Unexpected Sugar Crashes With Extended Fasting

Coming into this experiment, I wasn’t starting from a chaotic diet.

I had already been vegan for 7 years and had dialled into clean, whole food meals for over 6 months. Plus, in the four weeks before the experiment began, following a Diary of a CEO podcast with Water Fasting Scientist Dr Alan Goldhamer,  I had removed added oil, salt and sugar completely.

So when sugar was reintroduced during the early months, the result was not what I expected.

It didn’t immediately cause crashes.

In fact, between November and February, the opposite pattern showed up. When sweetness was tied to structured meals, it was usually tolerated well.

The real shift came later.

As my fasting and performance system became cleaner, more consistent, and more tightly aligned to compressed feeding windows, refined sugar started to stand out more clearly.

Initially as a random problem which evolved into detectable signals.

That is where the data became interesting.

This blog is not about sugar being “bad”.

It is about how my body's response to sugar changed as the system changed.

Key Insights

• Sugar wasn’t the consistent problem. The way it was introduced was
  
• Crashes appeared when intake became compressed or stacked
  
• Whole food carbohydrates grew more stable and predictable
• As the system became more responsive, it became less tolerant of unstructured eating
• Crashes became easier to recognise and anticipate over time
• A cleaner system made refined sugar responses stand out more clearly
• Feeling worse after sugar doesn’t automatically mean something is wrong. It can mean the system is reacting quickly
  
  

The Starting Point: Clean Eating, But Not a Clean System

Although the diet was already “clean” on paper, the system itself wasn’t.

Nutritional requirements were still being worked out, with some fasts not going to plan. Alcohol was still in the mix through December, which adds another layer of variability around hydration, recovery and energy stability. And even when food quality was good, meal timing and structure weren’t yet fully consistent, which made it harder to see clear cause-and-effect patterns.

On top of that, my body was still adapting to multiple changes at once, from diet shifts to fasting and training, which naturally creates a noisier baseline.

So while things looked controlled, the signals weren’t yet clear.

That context matters, because when I started reintroducing sweetness, the aim was understand whether it could exist within a structured approach without compromising progression.

The First Test Was Promising

On the 17th of December 2025, in week five of the experiment, I tested this properly for the first time.

After a structured meal of lentil dahl, a full block of tofu, and freshly made flatbreads, I added half a Rodeo donut to the end of the meal, fully expecting some kind of drop to follow.

It didn’t come.

There was no noticeable dip in energy, no rebound hunger, and no sense that I’d disrupted anything that had just been established. It felt, for lack of a better phrase, completely neutral.

That single moment shifted the direction of the experiment, because it suggested that sugar itself wasn’t the issue, but the way it was being introduced might be.

And over the following weeks, that pattern held. Sweet foods could be included, even fairly indulgent ones, as long as they were anchored to structured meals; and nothing seemed to break.

At that point, it would have been very easy to assume the problem had been solved.

Then the Pattern Started to Break

The first real crack appeared on the 27th of February, and it didn’t come from anything particularly unusual in isolation, but from how things were layered together.

Within the space of just over an hour, I had a donut, a homemade Snickers-style block, a banana, and then a protein bar, all sitting fairly tightly within the same window.

By mid-afternoon, somewhere between 2.30 and 3pm, the shift was obvious. Heavy eyelids, a drop in alertness, and that unmistakable sense that energy had fallen away rather than gradually tapered.

What stood out wasn’t just the crash itself, but the fact that the same types of foods had been tolerated before without issue.

The difference wasn’t what I ate.

It was how quickly, and how closely together, everything had been consumed.

This was the first point where things stopped behaving as expected, and it lines up closely with patterns I’ve seen elsewhere when structure starts to slip (I’ve written more about Why 48hr Fasts Fail).

The Moment It Became Undeniable

A month later, on the 7th of March, the pattern became impossible to ignore.

After a structured meal post-hockey, I ate 70 grams of sour sweets followed by a handful of fruitellas, and within ten minutes the response was immediate and strong enough that I had to pull over while driving and close my eyes for twenty minutes just to reset.

There was no ambiguity in that moment.

This wasn’t a subtle shift or something that could be explained away by fatigue.

It was a direct and immediate system response to refined sugar.

When It Stops Being About Energy

Then, towards the end of March, something slightly different happened.

After introducing carrot cake mid-afternoon on the 28th, the response wasn’t just a crash, but a prolonged period of continued eating that stretched across the rest of the day and into the evening, where the usual sense of control over intake just wasn’t there in the same way.

It wasn’t hunger driving it, and it didn’t feel particularly enjoyable either. It was more like a loop that had started and was difficult to step out of once it got going, and the same thing repeated itself again two days later when more cake was brought back into the environment.

Looking back at it now, it lines up quite cleanly with what else was happening in that window, with repeated sugar intake, higher fat combinations, and very little interruption between them.

At the time, it felt frustrating.

Now, it reads like a very clear signal.

What Changed Underneath All of This

By the time April came around, the baseline had shifted quite noticeably, even if it wasn’t something I could have articulated clearly in the moment.

Alcohol had been removed since mid-February, meals had become consistent, fasting windows were repeating week to week, and training demand had increased, all of which contributed to a system that felt far more stable on a day-to-day basis.

What became particularly noticeable was how whole food carbohydrates behaved.

Rice, oats, fruit, and the usual staples that formed most meals all produced the same kind of response, where energy felt steady, glycogen would refill, and there was no obvious spike or drop to speak of.

Then, when refined sugar was introduced, the contrast became much more obvious.

Not because sugar had suddenly become worse, but because everything else had become so consistent that anything outside of that pattern stood out immediately.

I've come to learn that this kind of shift in how my body handles fuel isn’t unique to this experiment, and aligns with broader work on metabolic flexibility from people like Dr Peter Attia, where the goal isn’t to eliminate certain inputs, but to improve how the system responds to them.

This is also where changes in glycogen and water handling became more obvious over time, something I’ve broken down in more detail in another piece about water retention and scale weight.

The Shift in Timing

The crashes didn’t disappear as the system stabilised, but they did change.

On the 4th of April, after a relatively balanced day of eating, a dessert of almond polenta with ice cream led to a crash that arrived around 30 to 40 minutes later, which felt slower and less abrupt than the earlier examples.

Then on the 23rd of April, after a well-structured morning and a midday meal built around white bread sandwiches with various spreads, the crash didn’t arrive until roughly two hours later, showing up as a strong need to sleep between 2.45 and 3.30pm.

That delay is important.

Earlier in the experiment, the response was immediate and obvious.

Now, it was slower, but still clearly linked.

What’s Actually Driving It Now

At this stage, it’s no longer useful to think about sugar in isolation.

The patterns that have emerged point much more towards combinations and context, where multiple sugar sources in a short window, refined carbohydrates paired with fats, lower hydration or sodium, and less structured eating environments all contribute to the outcome.

The 23rd of April is a good example of this, where protein had been present earlier in the day, but the combination of refined inputs and the setting itself overrode that stability.

What Actually Changed

The biggest shift across the experiment hasn’t been the removal of crashes, but the clarity around them.

Earlier on, it would have been easy to blame individual foods or assume something wasn’t working.

Now, the patterns are much easier to see.

Whole foods behave consistently. Structured meals hold steady. And when things drift outside of that, the response is far more predictable than it used to be.

At this point, it would be easy to assume that these crashes are a sign something is wrong.

But the data suggests something very different.

Why “Sugar Crashes” Don’t Always Mean Insulin Resistance

One of the easiest mistakes to make when improving your diet is assuming that feeling bad after sugar means something is broken.

It doesn’t.

People use the term insulin resistance quite readily, and I wanted to make sure this experiment wasn't endangering my body in subtler ways. So I dug into the topic and found that in many cases, feeling bad after sugar can mean your system and insulin regulation is better optimised.

Two very different systems

At a high level, the body can respond to sugar in two ways.

An insulin resistant system (often connected to diabetes)

• Needs more insulin to deal with the same food
• Blood sugar stays elevated for longer
• Energy feels slow, heavy, and prolonged

An insulin sensitive system (how my body seems to be operating)

• Responds quickly to small amounts of insulin
• Clears glucose from the bloodstream efficiently
• Energy is sharp, but less forgiving

If you’re interested in the broader thinking behind this, there’s some useful work from professionals you can read, looking at how the body handles fuel under different conditions. I've listed them at the bottom of this blog for you here.

The part most people get wrong

Both systems can feel bad after sugar.

But for completely different reasons.

• In an insulin resistant system, sugar lingers
• In a highly sensitive system, sugar can rise and fall quickly

That second scenario is where things get misread.

Because the experience can feel like a “crash”, it’s often assumed something is wrong.

But in reality, it can be a sign that the system is working too efficiently for the way the food was delivered.

What the data in this experiment shows

Earlier in the experiment, sugar was tolerated well when it was part of a structured meal.

Later, crashes began to appear, but only under specific conditions:

• When intake was rapid
• When foods were stacked too closely together
• When meals lost structure
• When food types had low satiety (feeling of fullness)

Not because sugar suddenly became a problem.

But because the delivery of that sugar changed.

The shift that matters

As the system becomes more sensitive, it becomes less tolerant of:

• Fast inputs
• Poor sequencing
• Unstructured eating

Which means the question is no longer:

“Is this food good or bad?”

It becomes:

“Is this being delivered in a way my system can handle?”

Where this leaves you

If you’re training regularly, fasting, and eating a clean diet, there’s a good chance your system is becoming more responsive.

That’s not something to fix.

But it does mean the margin for error gets smaller.

And the way you structure meals starts to matter more than the individual ingredients themselves.

FAQs

Do sugar crashes mean I’m insulin resistant?

Not necessarily. My data suggests the opposite, but if you're unsure, it's always worth checking a medical or nutritional professonal.

For me, “crashes” have happened when my body responds quickly to a rapid rise in blood sugar, especially when food is eaten quickly or without structure.

The experience feels negative, but the underlying mechanism is very different from insulin resistance.

Why does eating sugary foods sometimes make me feel worse than it used to?

For me, it's because the conditions changed. Look at what has changed in your overall eating, training and/or fasting behaviour.

If your diet has become cleaner, your training more consistent, or your fasting more structured, your system may respond more quickly than it did before.

That means foods that were once tolerated casually may now need more structure to feel the same.

Is sugar the problem, or how I’m eating it?

In this experiment, the pattern points more towards how than what.

The same foods produced very different outcomes depending on:

• how quickly they were eaten
• what they were paired with
• how close they were to other meals

That doesn’t make the food irrelevant. But it does shift the focus.

Why do crashes seem to happen more when I eat quickly or snack?

For me, it’s come down to the speed of intake and the type of snack.

I don’t tend to have issues with natural carbohydrate sources like fruit, where the energy release feels slower and more stable, likely because they’re tied to fibre and sit within a more structured context.

Refined or processed snacks, especially those higher in sugars or sweeteners, seem to behave very differently. The energy rise feels sharper, and when it drops, it’s much more noticeable. I’ve seen similar patterns with refined breads and certain sauces.

When those foods are eaten quickly, or stacked close together without much structure, the effect becomes much more obvious.

If I’m training and fasting, should I avoid sugar completely?

Not necessarily.

The data here doesn’t support complete avoidance as the solution.

Instead, it suggests that as the system becomes more responsive, the way food is introduced starts to matter more.

Removing foods entirely may simplify things, but it doesn’t address the underlying pattern.

Why did sugar feel fine earlier in the experiment but not later?

Because the system evolved.

Earlier phases were more structured, with clearer meal boundaries and fewer variables.

As new foods, timing changes, and different conditions were introduced, the same inputs produced different results.

This isn’t unusual. It’s part of what happens when you start paying closer attention.

What’s the main takeaway from all of this?

As the system becomes more consistent, the signals become clearer.

It’s not just about what you eat, but how that food sits within everything else you’re doing, and how that changes over time.

Tracking helps, but the real shift is recognising the patterns once they start to show up.

Additional Reading: Two Very Different Systems

Understanding how the body handles fuel differently depending on context

• Dr Peter Attia
  https://peterattiamd.com/ama74/
  Works on metabolic flexibility and how the body adapts to different fuel sources over time. This link is to a blog specifically on Sugar and sugar substitutes: weight control, metabolic effects, and health trade-offs.

How consistency and timing affect metabolic responses

• Dr Satchin Panda
  https://mycircadianclock.org
  Research on time-restricted eating and how consistent eating windows influence glucose control and energy stability.

Why glucose responses vary between individuals and situations

• Dr Eran Segal
  https://www.insidetracker.com/a/articles/longevity-by-design-dr-eran-segal
  Research showing that blood sugar responses to the same foods can vary significantly depending on the individual and context.

The role of internal system “noise” (inflammation, lipids, etc.)

• Dr Gerald Shulman
  https://medicine.yale.edu/profile/gerald_shulman/
  Work on insulin resistance and how internal metabolic state affects how fuel is processed.

Practical interpretation of blood sugar stability and food combinations

• Dr Stephen Cabral
  https://stephencabral.com
  Accessible explanations around how food timing, combinations, and lifestyle factors influence energy and blood sugar stability.

Photo by Mae Mu on Unsplash