BMI and GI – can we trust the numbers?

Body mass Index, or BMI, has been used by doctors and other health professionals as aindicator of healthiness. And Glycemic Index (GI) has been a popular tool for nutritioists to advise on the rate at which foods will release sugars into your bloodstream.

The trouble is, niether tool is perfect nor do they take into accout the differences in our indivdual body compostion or metabolism.

BMI is poorly used and understood

We’ve written about why BMI is a poor tool before. And a recent article at Prevention discusses five myths regarding this highly used tool.

Two things stand out in this article. A high BMI might be associated with some health conditions but it is not the cause. The old “coincindence is not causality” rule applies here. You can be extremely fit and healthy and have a high BMI. Likewise, you can have a poor diet and low levels of fitness and be in the healthy BMI range.

Perhaps the most damning part of the BMI puzzle is this.

The [US] federal government changed BMI guidelines in 1998, dropping the overweight category from 27.8 to 25. Overnight, a woman who was 5 feet 4 inches tall and 155 pounds was suddenly considered overweight.

At the time, Bacon was a PhD student working on a dissertation on body weight and health. Her advisor was on the committee that made the recommendation, so she asked her why would they recommend lowering the numbers when all evidence pointed toward the need to raise them.

“She said to me, ‘Linda, we came to the same conclusion [that the numbers should be raised], but government officials told us that we needed to make a recommendation that aligned with global standards.'” Bacon was floored—how could it be that politics played such a large role in this decision? So she took a look at how the global standards were set. It turns out that the task force that made that recommendation was funded by the only two pharmaceutical companies that made weight loss drugs at that time.

So, the reason the healthy range for BMI was reduced was to sell diet pills.

GI can’t be trusted

A new study conducted by the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts that was published in the American Journal of Clinical Nutrition found

The much-touted glycemic index has limited utility as a tool to predict how a food affects blood sugar levels

The issue, the researchers said, is we all react differently to foods so there can be a signficant variation in how much sugar is released from the food we eat. That difference can be between 20-25% said the researchers.

The abstract of the full study can be ready here.



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