Heart rate can affect your glucose levels because it affects the blood flow around your body, for example often your heart rate will increase if you have low blood glucose levels because your body is trying to pump more glucose around the body and into the cells.
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The opposite is also true: when you are doing exercise or have a stressful event, your heart rate will increase. Blood glucose levels will increase in response to a higher heart rate as your body needs to release stored glucose (or slow down glucose absorption) to support the muscles increased demands for energy.