A reader forwarded this 2019 study Dehydration affects exercise-induced asthma and anaphylaxis [Oct 2019]. Soem quotes:
PubMed was searched from April to July of 2019 using predefined search terms “dehydration,” “exercise,” and “allergy responses.” Based on the reference search, more than one-hundred articles were identified
Also, numerous mast cells and eosinophils were recruited, therefore isotype switching to IgE antibodies occur, this hypersensitivity activates mast cell degranulation. After degranulation, proteases, leukotrienes, prostaglandins, and histamine lead to many kinds of allergy symptoms.
A few months ago, I wrote Hydration and the Microbiome which reviewed some literature.
The event that caused me to start looking at hydration, was a cheap home hydration device — a modern blue tooth scale, the one below. Currently $20.
As a result, because I was definitely dehydrated (but did not feel dehydrated — because that had been my new norm!) and proceeded to work on increasing hydration. At the start, I was 45.4% and have slowly worked it up to the current 47.2%. My goal is at least 57%, the middle of the normal range (ideally over 65%). My significant other has mast cell issues and was sitting well below 45% and is working on this change. One of the challenges is finding alternatives for items she takes that are known to cause dehydration — including antihistamines.
- Antihistamines, Blood pressure medicines, Chemotherapy, Diuretics, Laxatives. (U of Michigan)
So there is the appearance of a feedback loop: dehydration triggers mast cells, which result in the need for antihistamines, which then causes dehydration. The person is trapped!
A longer list from Oxford Medical:
- Fybogel, Lactulose, Macrogol, Senna, Bisacodyl, Docusate
- Ibuprofen, Naproxen, Diclofenac
- Loratadine, Cetirizine, Promethazine, Hydroxyzine
- Lisinopril, Ramipril, Losartan, Amlodipine, Felodipine, Bendroflumethiazide
In terms of the microbiome environment, we can see the impact from this study
Dehydration was accompanied by cell changes in solitary lymphoid nodules and Peyer’s patches. The proportion between lymphocytes, macrophages, and mast cells in lymphoid organs depended on the stage of dehydration. The inhibition of cell mitoses, disappearance of mature plasma cells and mast cells (per field of view), significant decrease in lymphocyte count, 4-5-fold increase in the number of destructive cells, and low density of cells and lymphatic network of the small intestine (per unit area) were observed on days 6 and 10 of dehydration. Severe morphological changes were also revealed in other layers of the small intestinal wall (mucosa, submucosa, lamina propria, etc.).Effect of dehydration on morphogenesis of the lymphatic network and immune structures in the small intestine 
Hydration plays a very important role for health. Drinking 6 liters of water/day is the classic health advice. This may be a over simplification. This area is still being studied. “As the most effective sports nutrition supplement, sports beverage has different ingredients and formulas, and also has various effects. To provide clues for the development of sports beverage, this article reviews the types, components, effects, and mechanisms of sports beverage currently used in post-exercise fluid restoration.”[Research Progress on Application of Sports Beverage to Post-exercise Fluid Restoration, 2021] So some sports drink may help, some may have little effect.
An unexpected side effect, I did not change my diet habits but with the shift of hydration, I also lost 12 lbs (6 kilos) over 7 weeks.