His mortal body, after all, wasn't that of a summoning beast blessed with bloodline abilities—he couldn't sustain this kind of training for long.
Of course, it was also possible that his body hadn't fully adapted yet, or that there was a more perfectly synchronized breathing rhythm that could mitigate or even eliminate these side effects.
The root cause of these side effects was likely due to some unknown, mysterious element in his breathing. Alternatively, it could be his cells reacting to the absorption of food energy, triggering some kind of rejection.
Or perhaps it was a mix of both.
"Could that unknown element in my breathing be… natural energy?"
The thought sent a shock through Umino Yoru 's mind, but he quickly dismissed it. If it were natural energy, given his current chakra reserves and control, he would've already been corroded into an inert, lifeless husk.
"If it's not natural energy, then what is it? It can't just be oxygen, right?"
He shook his head, unable to figure it out, and decided not to dwell on it. He wasn't a scientist—he didn't need to uncover absolute truths. As long as it worked, he'd use it. Once his strength grew enough, he could always reverse-engineer the principles later. That would save him time and effort.
After resting for a long while, the numbing side effects finally began to fade.
"This isn't easy…"
For a moment, it felt like he had regained control of his body—as if he'd been exiled from it before, only now returning to himself.
Once back in control, he instinctively activated the Akimichi Digestion Technique to process the food in his system, replenishing his chakra and stamina.
"Huh?"
He noticed something unexpected—the numbing side effects were fading faster now.
"So that's how it is."
His eyes lit up in realization. If food energy could counteract the side effects, that meant the numbness was caused by his cells absorbing that unknown element. The food energy was neutralizing it.
This mysterious factor was clearly beneficial to the body—otherwise, it wouldn't be absorbed—but it also carried a strong corrosive effect. The digested food energy balanced it out, even assimilating its energy signature and converting it for his own use.
With this in mind, he quickly ate a large amount of food and pushed the Akimichi Digestion Technique to its limit.
After nearly half an hour, the numbness finally disappeared completely. Eager to continue, he resumed his training—this time deliberately guiding the food energy to synchronize with his breathing rhythm, adjusting it for deeper compatibility.
"Learn their strengths to surpass them."
What suited him best was what mattered.
As he fine-tuned his breathing, he confirmed his theory—the food energy could counteract the mysterious element in his breath, even absorbing and refining it into his cells.
Most of it still escaped with each exhale, but over time, traces remained, gradually accumulating.
The deeper he immersed himself in this training, the more entranced he became—until he hit a critical point.
"This… this is wind—"
In that moment, it felt as if he were submerged in an endless breeze, weightless, as though floating in emptiness. A fleeting sense of transcendence passed through him—and then he sensed it.
A trace of wind-natured chakra had manifested inside him.
"No way… Did I just unlock wind-natured chakra?!"
For a moment, he doubted his own senses. He had struggled for so long to develop a second chakra nature, yet now, through this bizarre breathing method, he had somehow unlocked it without even understanding its essence.
"So much for needing to comprehend chakra nature transformation first. The scrolls weren't entirely right, huh?"
This was the opposite of conventional wisdom. He hadn't studied wind chakra's properties at all, yet here he was—not only unlocking it but also gaining an instinctive grasp of its form manipulation.
While he still lacked deep insight into its cutting properties, the flow and pressure of wind now felt intuitive.
As he continued training, his wind chakra rapidly condensed—until the familiar numbness struck again, forcing him to stop.
Collapsing to the ground, it took him a long time to regain partial control of his body.
"Hah…"
He exhaled slowly, making a mental note: Next time, don't push to the limit. Shorter sessions mean faster recovery—and better results.
Once again, he maxed out the Akimichi Digestion Technique, gradually easing the numbness.
But when he reached for more food, he realized—he wasn't hungry. For the first time in a while, he felt full, even slightly bloated.
His body knew its limits better than he did, sending clear signals to his brain.
"Huh? My chakra reserves… increased?"
It took him a moment to notice, but his total chakra had risen slightly. Not by much, but enough to break past his previous ceiling.
"Unlocking a second nature shouldn't boost chakra capacity. So what's going on here?"
He rubbed his chin, puzzled, but ultimately shrugged. Whatever the reason, more chakra was always good.
"At this rate, I might reach jōnin-level reserves soon."
The thought filled him with excitement. His limited chakra had always been a bottleneck—now, the clouds were parting.
"Which means… it's time to start working on the Spiral Sphere."
Namikaze Minato had only just graduated from the Academy, meaning the Rasengan hadn't been invented yet.
If he developed it now, it would be his original technique—protected under Konoha's intellectual property laws.
Submitting it to the village would earn him an A-rank technique in exchange.
Of course, once shared, it wouldn't be exclusive anymore.
Konoha could add it to the mission exchange or seal it away in the Forbidden Scroll.
And without exclusive rights, teaching it to anyone outside his bloodline or students would require mission points and merit.