fanjo
If your blood level was 32 at the end of summer , when your vitamin D is notionally at maximum from summer sunlight , then you are quite likely to be even more deficient over winter etc.
Your body requires about 70 IU of Vit D per kilogram per day, of your weight. That would be from all sources , food, supplement , sunlight, sunbed, if any.
Whenever your blood level is below about 100 your liver quickly converts your supplement ( Vit D3 - cholecalciferol ) to 25hydroxyD3. You body cannot efficiently use whatever D3 you have, in the autocrine and paracrine cellular system. It would be to your great advantage to maintain a level above 100 ... preferably 120 - 140. At these levels you will start to store cholecalciferol, it will be available to your cells to fight infection , etc. You have about 200 anti-microbial , anti viral peptide "receipes" in your DNA ( genes) . In order to make these defensive proteins you need Vit D available to the cell. Without sufficient Vit D3 - cholecalciferol diffusing throughout your body you are cannot fight infections as effectively. So, yes you may very well find that bouts of cystitis will reduce. Vit D also has immune system regulatory functions. IF your asthma is due to overactive immunity/allergy , then, yes , it could help with that as well.
In order for you to maintain a blood level above 100 , you are likely to require about 3000 IU permanently .... every day. People do respond differently ... some with much greater response , others with less, on AVERAGE 1000 IU per day , permanently , will raise your blood level by 25 nmol/L above the level that your normal lifestyle seems to produce . i.e.32 nmol/L . I am referring to a 75 kg adult .
My advice would be to measure your level , say after 3 months , on 3000 IU , perhaps here at this NHS Lab , <a class="break-all" href="http://www.vitamindtest.org.uk/" rel="nofollow" target="_blank">www.vitamindtest.org.uk/</a> , or ask your doctor . Often your doctor will not be forthcoming ... you may need to organise the test yourself.
Bearing in mind that your level became so low , I would not drop down to 1000 IU per day , until you know your levels after 3 months of supplementing , IF you did drop down , I would test your level 3 months after dropping , to make sure that your blood levels does not fall below 100. If it does , then increase supplementation again , until your level is well into the 120 - 140 range.
Are you by any chance vegetarian ... or do you have any gut maladies .... that may be the reason your levels are low , i.e. you are not absorbing very well the small amount available in animal products. IF that is the case , then a sublingual spray bypasses the digestive system and may give better results than say a tablet or oil filled gel cap.
As others on this thread have suggested , it may prove helpful for you , if you were to check blood levels of , folate, iron, Vit B12, thyroid . Although low Vit D can have a great many effects , there may be other causes of your symptoms. A great starting point of your investigation however will be to get your Vit D to where it should be, and keep it there. Try to ensure that you are getting your other vitamins and minerals , particularly magnesium, a know co-factor of Vit D. i.e. plenty of veg , particularly dark greens , some fruit etc.
Best of luck , hope all that is helpful. <img loading="lazy" class="inline-flex mumsnet-emoji" alt="Smile" src="https://www.mumsnet.com/build/assets/smile-iCO8d7ST.png">
BTBH