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synergy · 1
Blood flow to the gut falls during exercise for two reasons: when dehydration lowers plasma volume, the body clamps down on blood flow to the abdominal organs, and exercise intensity itself shifts the heart's output away from the gut toward muscle and skin. The two can stack, but they are not the same thing.
cofactor · 2
In the heat, the body cools mainly by evaporating sweat, and sweating removes both water and sodium. Fluid replacement is risky at both ends: losing more than 2% of body weight to dehydration clearly lowers performance and adds heat strain, while drinking more plain water than you sweat out dilutes blood sodium too far.
Fiber needs water to work: soluble fiber absorbs water and forms a gel, and insoluble fiber adds stool bulk by holding water. Eating more fiber without drinking more can make constipation worse.
regulates · 4
Where water goes in the body depends largely on sodium and potassium: sodium stays mainly outside cells and potassium mainly inside, and water follows them. So drinking water and taking in salt have to be considered together.
Blood volume, the kidneys' handling of sodium and water, and whether you get enough potassium together shape the load on the heart and blood pressure. Too much sodium makes the body hold more water, and potassium helps it get rid of extra sodium.
Thirst comes later and weaker in older people: after 24 hours without water, healthy older men had more concentrated blood and higher antidiuretic hormone, yet felt less thirsty, drank less and could not concentrate their urine as well as younger men. So older people should drink at set times rather than wait for thirst.
Dehydration may trigger migraine in some people, but migraine itself is a neurovascular disorder in which a signaling molecule called CGRP is a key link, and a newer class of drugs for preventing and treating attacks is designed to target it. Relying on drinking more water alone misses the treatments that actually work.
contrast · 1
Calf cramps late in exercise are often blamed on dehydration or low salt, so people reach for electrolyte drinks. The systematic review by Schwellnus 2009 concluded that the main cause of cramps during exercise is disturbed nerve control of a local muscle that has been worked too hard, not dehydration or electrolyte loss: a cramp strikes only the muscle that is already exhausted, and a whole-body imbalance cannot explain something that local.