It was becoming increasingly apparent to Sakura that she had discovered in her half-human, half-Ōtsutsuki physique the secret to perpetual motion and infinite energy… naturally referring to the perpetual jiggling of her blubber and her stamina reserves; which ironically enough, kept skyrocketing even though she could no longer run forwards a few steps in her tubby form without exhausting herself.
Although chakra appeared to violate the Laws of Conservation of Mass and Energy, since the melding of spiritual energy with the limited physical energy contained within one's cells resulted in a substance that somehow contained far more energy than the sum of its parts, Sakura refused to believe that such a violation truly existed.
After all, the Spiritual Plane was a real place that Sakura had visited and drawn imaginary friends from. Spiritual energy had to be some kind of mysterious, extra-dimensional form of power attracted to sentient beings' consciousness, that only became tangible and measurable when mixed with the Physical Plane's energy… and thus, the laws of science that she cherished so dearly still held true.
…and her strange physique was no different.
Any excess chakra floating inside Sakura's chakra pathways would be reabsorbed into her body in the form of a veritable shower of calories, which would end up stored as fat… and from these energy-rich fat cells, she could once again draw physical energy and spin it into chakra with extra-dimensionally sourced, never-ending spiritual energy into even more chakra…
The equation was balanced, and yet…
"I'm dying…!" Sakura wheezed as she collapsed onto her enormous rear. Her belly had split into multiple rolls, she had more chins than she could count, and her legs kept rubbing together when she ran. "Can we… take a break…? I need to… move this fat… into my chest…"
"Again?" Naruto complained. "It's getting worse with every step, ya know!"
The more chakra Sakura wasted, the fatter she got. The fatter she got, the more chakra she had to produce in order to keep up with Kakashi-sensei and Naruto as they streaked through the countryside, and the more powerful she got, the more uncontrollable her chakra became…
Could this be the ultimate secret that Ōtsutsuki Shibai had discovered on Earth millennia ago, Sakura wondered? That the Akimichi clan's physique was actually the strongest in the world, and that to achieve apotheosis and attain godhood, one needed to combine humanity's power with that of the Celestials?
"At this rate, we'll never catch up to Sasuke," Kakashi said thoughtfully. "It might be time for a little training session… you said the problem was your chakra control, I believe?"
At this point, it didn't matter whether she had the Akimichi clan's physique or not. The number of calories flooding her system each time she produced the slightest drop of chakra was enough to wreck even the most performing of metabolisms.
"I need to reapply my breast lotion too, all over my body…" Sakura gasped. "My skin is really stretchy, but this is asking way too much of it… I'll get stretch marks…"
Sakura briefly entered Sage Mode and activated Organic Reincarnation to relocate her fat stores to her breasts, and once again, Thin Sakura emerged from her cocoon.
"It's not getting any better," she said morosely. "With each gram of fat I gain, my chakra control gets worse, and my tendency to gain weight only increases… at this rate, I'll never be able to adapt… it's a vicious cycle."
At the very least, she needed her System Panel to start using S units for her chakra control again. S for Sakura, not N for Naruto.
"I see the problem…" Kakashi said pensively. "You need to get used to your ever-increasing chakra pool, but training increases your chakra reserves faster than you can get used to them…"
Talk about problems only the truly gifted could have, Kakashi ruefully thought to himself. He could only slowly and painstakingly increase his chakra reserves by strengthening his body through rigorous regimen of exercise, and the results were only barely noticeable…
"Do you have a solution, Kakashi-sensei?" Sakura asked hopefully. "Pretty soon, I won't be able to throw even a single punch before blowing up into a whale…"
Kakashi folded his arms and drummed his fingers on his biceps, deep in thought. Shadow Clones would only multiply Sakura's chakra expenditure and thus exponentially increase her rate of unstoppable growth, so the solution that had worked for Naruto wouldn't work for her… hang on.
The solution that had worked for Naruto…
Naruto…
Naruto…
Naruto…
"I've got it, Naruto himself is the solution," Kakashi said, snapping his fingers. "Do you happen to know the Mind Body Switch Jutsu?"
Sakura immediately understood where Kakashi was getting at. Together, Naruto's body and the Nine-Tails could provide the perfect training environment for her, since they were about the only beings on this star with greater chakra potential than she had. Kurama could empower Naruto's cells to simulate the high-energy environment of her own body and alter the difficulty of the training at will by choosing to interfere with the moulding of her chakra!
"Yeah, I do, actually!" Sakura replied brightly, her eyes lighting up. "Naruto, wanna swap bodies for a while?"
Used offensively, the Shintenshin no Jutsu could suppress the host's consciousness and keep it prisoner within their own body while the invader took the reins, but when used co-operatively, the caster could instead choose to fully swap minds with the targeted comrade, allowing them to pilot the body they had left behind.
"You mean… I get to be Hokage until we switch back?" Naruto said excitedly. "And… I get to fool around with Sakura-chan's body too?"
"Within reason!" Sakura spluttered. "I'd better not find weird things stuck inside me down there when I get back, got it?"
Since Naruto didn't seem opposed to exchanging bodies with her, Sakura took careful aim at him with a window formed with her thumbs and index and middle fingers, while Kakashi watched carefully with his exposed eye. Little did they know, he was reporting their every interaction to Jiraiya as inspiration for his newest book series, The Tale of the Udderly Busty Kunoichi… an erotic retelling of his failed novel before Make Out Tactics, The Tale of the Utterly Gutsy Shinobi.
"Ninja Art: Mind Body Switch Jutsu!"
Wooo…
Sakura blinked as she found herself looking at her own body from the outside… and almost immediately, her hands shot down to her crotch as a familiar sensation bloomed in her nether regions. At long last, her little brother was back, standing at attention!