How do the Hashiras stay healthy? Specifically how do they manage to keep up their calorie intake and do they do any optimized exercises or something to stay healthy?
How on GODS' green earth did I only see this after a YEAR!????
Answering your question, let's consider their resources.
Hashiras and all demon slayers, for that matter, have accessible health care. Whether in the form of a Wisteria House, allowing rest and recuperation with visits from doctors, or the Butterfly Mansion, which appears to be consistently modernized.
Master Ubuyashiki considers the health of his slayers highly important, and logically so, since there are so few. Funding and access to new and Western medicines/techinques must've been high. Along with the training all Kakushi received to ensure instant, lifesaving care.
Personally, I believe the Kakushi were ordered to stay close to the Hashira. Not because the Hashira are incapable of keeping themselves safe, but because they are sent towards high-ranking demons or as a last resort. Meaning the destruction was usually massive, and so were the casualties. They are responsible for carrying all the tools on the field, such as bandages, alcohol, ointments, sutures, and whatever else. Therefore, all injuries, minor or major, receive instant care.
As for meals and calorie intake, it wouldn't be unusual for Hashiras to have some sort of meal plan. With the Kakushi at their disposal, they could have routine heavy meals accounting for all the calories lost in their training.
Some might not utilize this; they might just stack up on meals when they have the chance. Like Rengoku eating several bowls of udon and bentos before a mission. Though most demon slayers don't appear to carry much on their travels, Hashiras likely have some nutrient-dense field rations provided.
I'd compare them to imperial Japanese rations. According to Wiki (my beloved), "Rations had to be stout, durable, simple, sturdy and had to survive without refrigeration for long periods of time. Typically, each ration was served in the field in canned food boxes, and cooked near the battlefield. The mess tin was known as a han-gou.
The rations issued by the Imperial Japanese Government usually consisted of rice with barley, meat or fish, pickled or fresh vegetables, umeboshi, shoyu sauce, miso or bean paste, and green tea. A typical field ration would have 28 oz (790 g) of rice, usually mixed with barley to combat nutritional deficiencies such as beriberi. Often, soldiers would forage for fresh fruit to provide vitamins."
As for exercises, all Hashiras maintain their fitness through their swordmanship, sparring, and drills.
There's the more basic yet demanding training all demon slayers go through once in the Butterfly Mansion: Rehabilitation Training. This consists of full-body stretches, reflex tests, and an enhanced game of tag.
Additionally, they likely maintain the regimens their cultivators taught them during their training to become demon slayers.
We see an aspect of their individual training regimen in Season 4, The Training Arc. Tengen focused on Fitness and Stamina, Muichirou on Rapid Movement, Mitsuri on Flexibility, Obanai on Sword Skill Revision, Sanemi on Infinite Striking, and Gyomei on Muscle Amplification.
Overall, while the Demon Slayer Corps cares for their slayers, the Hashiras all seem capable of understanding their physical needs and their importance.
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