved. So in this example where you started out with 32Kg of fat - your short term goal should be to initially lose 3Kg of body fat.

    Okay, I hope I haven't totally confused you. with all this maths. It is a rather complicated issue. And a lot more needs to be considered than just refering to a BMI table.

    Good luck.

    Onwards & Upwards
    Martin


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  • Elizabeth Newsham-West

    Dietitian

    Healthcare professionals use a number of  guidelines to assess a healthy body weight for you personally. Growth charts for children, the WHO BMI and waist circumference provide some information on what your body weight should be for you. These guidelines are used for the general population and provided to give you a general measure of your risk for  obesity related disease which Weight Watches would use.
    However healthcare professionals do not use these alone.  We interpret these guidelines considering other individual factors such as individual  medical information and history , familial risk , ethnicity and age. This is why your doctor will have recommended a weight that may be different to Weight Watches and one that is more healthy for you.

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