Backache during fasting is common, when I look for the reason all I get is woo about "toxins".

[astro]

I’ve done two day long fasts in the last few months and each time my lower back hurt toward the end of the day I fasted unrelated to activity. The backache vanished when I began eating again. When I googled this all I get is pure fasting woo re “toxins being purged” etc. What is the real reason?

Is it something to do with the kidneys?

[Johnny_Bravo]

Just a WAG, but you could have a mild vitamin deficiency that’s being exacerbated by the fasting.

[Bibliovore]

I sometimes get the same thing when fasting for Ramadan and I suspect it’s your kidneys. Try drinking plenty of water before you start the fast and then see if that helps.

[DSeid]

Fasting theoretically could increase the risk of urinary stone formation but lower back? Not usually the spot and stone pain tends to be, shall we say, more dramatic.

Caffeine withdrawl?

Muscle pain/stiffness (descriptors: muscle pain/stiffness,
muscle joint ache, general muscle pains, and leg muscle
pains) Muscle pain/stiffness has been demonstrated in 4 of
15 experimental studies (27%). The incidence of muscle
pain/stiffness was 43% in one study (Table 4). Muscle
pain/stiffness has been demonstrated in studies comparing
acute caffeine abstinence with a preceding baseline
condition, a caffeine administration condition, and a
chronic caffeine abstinence condition (Table 4). One
study also showed that abstinence-induced muscle pain/
stiffness is time limited (Table 4). …

[astro]

@DSeid

Fasting theoretically could increase the risk of urinary stone formation but lower back? Not usually the spot and stone pain tends to be, shall we say, more dramatic.

Caffeine withdrawl?

No I was hydrating all day with lots of diet lipton bottled diet green tea and hot tea. I was best buds with the urinal all day long. Honestly it kind of concerns me I was sans food for for only about 12 hours when it started cropping up. It was definitely kidney pain. Other muscular aches and pains started in as well. All went away when I started eating again this morning.

I wonder what they were complaining about? Not having expected the chemicals / electrolytes in bloodstream? Having to burn fat as fuel with no food being taken in? Who knows?

[DSeid]

@astro

… It was definitely kidney pain. …

Well since you already know why were you asking?

(Not saying it wasn’t mind you, although again, kidneys are usually upper flanks not lower back, with referred pain to inner thigh sometimes.)

[Amateur_Barbarian]

You could stop fasting, as it has no known medical value except among those who run in mortal fear of <looks furtively about> toxins.

[astro]

@DSeid

Well since you already know why were you asking?

(Not saying it wasn’t mind you, although again, kidneys are usually upper flanks not lower back, with referred pain to inner thigh sometimes.)

I looked further online after I posted the initial question and found fasting and kidney pain are related which made sense based on the location of the pain and the fact that after I started eating again it vanished. In reviewing that info and thinking about the context I came to the conclusion it was kidney pain.

So far as I know my kidneys are perfectly healthy, why they would become painful after just 12-14 hours with no food is a mystery to me.