# How exactly do BMIs work, practically?

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Posts: 1 Member
I'm 172 centimeters, 5"7 I think. According to the Australian Heart Foundation's BMI calculator, at 118 pounds I would be healthy at 18.7, but at 117 I would be underweight at 18.3. Why would losing one pound take away .4 from the total number? I'm 123 currently and don't look anywhere near underweight physically, although that could be lack of exercise. That's not much relevant though, since what I'm more confused about is how exactly the totals work, in a practical sense.
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## Answers

• Posts: 3,365 Member
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The formula for BMI is weight in kilograms divided by height in meters squared. So there is some maths in play. Also while you may think you don't think you look anywhere near underweight that may be due to your biasagainst yourself and / or body dysmorphia issues.

Maybe worthwhile to check in with a doctor or another medical person or perhaps have a DEXA body scan (which will give you an indication of body fat %).
• Posts: 14,054 Member
edited May 1
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Health and statistics. Rules of thumb. Then you look at the individual situation.... individually past that.

Is there blood in your poo or pee? Then your health deserves the time investment by you and your health provider to check for internal issues.

Are you under BMI 20 (or significantly over 25), NOT of Asian/oriental/whatever the current non offensive term denoting the appropriate origin where the BMI goalposts slightly change, and either an adult male, or an adult female of reproductive age?

(You may notice that I actually moved the goal posts from the 18.5 minimum because BMI 20 already introduces concerns in certain specific situations)

Then your health deserves a second look by you and your health provider to ensure that you're at an optimal weight for you.

A reminder that BMI does not mean that an individual is optimally healthy anywhere in the normal range.

All it says is that most individuals who are healthy when it comes to weight for their height happen to be in the normal range.

Which means that your own healthy point is likely to be inside that range but not necessarily at any one point in it.

For Anne it may be BMI 21. For myself it may be 23 or 24. For someone else it may be 24.9.

Generally speaking even a little underweight than optimal for a particular person introduces more health problems faster than a little (or even more than a little) overweight for the same person

• Posts: 14,054 Member
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There is an implied question here. Presumably you're looking at these numbers because you're at 123lbs already, yet are looking to lose more weight to achieve a certain look

But as you discovered you have at most a few lbs to play with and you have effectively already identified as issue with lack of exercise.

Might appropriate exercise instead of trying to lose weight get you closer to where you want to be?
• Posts: 28,047 Member
edited May 3
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I'm almost your height. I agree with the recommendation to get an objective option about your weight goals from your doctor.

Also, see Kelsey Wells' story. She thinks, and I and many others agree, that she looks better at a higher weight than her goal weight of 122. She's done body recomposition through exercise. She's 5'6".

https://www.popsugar.com/fitness/bbg-before-after-pregnancy-kelsey-mysweatlife-42105801
• Posts: 10,050 Member
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ddsb1111 wrote: »
What’s more important to you, the way your body looks or the number on the scale? If it’s the number it might be a good idea to ask yourself why.

I weigh the same now as I did in college, but I carry it completely differently because I have less muscle and more fat, so I’m much bigger now. The answer for me to change that isn’t to lose weight but change my body composition, right?

The problem is that some people don't have a realistic perception of the way their body looks. I think it's worth considering that possibility with respect to someone who is nearly 5'8" and who weighs 123 lbs and seems to want to lose additional weight and is questioning whether they should ignore their BMI to continue losing below 118 lbs.
• Posts: 14,054 Member
edited May 3
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ddsb1111 wrote: »
What’s more important to you, the way your body looks or the number on the scale? If it’s the number it might be a good idea to ask yourself why.

I weigh the same now as I did in college, but I carry it completely differently because I have less muscle and more fat, so I’m much bigger now. The answer for me to change that isn’t to lose weight but change my body composition, right?

The problem is that some people don't have a realistic perception of the way their body looks. I think it's worth considering that possibility with respect to someone who is nearly 5'8" and who weighs 123 lbs and seems to want to lose additional weight and is questioning whether they should ignore their BMI to continue losing below 118 lbs.

@lynn_glenmont I don't read what you say above, what @ddsb1111 wrote further up and you quoted in your post, or what @kshama2001 or myself wrote as saying anything essentially different... seems to me that all four of us are highlighting different aspects and approaches to the same potential issue.