system
Given the following post, respond as if you're commenting on Reddit:
human
[Dragon Ball Z] What exactly happens to Goku's body when they go Super Saiyan?

On a cellular level and what actually causes the changes in his body.
gpt
Saiyan biology is fairly complex.  Compared to most species, they need to drink very little, and tend to eat in massive meals that last them a long time compared to smaller meals throughout the day.

A transformation in any creature represents their bodies and cells restructuring into different shapes.  With a Saiyan body, this is much less obvious - the only external difference is hair color and shape, and a bit of muscle mass.  What's actually going on is that their cells are changing how they output the energy in the body, multiplying its potency several times over.  All creatures have a cell structure that manipulates the Ki coming into their bodies in different ways, and Saiyans are no exception.  Their default state absorbs and then outputs Ki similar to a human.

This is visible in the way that Ki flows around their body.  Ki, when focused to a certain degree, enters the visible light spectrum, but in a way that isn't fully understood.  At lower energy levels, this usually becomes a white aura around the user, or sometimes blue of focused differently.  

The "Kaio-ken" technique, perfected by King Kai, alters the cells of the user to multiply the output similar to a Super Saiyan transformation.  This is very stressful, as it is an unnatural transformation, but it can be seen by how it changes the visible aura to a red glow.  The Super Saiyan transformation does the same thing, but on a "perfect" level, bringing the aura to a vibrant yellow, that seems to grow in intensity with each "level" of Super Saiyan.  Past Super Saiyan 2, the cells allow energy throughput with such potency that bio-electric surges may emanate from the user.

Worth noting is how other unique entities show this power.  Dr. Gero's androids appear to have modulators that allow them to change what kind of energy they output, from weaker white and blue blasts to potent yellow and pink.  The creature known as Perfect Cell attained a deep purple aura constantly, which seems to be a higher rank of Ki than Super-Saiyan yellow, although Gohan was able to draw more total energy and thus overpower him (before he was injured).

Majin Buu's unique cellular structure created a bright pink aura that was more potent than anything seen before.  The Kais seem to manipulate energy in a similar way.  Additionally, when Gohan's potential was fully unlocked, he created a white glow again, signifying that his Ki was not being magnified in any way, but he simply had a massive amount to use and focus.