A new study has cast doubt on the accuracy of the glycemic index (GI).

The GI is a commonly used rating scale for how foods affect blood sugar—too low on the scale, and you may feel hungry and lethargic; too high, and your pancreas will secrete more insulin, prompting the excess sugar to be stored as fat.

But when scientists at Tufts University tested 63 adults for six weeks, they found that when subjects ate the same foods at regular times, their blood glucose still varied by up to 20%. An even bigger variation was seen when the same food was eaten at the same time by different people.

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