Most cases of the disease could either be prevented or reversed if someone's BMI was kept below their personal cut-off point at which abnormal blood sugar levels are triggered, an expert behind the study said.
Everyone will have a different threshold which sees them at risk of becoming diabetic, explaining why some people with a healthy weight develop the condition and some who are overweight do not, the Cambridge University professor said.
Professor Brian Ference said the findings of the study could have “significant implications” for the approach to screening for, preventing, treating and even reversing the condition.
The study of 445,765 participants of the UK Biobank saw people divided into five groups according to genetic risk of diabetes and five groups according to BMI.
Participants, just over half of whom were women, had an average age of 57 and were followed up until an average age of 65.
During that period 31,298 of them developed type 2 diabetes.
Those in the highest BMI group had an 11-fold increased risk of diabetes compared to the lowest BMI group, researchers said, and a greater likelihood of developing diabetes than all other BMI groups, regardless of genetic risk.
Investigators also discovered that the length of time a person had a higher BMI did not have an impact on the risk of diabetes.
Professor Ference said: “This suggests that when people cross a certain BMI threshold, their chances of diabetes go up and stay at that same high-risk level regardless of how long they are overweight.”
Explaining further the significance of the study, he said: “You can prevent most cases of diabetes by keeping BMI below a person's threshold.
“But it (the study) also implies something that we haven't focused on in the past and that is we can also probably reverse most cases of diabetes if we lower somebody's BMI aggressively below their BMI threshold relatively soon after they develop diabetes.
“I think the fact that BMI appears to have a threshold rather than a cumulative effect on the risk of diabetes really has potentially significant implications for how we think about changing screening, preventing, treating and reversing diabetes.”
He said work is ongoing to be able to allow researchers to estimate what someone's threshold is and that he hoped for a breakthrough on that by the end of this year or early next year.
The findings, presented at the European Society of Cardiology Congress, highlight the need to regularly measure BMI and track blood sugar levels of people at high risk, the British Heart Foundation said.
Professor Jeremy Pearson, the organisation's associate medical director, said: “This important study of nearly half a million people shows that BMI is a more vital risk factor for type 2 diabetes than we previously realised.
“When someone's BMI goes above their personal threshold, blood sugar levels increase, triggering the onset of type 2 diabetes, which can lead to damaged blood vessels and increased risk of developing heart and circulatory diseases.
“If you are overweight, making small, long-term changes to your lifestyle such as reducing portion sizes and being more physically active can help lower your BMI, which is good for your heart and blood vessels.”
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