Naruto yawned.
Like a young married couple, he and Sakura had ended up fornicating all night until the sun rose over the horizon. Although he was still good to go thanks to the endless stamina the Nine-Tails granted him, he was getting rather sleep-deprived.
"C'mere, I'll finish you off with my tits, I'm a bit sore down there," Sakura said wearily, lifting up her breasts for the offering as she sensed Naruto approaching orgasm. She glanced out of the window with bleary eyes as Naruto pulled out of her. "Hold up, is it really morning already!?"
In order to expedite her weight-loss journey, Sakura had spiced up their bedtime fun with all sorts of ninja arts, using illusions, Shadow Clones and even the Multi-Size Jutsu to expend as much of her physical energy as possible. Getting rid of the fat she had already accumulated would necessitate further dieting, but for now, Sakura was confident that she'd burned any extra calories she might have consumed eating expensive sweets all week long.
"Can you do that thing with your boobs and your mouth and your tongue?" Naruto said eagerly as Sakura sandwiched his prick between her enormous breasts. "Ya know?"
Sakura blushed.
The last time they'd had sex, she'd forgotten all about her pride and got carried away by Naruto's pheromones, and she had done something she'd promised herself she would never do unless it was his birthday or Rinne Festival season. Even now, the smell of it so close to her Inuzuka nose as she rubbed its scent all over her cleavage was driving her wild…
"Maybe in the bath after you've thoroughly scrubbed it," she said, wiping a trickle of slobber dribbling from the corner of her mouth. "And that's if and only if I still feel like doing it then—"
Sakura flinched as Naruto splooged his load all over her face without warning. She sighed as she pulled a rope of stringy emissions from her eye. Once again, she had sticky Naruto extract in her long hair, which she would have to wash out with cold water to keep it from clumping together like curdled milk.
"Never mind, I'm done," Naruto said happily. "Bath time!"
"No kidding," Sakura sighed again, feeling slightly disappointed despite herself.
A few minutes later, the two young lovers were in the bath, scrubbing each other's backs and washing each other clean without any further friskiness, and a few minutes after that, they were falling asleep in each other's arms in Minato's bed, which unlike Naruto's, wasn't a complete mess, mercifully. Cleaning would just have to wait until tomorrow, Sakura thought tiredly to herself as she drifted off to sleep, she'd have a Shadow Clone do it while she went off on her adventure.
…
"GYAAAAAHHH!!!"
Naruto's peaceful slumber was brought to an abrupt and rather violent end as a shrill scream pierced his ear from no more than a pillow's length away. He sat up in bed with a start, reflexively clapping his hands over his ears as he looked around with wide eyes.
"…Sakura-chan?" Naruto said blearily as he quickly realised there was no immediate danger. "It's so early…"
More accurately, it was midday, but Naruto was used to sleeping in on his days off.
"What's wrong, Sakura-chan?" Naruto asked as he rubbed his eyes. "…whoa, did you eat an entire ostrich in your sleep, or what, ya know!?"
"Why is this happening to me!?" Sakura howled, burying her face in her hands as she sobbed uncontrollably. "This isn't possible, humans aren't supposed to get this fat this fast!"
In the span of six hours, the length of time they had slept, Sakura had somehow doubled her weight gain from the past week of inaction, bringing her body weight to around 90 kilos… discounting the more than five tons of breast flesh that Karin's fūinjutsu had sealed away. In a matter of days, she had gone from a slightly taller-than-average, athletic and very curvy teenage girl to a thicc, pudgy one… and then to a downright overweight one.
"Sakura-chan…"
"Turn around! Don't look at me, I'm huge!" Sakura shrieked. "Just… just give me a moment to think!"
There had to be a logical explanation for this, Sakura thought to herself in a panic. Although she could easily suppress the symptoms of whatever was ailing her by using Shikkotsu Forest Sage Mode's Organic Reincarnation Sage Art and transfer the excess body fat to her breasts, instantly restoring her killer figure, it would be unwise to initiate treatment without first confirming what was actually wrong with her.
There was nothing she didn't know about human biology, so surely…
And then it hit her:
Humans weren't supposed to get this fat this fast.
Human biology.
Human.
She wasn't exactly human any more, was she? She was a half-human, half-Ōtsutsuki hybrid, just like Hagoromo and Hamura: neither quite celestial being nor quite lowly worm of a mortal, but rather somethingin between.
"Think, Sakura, think…"
She could vaguely remember reading about one of Sakura's and Sasuke's adventures; Naruto had been ill with something that had once afflicted the Sage of Six Paths, so they had gone off on a quest to find him a cure… and there had also been dinosaurs, perhaps.
At any rate, whether or not velociraptors and diplodocuses still existed was beside the point. What mattered was that even Ōtsutsuki hybrids were still susceptible to disease… perhaps they were vulnerable to illnesses from both worlds, even.
"I just don't know enough about Ōtsutsuki biology," Sakura muttered to herself. "By all rights, necrosis should have set in within hours of my chest surpassing my body's ability to support it… but then I discovered I no longer needed to eat or drink or even breathe… wait, I think I've got it."
The Ōtsutsuki were a race of spacefaring parasitic alien; they would descend upon stars with the capacity to bear life and absorb their nature energy using God Trees, harvesting Chakra Fruits until they were sucked dry. Their kind needed to cover incredibly vast distances in the inhospitable void of space to find suitable stars, and thus they didn't need to eat, drink or even breathe; or at least, not in the conventional sense.
"Chakra…" Sakura murmured to herself in horror. "That's why…!"
As parasitic life forms, the Ōtsutsuki subsisted on internal reserves of stolen chakra, amassed through the consumption of Chakra Fruits or by directly absorbing energy from their victims or a Ten-Tails. Even at the start of the life cycle, Ten-Tails were creatures capable of drawing in airborne nature energy and converting it into usable chakra, which was why the Ōtsutsuki often kept them around as pets, in case they grew peckish between their main meals.
Indeed, Sakura's unique Rinnegan ability was an extension of the Ōtsutsuki clan's basic ability to consume chakra for sustenance. Just like Urashiki, she could eat incoming attacks and turn them into her own power.
However, the human species was completely different: shinobi could metabolise their own chakra by converting nutrients into physical energy, which could then be spun into chakra by melding it with spiritual energy; as long as they had food to eat and water to drink, humans were infinite, self-sustaining chakra factories… albeit on a very minor scale compared to God Trees.
"System, Status," Sakura silently recited. "Remind me of my chakra capacity and control values?"
[Ding! Chakra Capacity: 14 891 -> 14 897 KK. Chakra Control: 0.01 N -> 0.0096 N]
Around fifteen thousand KK was an inhuman amount, comparable to the lowest-tier Tailed Beasts. This was Kisame-level chakra, around a hundredth of the maximum output of the Nine-Tailed Fox or Senju Hashirama.
However, the real problem was the second number: a mere fraction of Chapter One Naruto's chakra control was laughably incompetent. Sakura was not used to have access to such incredible amounts of physical energy; attempting to use ninjutsu like this was like trying to crochet a scarf using ship-anchor chains instead of woollen yarn, all while hornets stung her fingers.
"I'm just like Naruto used to be!" Sakura realised in horror. "When I convert my stamina to chakra for ninjutsu, I produce way more than necessary… and then I only use a fraction of that to actually power my techniques!"
When ordinary humans moulded chakra and performed ninjutsu, anything left over would dissipate and go to waste. There were not one, but two places where inefficiencies could arise, which was why chakra control was an essential skill for any shinobi to master, given ordinary human limitations.
Since her body had changed radically in a very short time, Sakura's ability to control her own chakra had deteriorated drastically, which meant huge losses both during the generation of chakra and the weaving of ninja arts, creating massive amounts of wasted chakra that recycled back into her waistline…
After all, Sakura was half-Ōtsutsuki, and that meant her body would automatically convert any unused chakra floating around her pathways into sustenance, assuming it was meant to serve as food. And since she was also half-human, any extra energy left over after that would then be converted into fat reserves.
Combined with her Akimichi bloodline that promoted the proliferation of fat cells, this meant that instead of burning calories by using ninjutsu to spice up sex last night, she had been doing the equivalent of stuffing her face nonstop for hours on end!
"Instead of losing weight when I cast ninjutsu… I gain weight!?" Sakura cried. "This is a nightmare! Somebody pinch me… Ouch!"
"You said to pinch you," Naruto said innocently as he hugged her from behind. "Whoa, you feel so soft… it's kinda nice, ya know? You're like a big plushie."
Sakura grumbled under her breath as Naruto jiggled her tummy fat with the fascination of a child who had just received a brand-new toy. What was she supposed to do now? Stop eating altogether, since she no longer needed to? Was there some kind of training that would help?
And if so, what kind of training? Tree climbing? Water-walking!?
She wouldn't grow any fatter as long as she didn't make any more chakra, but she was the Hokage; she couldn't just avoid her responsibilities! And training still meant expending chakra! If she began training to get used to her immense chakra reserves, there would soon be no tree left capable of supporting her rapidly increasing weight, and no amount of surface-tension-altering chakra would let her fat feet stand on water!
"The sticking-leaves-to-forehead exercise it is," Sakura said decisively… and then she remembered the promise she had made to Naruto before going to sleep. "Oh, but today's the day we're supposed to go out looking for our missing comrades, isn't it…"
"Ya know it!" Naruto said brightly.