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I am not someone from Medical field. I am asking this question out of curiosity. My question is about sugar levels in blood, particularly low blood sugar (hypoglycemia).

I can understand the problem with high blood sugar levels (hyperglycemia) as we want sugar to be in the cells so that it can be used in respiration for generating energy.

But I can't quite understand why is hypoglycemia a problem.

Google search indicates that dangerously low levels of hypoglycemia can even cause brain death. Does not hypoglycemia mean that sugar is where it should be i.e. in the cells. When sugar is present there in brain cells instead of being in blood, how can it cause brain death as brain cells have enough fuel to burn for powering them?

Those stores of energy will only get depleted after some time (should last longer if the person in question is not active enough. Only if one does not replenish these depleted stores, this should be a problem (sort of starvation). But replenishment of stores is quite likely to occur in the meantime. Why is hypoglycemia a big problem then (even regarded more serious than hyperglycemia)?

Please enlighten me about this issue. Please forgive me if you found my question silly or naive.

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Your tissue gets sugar from the blood (along with the other needed nutrients, and oxygen). Sugar is not stored in places like the brain, brain cells only take up enough sugar for what they need in the short term, because they ordinarily get constant fresh delivery of nutrients and oxygen from blood.

When you have hypoglycemia = low blood sugar, that means there isn't enough sugar available in the blood for tissues to use. It does not mean that the sugar is already in the cells. It's like if you go to the grocery store where you get your food and find they are out of food, it does not mean you have enough food already at home.

Similarly, the reason that hyperglycemia is a problem is not because the blood is keeping it unavailable to cells. Quite the opposite: when there is a lot of sugar in the blood, there's a lot of sugar in the cells, too: too much of it. Sugar in high concentrations is toxic to cells, causing unwanted chemical reactions and oxidative stress.

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    I wanted to comment the exact same analogy - beat me to it! Have my +1. Might be worth mentioning that OP got the reasoning for why hyperglycaemia is dangerous also slightly wrong. An interesting follow-up would be: If cells had magically large stores of glucose, would hypoglycaemia cause other „off-target“ issues?
    – Narusan
    Commented May 11, 2022 at 18:01
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    A diabetic ,after insulin overdose, experiences hypoglycemia. So earlier he/she was grappling with high sugar levels and with administration of insulin, sugar levels suddenly plummet. Where does the sugar disappear? Has it changed in some other form?
    – HARVEER RAWAT
    Commented May 11, 2022 at 18:12
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    Think of it this way, maybe: The liver and muscle store the glucose for long-term use, and the blood transports it to the tissues that need it. If you increase insulin concentrations, more glucose is taken up and stored by the liver and the muscle. If you increase glucagon, more glucose is freed by the liver to the blood.
    – Narusan
    Commented May 11, 2022 at 19:17
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    It should probably be noted that hyperglycemia is usually a problem related to a long-term increase in blood sugar - just eating too much that one lunch isn't going to hurt you. The body mainly has trouble when this happens regularly. But hypoglycemia absolutely can kill just from being really physically exhausted once. There are limits to how much glucose the body can produce even if you have plenty of reserves - and if it can't produce enough to sustain the body, that's going to cause serious damage (of course especially dangerous when you're in extreme environments).
    – Luaan
    Commented May 12, 2022 at 10:36
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    @Luaan As most things, it depends on the extent. Mild, temporary hyperglycemia from a meal is benign. Chronic, moderate hyperglycemia causes sustained damage. But acute, severe hyperglycemia can certainly be an immediate medical emergency and, untreated, lead to death. Death only needs to happen once to be serious, I would not underestimate the danger of hyperglycemia.
    – Bryan Krause ♦
    Commented May 12, 2022 at 13:20

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