What counts as a healthy body weight? Major study rewrites the rules

Last Updated on July 10, 2023 by Keir Watson
Read Time: 6 min

Looking at the image above, who would you think had the healthier weight? Lean Jack Sprat or his fullsome wife?

Under the current medical model, to answer that properly, you would need to calculate the Body Mass Index (BMI) of Mr and Mrs Sprat, then compare those figures to the reference ranges for Underweight, Healthy Weight, Overweight, Obese or Very Obese. The further their BMI is found to be from the Healthy Weight range the higher their risk. From this analysis, they can then be advised accordingly. Knowing what constitutes the “Healthy Weight” range is therefore vitally important.

A new study, however, demonstrates that we have had the boundaries wrong for decades. In this article, we explore the ramifications of this new data, using poor Mr and Mrs Sprat as our guinea pigs.

Read time: Main article 5 minutes (1000 words) | Additional analysis 2 minutes (400 words)

Current “Healthy Weight” Guidelines

Technically, BMI is calculated by taking a person’s mass in kilograms and dividing it by the square of their height in metres. This munber is then used to categorise a person as Underweight, Healthy Weight, Overweight, Obese or Very Obese. If you can’t be bothered to work out your BMI you can use the NHS chart below to find out where you would fall.

NHS BMI calculation chart

While the chart can tell you which category you are in if you want an accurate BMI you can put your height and weight into the NHS BMI calculator page. It will then give you a readout like the one below that I made for our imaginary friend, 55-year-old Mrs Sprat (who, remember, could eat no lean). Let’s make her 5′ 8″ tall, with a weight of 12 stone 6 lbs. A strapping lass who will tell you firmly that she is “well built” but definitely not fat!

As you can see, the NHS considers a healthy BMI to be between 18.5 and 25 kg/m². So they advise Mrs Sprat to lose weight. Mrs Sprat is quite bothered about this as she knows it is difficult to do, but now the doctor has warned her she can’t help but worry about it.

Meanwhile, at the svelte end of the “healthy” NHS scale, her husband Jack Sprat (who could eat no fat — gall bladder problems I reckon) at 6’1″ and 10 stone zilch, clocked in at a svelte 19 kg/m² getting the big thumbs up from the NHS as he fell within their healthy range. Somewhat smugly, we went away feeling that he had a long life and good health to look forward to. If only his wife could learn to eat like him!

But wait! New research suggests that the advice they just received from the NHS may have served them both badly…

Jack Sprat could eat no fat.
His wife could eat no lean.
And so betwixt the two of them
They licked the platter clean.

(Traditional English Nursery Rhyme)
Picture by Frederick Richardson – Wikimedia

Study: Association of BMI with overall and cause-specific mortality

Here is the paper, published at the end of last year in the Lancet, that has challenged our assumptions about the healthy weight range: Association of BMI with overall and cause-specific mortality: a population-based cohort study of 3·6 million adults in the UK (its open source, so you can read the full thing free)

In this study — which was huge by any standards — 3.6 million people from the UK were enrolled beginning in 1998. BMI was recorded at enrolment and then again at the end of the study in 2016, or at death. Most of the analysis in the paper focused on 2 million people who had never-smoked (reducing a major confounder). During the 18 years, there were nearly 200,000 deaths. From these data, they were able to identify the link between BMI and all-cause mortality. Here’s what they found:

What you can see straight away is the typical ‘U’ shaped curve showing a greater risk of death as you get more overweight or more underweight. That’s as expected. The big difference, though, is where the sweet-spot falls, and it isn’t in the same place as that healthy BMI given on the NHS chart.

In the light of the Lancet findings, the advice the NHS and others have been giving has also been wrong. In some cases, following their advice would simply have been unnecessary, but in others it would this would have increased the chance of early death, as we shall see below.

Under the NHS, men with a BMI between 25 and 27 kg/m² and women between 25 and 29 kg/m² would be classified as overweight, and be encouraged to shed some pounds, but as we can see, such advice would be unnecessary. The risk only begins to rise significantly once one is in the NHS obese region (over 30 kg/m²). If the NHS corrected their advice, a lot of people would be spared some degree of unnecessary anxiety — a weight off their mind, so to speak.

The authors found the lowest all-cause mortality at 25 kg/m2. This sweet spot is at the very top end of the NHS “Healthy” range. Telling people with a BMI closer to the low end (between 18.5 and 22 kg/m2) that they are “healthy”, when, as the Lancet study shows, they are really underweight and carry an increased risk of death, has serious implications for their health.

Returning to the Sprats, the NHS has worried Mrs Sprat needlessly as the Lancet study shows she is actually in fine fettle. Jack, on the other hand, is the one at increased risk and would be advised to put on some weight. In fact, to reach the healthy range indicated by the Lancet data, Jack would need to put on at least 2 stone, for at 6’1″ and 12 stone his BMI would be 22.2 kg/m2, much closer to the ideal 25 kg/m2.

The error of classifying an underweight person as “healthy” is particularly problematic. You can see why from the Lancet graphs, as Jack Sprat, with a BMI of 19 kg/m2, shares the same increased mortality risk as an overweight person with a BMI of 31 kg/m2 who the NHS would currently class as obese and no doubt have considerable concerns about.


Studies of this scale only come along once in a generation. They are very expensive and designed to emphatically determine a specific question — in this case the link between BMI and mortality — which is why this paper does not finish with the usual line “this issue warrants further investigation”.

For this reason, we have to take its conclusions as emphatic. That means that our ideas about what constitutes a healthy weight have been wrong for decades and need to be changed, as does the advice given to patients, particularly those in the lower half of the accepted (NHS) “Healthy Weight” range, who should properly be classifed as underweight and encouraged to increase their weight.

Part2: Additional Analysis » For those of you who like to dig into the details, I’ve got some nice graphs and juicy details from the Lancet BMI study to share on below

Additional Analysis (if you like the detail)

Effects of age on BMI mortality association

The hazard curves change with age, with the harms of excess weight decreasing with age. The harms of being underweight, on the other hand, remain constant across all age groups (except in the over 80s when age rather than BMI begins to take its toll).

Another change that occurs with age is the lowest point of the curves — representing the ideal BMI — which moves gradually to the right, i.e. implying that a gradual increase in weight as one gets older is protective. A person who boasts in their 60’s that they are the same weight now as they were in their 20s might like to think twice about the basis for their smugness.

Causes of Death

When the researchers analysed the relationship between BMI and various causes of death some interesting results were found. Whilse for most causes of death the same U shaped graph was found, there were notable exceptions.

Underweight risks

Excess Risk of Death vs Underweight BMJ

As the graphs above show, being underweight increases the risks for suffering various neurological and psychological conditions. Of course, these graphs only show associations, so it is quite possible that depression or other psychological problems might result in weight loss, which would skew the results this way. However, the fact that the researchers measured BMI at the beginning and end of the study period makes this reverse-causality less likely.

Overweight risks

Graph showing excess risk of death associated with increased BMI (from the BMJ)

Conditions where being overweight is more significant than being underweight perhaps unsurprisingly include cardiovascular disease and diabetes. With cancers, liver, uterine and kidney cancers stand out with risk increasing rapidly with excess BMI (graphs not shown). Digestive conditions, liver cirrhosis and urogenital conditions also show a large increase in risk with excess weight.

So this is an interesting conundrum for us mortals. To me, it looks as though one would do best to aim for the upper end of the NHS ‘healthy BMI’ range, and to allow your weight to creep up over the years, rather than down. Dividing the task between husband and wife does not appear to be the best strategy after all!

1 thought on “What counts as a healthy body weight? Major study rewrites the rules”

  1. Very helpful. I used the NHS Calculator and can confirm I am in the healthy weight bracket – all thanks to you Afifah!


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