Let’s start with an interesting fact most people don’t know. At any given moment, the total amount of sugar circulating in your blood is roughly 4-5 grams - about one sugar cube. Even if you eat something like a piece of cake with 40 grams of sugar, a healthy body won’t let all of that stay in your bloodstream. Blood sugar is tightly controlled and typically kept within a narrow range of about 4-5 grams, corresponding to roughly 90-100 mg/dL. That tiny amount is one of the most tightly regulated variables in your body!

Your system is constantly balancing blood sugar production, blood sugar use, and hormonal control - mainly insulin and glucagon. Even small deviations trigger immediate corrections.

Blood sugar in the fed state

So, in a typical fed state, blood sugar is usually in the 90-100 mg/dL range. After you eat, it rises, insulin is released, and sugar is either used for energy or stored as glycogen. Then it comes back down. This cycle repeats throughout the day and becomes your “normal.”

So when you stop eating and those numbers change, it feels like something is off.

What changes during fasting

Once you stop eating, your body doesn’t run out of sugar - it changes how it manages it. During the first 24 hours, your liver uses stored glycogen to maintain blood sugar. After that, glycogen runs low and your body shifts to gluconeogenesis - making sugar internally from things like amino acids, glycerol, and lactate.

At the same time, your body starts needing less sugar. After about 2-3 days:

your brain begins to rely less on sugar
your body uses more fat for energy

So instead of keeping blood sugar at 90–100, your body resets to a lower level. That new steady range is typically 60-70 mg/dL. This is not a problem. It’s how the system adapts.

What this looks like in real life

Blood sugar during a 10-day water fast, dropping from about 100 mg/dL to 60-70 mg/dL, with a brief dip to 42 mg/dL and a spike after refeeding.

In my recent 10-day water fast, this pattern showed up very clearly. I started at around 100 mg/dL. Over the next few days, blood sugar dropped and then stabilized. For most of the fast, it stayed in the 60-70 mg/dL range.

What stands out is that it wasn’t flat. Blood sugar moved:

slightly higher after physical activity
lower during prolonged heat and stress
with normal day-to-day variation

That’s exactly what you want to see. It means your body is actively regulating things.

Can blood sugar go even lower?

Yes! And this is where things get interesting. On day 6 of my fast, my blood sugar dropped to 41-42 mg/dL, the lowest I’ve ever recorded.

That day wasn’t typical - 95F heat, direct sun for hours and a lot of walking and standing. Here’s how it played out over a few hours:

1:04 pm - 41
1:05 pm - 43
1:31 pm - 52
2:16 pm - 58
2:17 pm - 56
3:23 pm - 54
4:29 pm - 67
7:38 pm - 54

Within a relatively short time, blood sugar moved from the low 40s back into the 60s. And importantly I felt fine, no dizziness and no confusion. By the next morning, it was back above 60. This was a temporary dip under combined stress, followed by normal recovery.

From what I’ve seen in the research and anecdotal reports, some fasting-adapted individuals can dip into the low 30s mg/dL without symptoms, so my 41 was still well above that range. And just to be clear, this isn’t something to aim for - it just happens.

Why low blood sugar during fasting is not the same as hypoglycemia

This is where most people get confused. In a fed state, a blood sugar reading in the 40s would usually be considered hypoglycemia. But fasting changes the context. During an extended fast:

insulin levels are low
your body is still producing sugar
and you’re running on an alternative fuel source

Ketone levels during a 10-day water fast, rising from about 0.4 to 7-8 mmol/L and dropping sharply after refeeding.

In my case, ketones rose from about 0.4 to 7-8 mmol/L during the fast. As ketones increase:

your brain relies less on sugar
overall sugar demand drops
energy supply remains stable

So even when blood sugar is low, your body is not running out of energy. That’s the key difference!

What happens after the fast

After breaking the fast, my blood sugar briefly rose to about 123 mg/dL, then settled back around 100. This is expected.

During fasting, glycogen stores are depleted. When you start eating again, your body prioritizes refilling them, which can temporarily push blood sugar higher. This is a normal transition back to a fed state.

Bottom line

Blood sugar during extended fasting follows a clear pattern. It drops from fed-state levels, stabilizes at a lower range, and remains tightly regulated. Dips below that range can happen, especially when you add extra stress like heat, activity, or dehydration. What matters is not a single number, but the overall trend, the context, and how you feel.

Blood sugar is not supposed to stay at 90-100 mg/dL during a fast. Your body lowers it because it needs less. Once you understand that, the numbers stop looking scary and start making sense.

Blood Sugar During Fasting: Why It Drops to the 60s (and Even the 40s)