News · Heart & Metabolic
Sitting at the top of the 'normal' BMI range carried 78% more type 2 diabetes across 1.8 million people
Guidelines draw the risk line at a BMI of 25 and treat everything below it as fine. Pooling 11 cohorts, researchers found people at 22.5 to 24.9 already carried substantially more diabetes than people lower in the same band.
- Sitting at the top of the healthy weight range carried 78% more type 2 diabetes.
- The comparison is against people lower in the same healthy band, not against overweight.
- Guidelines draw the line at a BMI of 25 and treat everything under it as fine.
- The studies disagreed with each other a great deal, so the size of the figure is soft.
- Pooled from 11 cohorts, 1.8 million people and 84,414 cases of diabetes.
A body mass index of 24.6 gets you told you are fine. A body mass index of 25.1 gets you told you are overweight. Nothing biological happens in between.
That gap has been an open joke in obesity research for years, and it has rarely been quantified. Writing in the International Journal of Obesity, researchers pooled cohort studies that had reported risk separately for the upper and lower halves of the official normal band.
Compared with lower BMI values within the normal weight range, high normal weight BMI values were associated with a greater risk of type 2 diabetes, at 78% higher.
Where the threshold came from
Body Mass Index measures how much you weigh compared to how tall you are, and an adult who has a BMI of 30 or more is considered to have obesity.
The 25 line beneath it is doing a different job from the one most people assume. It is a convention drawn across a continuous distribution, useful for comparing populations and clumsy for describing an individual. Risk of diabetes does not sit flat until 25 and then rise; it climbs the whole way.
What has been missing is a number for how much of that climb happens inside the band everyone is told not to worry about.
What the pooling shows
Eleven cohort studies were included, with 23% of participants and 17% of cases being classified as having high normal weight BMI values, meaning roughly 22.5 to 24.9.
The diabetes estimate rests on 10 studies with 1,818,036 participants and 84,414 cases, which is a large evidence base by any standard, and the authors grade it moderate certainty.
Blood pressure moved too, though on much thinner ground. Evidence for hypertension was limited, based on three US cohort studies, at 42% higher and graded low certainty. Three cohorts is three cohorts, and the article should not lean on it.
The number that undercuts the headline
Heterogeneity for the diabetes result was 98%.
That statistic describes how much the individual studies disagreed with each other, and 98% is close to the ceiling. It means the true effect almost certainly differs between the populations studied, and that the pooled 1.78 is an average across estimates that were not measuring the same thing in the same people.
The direction survives that. Every included study pointed the same way, which is why the certainty grade is moderate rather than very low. The size does not survive it, and anyone quoting “78% higher” as a personal risk figure is over-reading a pooled average with a wide interval.
Interestingly, the hypertension estimate had the opposite problem: almost no heterogeneity, and almost no studies.
What the design cannot do
This compares people, not changes.
Someone with a BMI of 24 and someone with a BMI of 21 differ in more than three units of index. They differ in muscle, in waist, in fitness, in what they eat and in how much of their weight sits around the organs. Cohort studies adjust for what they measured and inherit everything they did not.
Crucially, nobody in these studies was followed through a move from 24 to 21. The review cannot say that shifting down the band lowers risk, only that the people already lower in it developed less diabetes.
What to do with a normal BMI
Obesity increases your risk for many other diseases and health problems, especially if the extra body fat is carried around the waist, and that last clause is the one that transfers here.
A BMI of 24 with a large waist, a family history and a fasting glucose creeping up is not the same clinical situation as a BMI of 24 with none of those, and no single number captures the difference. The useful reading of this review is not that 24 is the new 26. It is that “your BMI is normal” answers a question about a population, not about you.
The authors put the implication carefully: if replicated, the findings may justify reconsideration of BMI thresholds. That is a question for guideline committees. For an individual, the operative fact is simpler, which is that the reassurance a normal BMI provides has always been softer than it sounds.
People also ask
What did the review find?
Compared with lower BMI values within the normal weight range, high normal weight BMI values were associated with a greater risk of type 2 diabetes (RR 1.78, 95% CI 1.30 to 2.45, I-squared 98%; 10 studies, 1,818,036 participants, 84,414 cases; moderate certainty). Evidence for hypertension was limited, based on three US cohort studies (RR 1.42, 1.33 to 1.51, I-squared 0; 129,046 participants, 22,254 cases; low certainty).
What counts as high normal weight?
A BMI of roughly 22.5 to 24.9, compared with lower values inside the same official normal band. In the pooled studies, 23% of participants and 17% of cases sat in that upper slice.
Why does the 25 threshold exist at all?
It is a convention that came from population distributions rather than from a biological cliff, and it has been criticized for decades. Risk does not step up at a round number; it rises continuously, and any cutoff draws a line through a gradient.
How seriously should the 78% figure be taken?
As a direction more than a magnitude. The interval runs from 1.30 to 2.45, and heterogeneity was 98%, meaning the individual studies disagreed almost completely about the size. The authors still grade it moderate certainty because the direction is consistent.
Why were Asian cohorts excluded?
Because Asian populations develop diabetes at lower BMI values, and international guidance already uses lower thresholds for them. Mixing the two would have blurred exactly the question being asked.
Does this mean people at BMI 24 should lose weight?
The review does not test that. It compares people who happen to sit at different points inside the normal band; it does not follow anyone who moved between them. Whether losing 3 kg from a BMI of 24 changes anything is a separate question.
What is the practical version?
That being told your BMI is normal is not the same as being told your risk is low, particularly alongside family history, waist size or a raised fasting glucose. This is general information rather than medical advice.
References
- High body mass index within the normal weight range and risk of type 2 diabetes and hypertension in US and Europe: a systematic review and meta-analysis. International Journal of Obesity, 2026.
- MedlinePlus. Obesity. US National Library of Medicine.
- MedlinePlus. Diabetes Type 2. US National Library of Medicine.