While I can grasp the idea that low blood sugar means “low immediate energy supply”, how come that too much sugar in the blood can cause harm?
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You’re asking why high sugar is bad for diabetic patients.
This is a complicated question, with a lot of complex biochemical pathways involved. To be honest, the state of current medical understanding is evolving as scientists discover new processes by which the body is damaged by high sugar. I won’t get into the details of the chemistry here, partly because I don’t fully understand the most up to date science. The final common pathway, though, is that prolonged exposure to high sugar damages blood vessels.
Hyperglycemia accelerates atherosclerosis, a process by which cholesterol-rich plaques build up in the lining of blood vessels and gradually obstruct the flow of blood. This can happen in small microscopic vessels (micro vascular complications), such as those in the kidney and retina. The dysfunction of these vessels can lead to blindness and renal failure.
When larger vessels get affected, that’s called macro vascular complications. In diabetics, commonly involves blood vessels are those that supply the heart (coronary arteries), leading to heart attacks and heart failure. Vessels that supply the leg can be affected, leading to painful ulcers on feet/toes, bad infections (without blood supply, immune system can’t reach the wounds), and loss of limbs. Gangrene and amputations are a known complication of diabetes. If blood vessels supplying the brain are affected, that can cause strokes, which can be permanently disabling or life threatening.
High sugar also damages small nerve fibers. This is why diabetics must take such meticulous care of their feet. The nerve fibers are damaged, so they can’t feel their feet well, and they can’t tell when their feet have minor scrapes or wounds. Then, because of the blood supply issues above, those wounds get infected and spiral out of control, leading to amputations.
Honestly, an exhaustive answer to this question really requires a degree in medicine or physiology. I hope this helps at least a little.
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