Updated: Aug 28, 2020
“I need help to lose weight, I need to get to a healthy BMI”. I hear that so often from my clients.
But who is to say what their healthy weight is?
One thing is sure, BMI is not the right tool for assessing what is a healthy or unhealthy weight for one individual.
Before I delve into the reasons why, let me remind you what BMI is. It stands for Body Mass Index and is calculated by taking one’s weight (in kilograms) and dividing it by one’s height (in meters), squared. (For example, if an individual weighs 80kg and measures 1.7m, their BMI will be 80/1.7sq = 27.7).
Based on this indicator the World Health Organization  drew the following categories for adults:
BMI below 18.5 – underweight
BMI 18.5 to 24.9 – healthy weight
BMI 25 to 29.9 – overweight
BMI above 30 – obesity
Now, let’s look at why is BMI a useless tool when dealing with individuals.
BMI is a very basic and simplistic tool
BMI only takes in account height and weight and ignores a whole bunch of very important parameters such as age, ethnicity, gender, body composition (fat vs muscle), genetics. Yet all these factors will have a huge impact on one’s weight, as well as what would be their healthy weight.
For example, with age, it is much better to carry some extra weight as it makes you far more likely to recover from a serious illness or injury. Therefore, for people above 65, the healthier range is 25-29.9 (“overweight”). 
Another interesting example is the case of athletes . Many have a high BMI, putting them in the overweight and sometimes obese categories, when in fact they are much healthier than most people fitting the BMI “healthy weight” range. Take Arnold Schwarzenegger at his prime age of bodybuilding (age 27). His BMI was 30.6 – he was officially obese!
And, as it happens, having a healthy BMI can actually hide an unhealthy body composition. In nutrition we call these people T.O.F.I. (pronounced toffee) which stand for “Thin Outside, Fat Inside” because despite their light weight, they carry very little muscle mass and a lot of fat around their organs.
Could BMI be useful for an average person though?
Now that we have talked about the limitations of BMI for some out-liars such as older or muscly people, we might be tempted to think that for the average Jane and John Doe (say, middle aged, exercising a bit, otherwise healthy) BMI is a “good enough” tool to estimate what the right weight for them might be.
Unfortunately, that would be wrong too. This is because, as I mentioned earlier, a huge factor of our weight is our genetic making. In fact, it is estimated that 40 to 70% of our weight could be dictated by our genes! And, that 10% to 25% of people with genetic obesity are totally healthy. 
In a nutshell, BMI is a very blunt and unsophisticated tool which gives a very poor assessment of one’s health in relation to weight. Which leads us to wonder: why it is used so widely?
BMI: a powerful tool for public health and epidemiology studies
Whilst useless for an individual, BMI is actually a great tool to assess the health of a whole population.
Great because, when used in the general population (e.g., a country, a region) with a large sample, it becomes a reliable indicator of whether that population is at a healthy weight. And if you look at this indicator over time it helps to assess the impact of public health policies. (check these infographics  to see how obesity is evolving overtime throughout the world. No doubt that something needs to be done about it – and that what we have done so far hasn't worked yet!)
Great also, because it is so simple! You just need 2 variables (height and weight) from your population and with that you can quickly get a sense of whether your population’s health is at risk.
The story of BMI's origins is enlightening – it was created by life insurers who had a large dataset made available to them. They were trying to find a way to easily gauge the risk of mortality of their clients and thus assess their own risk as life insurers. They noticed that they reliably had info about height and weight and, after a few trial and errors, came up with the BMI as a useful tool to predict mortality rates in their sample.
For more info about the concept of BMI, this video by Vox “What BMI doesn’t tell you about your health” sums it up really nicely.
So, what is your healthy weight?
That is very good question for which we do not have a straight answer.
We have lots of tools available such as Waist-to-Hip ratio, Waist-to-Height ratio, Body Fat Percentage. These are all useful indicators but they are not completely satisfactory.
Instead, I prefer this slightly more subtle and less categorical definition:
Your healthy weight is the one that you can maintain over long term and that enables you to live your life to the full, with minimal impact on both your physical and mental health.
If you think that you need to lose weight and want to do it in a sustainable way, with the support of an evidence-based expert specialised in nutrition and behavioural changes, have a look at my online Worth the weight programme and join the free introductory session scheduled on the 11th September - register here.
 Winter JE, MacInnis RJ, Wattanapenpaiboon N, Nowson CA. BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr. 2014;99(4):875-890.
 Herrera BM, Lindgren CM. The genetics of obesity. Curr Diab Rep. 2010;10(6):498-505. https://dx.doi.org/10.1007%2Fs11892-010-0153-z
 Mike Gibney, Ever Seen a Fat Fox? Human Obesity Explore