Why do Humalog, NovoLog, and Fiasp feel so different if they’re supposed to do the same thing? I went down an insulin rabbit hole looking for answers.

Little irritated areas under my skin that I’d never had before. Right where my Fiasp injection site was.

This was not "pooling" because this was happening with almost every single Fiasp injection, but not Humalog.

It had me thinking:

Why does one insulin suddenly irritate me while another doesn’t?

And then, I remembered the age old Humalog vs NovoLog controversy. Some swear by one and others don't notice a difference between the two.

In theory, since these are all rapid-acting insulins, they should all work the same...so what was going on?

I NEEDED to know.

What I found was WAY more interesting than I expected.

The entire rapid acting insulin revolution basically came down to scientists figuring out how to stop insulin molecules from clumping together. Because the faster you can get it to un-clump, the faster it works.

This applies to insulin the body makes too. If the insulin our body makes were to sit out or be stored in a vial, it would clump.

What prevents human insulin from clumping though is the fact that your pancreas creates it, then immediately pushes it into the blood stream, which is like a rushing river... No clumping is happening once it's in the blood stream. It can't. The current is just too fast.

Man-made insulin on the other hand is stagnant. It sits in a vial (or pen). Even when it's injected into the body, it's not going into the fast moving current of the blood stream.... it's going into interstitial fluid (aka tissue fluid) which is more like a pond.

The scientists who were working on Humalog, NovoLog, and Fiasp each had to figure out how they were going to get insulin NOT to clump... Because clumping is what was slowing the insulin down.

How each brand solved the clumping issue is what makes one faster, another smoother, and another turbo charged ...More on that in a minute... because my next question (and maybe yours too) was:

If clumping is such a problem, why does insulin even do it?

Because clumping is actually useful.

Inside the pancreas, insulin is stored in tightly packed little clumped bundles, which helps keep it stable and protected until your body needs to release it.

OK. Now back to how each brand fixed the clumping issue and what that means for how it works in your body...

Scientists sought to to make insulin molecules less clingy.  And that changed diabetes care forever.

In 1996 Humalog came to the market.

Humalog was THE FIRST rapid insulin analog.

Scientists changed TWO tiny building blocks near the end of the insulin molecule.

In normal human insulin:

Amino Acid B28 = Proline
Amino Acid B29 = Lysine

Humalog simply swaps them.

That’s it.

No new amino acids added.

Just rearranged.

This tiny swap makes insulin molecules:

• not clump easily
• separate faster
• absorb faster

Which is why lot of people describe Humalog as:

• sharper
• faster
• stronger up front

Some people feel it “hits harder.”

And chemically, Humalog is considered the closest to natural human insulin because it keeps the same amino acids, it just rearranges them.

That’s why some people think of Humalog as the “cleanest” molecular design.

Four years later, in 2000 Novolog came to the market. NovoLog took a different approach.

Instead of rearranging pieces, scientists replaced ONE amino acid.

At B28: Proline was replaced with Aspartic Acid

Aspartic acid has a negative electrical charge.

So now insulin molecules kind of go:

“Don’t stand so close to me.”

They repel each other a little.

This reduces clumping which in turn speeds absorption.

People often describe NovoLog as:

• smoother
• gentler
• more even

In 2018 Fiasp came to market. Fiasp is basically NovoLog with TURBO BOOSTERS added.

Fiasp uses the SAME insulin molecule as NovoLog.
It still changes B28 Proline → Aspartic Acid

But scientists added extra ingredients around the insulin: Niacinamide (vitamin B3)

This helps insulin absorb faster by:

• increasing local blood flow
• helping insulin move through tissue quicker

Think of it like opening extra doors so insulin can run into the bloodstream faster.

Fiasp also contains L-arginine, which helps to stabilize the formula.

People often describe Fiasp as:

• working almost instantly
• more aggressive early

Lot's of people love it.

Some say:

• it burns
• it irritates their skin
• it works great at first then seems inconsistent
• it causes pump site issues

And that postpartum skin irritation I mentioned earlier? Apparently that’s not uncommon. Because Fiasp contains extra absorption boosters, some people (like myself) seem more sensitive to it.

If one of these insulins doesn't work well for you, the key to making it work might be the pre-bolus.

• Pre-bolusing HUMALOG:
  Many people find Humalog kicks in a bit quicker than NovoLog. So you may only need a 10-15 minute pre-bolus. Again, everyone is different. But if Humalog feels stronger for you, that shorter timing can make a difference.
• Pre-bolusing NOVOLOG:
  Because NovoLog can feel smoother and less aggressive early on people often need a longer pre-bolus than Humalog. So with NovoLog you may need up to a 20-30 minute pre-bolus.
• Pre-bolusing FIASP:
  Fiasp hits hard and fast. I find that I don't even need a pre-bolus with Fiasp.

So which insulin is the cleanest and which one most closely resembled natural human insulin?

The closest to natural human insulin is probably Humalog, because it keeps the original amino acids, it just swaps their positions.

NovoLog and Fiasp actually replace one amino acid with a different one.

Fewest extra additives? Again, probably Humalog.

Fiasp intentionally adds absorption boosters and stabilizers.

It still blows my mind that the huge leap from old-school insulin to today’s rapid insulins came down to changing just ONE or TWO tiny parts of the molecule.

And once I understood how Humalog, NovoLog, and Fiasp each prevent clumping a little differently, it finally made sense why people can have such different experiences with them.

Some people love Humalog because it hits harder and faster.

Some love NovoLog because it feels smoother and steadier.

Others feel Fiasp is a total game changer because it starts working almost immediately.

At the end of the day, diabetes is chemistry. And learning the “why” behind these insulins will help us stay better informed so we can make better blood sugar decisions.