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when the histamine receptor is blocked, lowering metabolism. These biological changes result in increased appetite, overeating, slower fat breakdown and weight gain. Antihistamines can also make you feel tired and unmotivated to exercise.

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  • The quote you have suggests that blocking histamine receptors results in increased appetite; why would you think that less histamine would reduce appetite instead?
    – Bryan Krause ♦
    Commented Aug 19, 2022 at 15:49
  • If the histamine receptor is blocked, less histamine would be metabolized and there would be more appetite.
    – houdy
    Commented Aug 19, 2022 at 16:24

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That's not how receptors work. Receptors for hormones, neurotransmitters, etc. convey some biochemical signal. The strength of histamine signal is given by the presence of available receptors (i.e., those that aren't blocked) and the concentration of histamine.

Blocking the receptor or reducing histamine have generally the same effect: less of that biochemical signal. Histamine isn't "metabolized" when it binds a receptor, it just binds for a bit and then floats away (perhaps to bind another receptor somewhere else).

The H1 histamine receptor specifically is a G-protein coupled receptor with downstream signaling through phospholipase C and IP3. This can have all sorts of downstream effects in different cell types, including changes in gene expression.

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  • Blocking the receptor or reducing histamine do you mean that either we block the receptors for histamine OR making that less histamine would be biosynthesized, will have the same effect of increased appetite?
    – houdy
    Commented Aug 19, 2022 at 16:27
  • @houdy It could be more complex than that, because there are multiple histamine receptors and antagonists don't block them all equally, whereas reducing histamine will reduce activation of all the receptors. But yes, generally if you find that blocking a histamine receptor increases appetite, you would expect that reducing histamine would do the same thing, they are both ways to reduce signaling through the same pathway.
    – Bryan Krause ♦
    Commented Aug 19, 2022 at 16:30
  • So I figure that with the current data, as a general rule, less histamine in a human body means more appetite.
    – houdy
    Commented Aug 19, 2022 at 16:51
  • @houdy Yes, that seems to be the case, though I doubt that experiment has actually been performed, at least in people. It's a lot easier to have a drug that blocks a receptor than a drug that just reduces histamine, which is why we use histamine receptor antagonists as anti-histamines.
    – Bryan Krause ♦
    Commented Aug 19, 2022 at 19:56
  • Thanks Bryan, I think your last comment should be in the answer (in case comments get deleted).
    – houdy
    Commented Aug 19, 2022 at 21:43

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