A new genetic study provides evidence that a higher insulin level is a key causal factor in weight gain.
The way the study is run provides useful food for thought about why some people develop obesity whilst others do not. It also adds to the discussion about what can be done to limit the weight gain that is occurring in nearly every country.
There are two theories as to why people grow obese. One is that high calorie intake and too few calories being burned off leads to weight gain. The other proposes too much carbohydrate which promotes high levels of insulin, the fat storage hormone, causes weight gain.
The calories i, calories out theory is the theory most talked about by medical professionals, charities and the media. However, the theory has its critics as it does not explain why some people have a high calorie intake yet remain slim whilst another person may have a lower calorie intake and put on weight despite being just as active.
The carbohydrate-insulin model is less discussed in the media but has been gaining traction in research circles. Indeed, the carbohydrate-insulin model is the basis behind Diabetes.co.uk’s Low Carb Program.
In the genetic study, the researchers from Boston Children’s Hospital used a technique called Mendelian randomisation to explore how different genetic traits influenced obesity.
Mendelian randomisation is an effective research method for exploring causation as it is stronger at controlling against reverse causation and confounding factors. Reverse causation and confounding factors are two main reasons why many studies into possible causal effects can only give associations rather than strong causal evidence.
The researchers assessed genetically determined higher insulin secretion on BMI and genetically determined higher BMI on greater insulin secretion. The results showed that being genetically predisposed to have higher insulin secretion led to a greater a BMI. However, being genetically predisposed to have a greater BMI did not lead to greater insulin secretion.
Researcher Dr David Ludwig commented of their findings: “We found that genetically-determined insulin secretion predicted body mass index with extremely high confidence and a potentially large effect across the population.”
By running the analysis from two directions, the researchers were able to show that higher insulin leads to high BMI whilst reverse causality was shown not to apply. This provides stronger evidence that a higher insulin level leads to weight gain than would be possible with regular epidemiological studies.
Dr Ludwig added: “Of particular importance, the ‘reverse’ relationship was null. That is, genetically determined body mass index did not predict insulin secretion to any degree.”
The study is published in the Clinical Chemistry journal.

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