First Meal After a 48-Hour Fast: Why Blood Sugar Spikes

The first meal after a two-day fast raises your blood sugar more than the same meal would on a normal day. A randomized crossover study published online on 5 July 2026 in The Journal of Physiology measured this in 16 healthy adults, and found that adding daily exercise to the fast did not prevent it. The effect had shrunk a day later but had not fully gone.

What the study did

Researchers at the University of British Columbia Okanagan recruited 16 healthy adults, 8 males and 8 females, with an average age of 24. Each person completed a 48-hour fast twice: once with no exercise, and once with daily cycling. The order was randomized, so every participant served as their own comparison.

Each exercise session combined 50 minutes of moderate-intensity continuous cycling with five 1-minute high-intensity intervals, performed at baseline and again 24 hours into the fast.

To see how the body handled food, participants took a mixed meal tolerance test: a standard meal, followed by two hours of blood sugar measurements. They did this before the fast, at the end of the fast, and again 24 hours after refeeding. Blood sugar was tracked two ways, with fingertip (capillary) samples and with a continuous glucose monitor (CGM).

Study at a glance
Design	Randomized crossover
Who	16 healthy adults (8 males, 8 females), average age 24
Fast	48 hours, done twice
Compared	Fasting alone versus fasting with daily cycling
Test	Mixed meal tolerance test before the fast, after it, and 24 hours after refeeding
Main result	Higher blood sugar response to the meal after fasting, with or without exercise

What it found

The first meal hit harder. The two-hour blood sugar response to the test meal was higher at the end of the 48-hour fast than at baseline. This showed up in both the fingertip samples and the CGM readings (P<0.001).

Exercise did not change it. The rise was the same whether or not participants had cycled during the fast. Statistically, there was no interaction between condition and time (P>0.364). The authors conclude that this combination of moderate and high-intensity exercise does not attenuate the effect.

It lingered for a day. Twenty-four hours after refeeding, the CGM response to the meal had come down compared with the end of the fast (P<0.001), but it was still elevated relative to baseline (P = 0.01). In the authors' words, glucose intolerance persists 24 hours after refeeding.

The abstract reports the direction and the statistical strength of these changes. It does not give the size of the rise in everyday units, so this post does not either.

Why would a fast make you worse at handling a meal?

It sounds backwards, but the paper opens by noting that short-term fasting of 48 to 72 hours impairs insulin sensitivity and glucose tolerance. In general terms, a body that has run on fat for two days has turned down its use of glucose, and it takes time to turn that back up when carbohydrate arrives again. You can read more about that shift in our guide to ketosis and fasting.

The researchers expected exercise might help, because a bout of aerobic exercise is known to make the body more sensitive to insulin. It did not help here. Their suggested explanation is that fasting-induced insulin resistance works differently from the pathological kind seen in disease, so the usual remedy does not apply. That is their interpretation, not a tested result.

The sex difference is exploratory

The team also looked at males and females separately. Blood sugar rose more after refeeding in females than in males on the fingertip measure (P = 0.020), the CGM measure (P<0.001) and plasma glucose (P<0.001), and females showed greater impairments in CGM metrics over the 24 hours after refeeding (P<0.01).

The authors label these analyses exploratory, and that label matters. With 8 people in each group, in a study designed to answer a different question, this is a signal worth testing in a larger trial. It is not an established difference between men and women.

What the study does not show

• It does not show harm. The study measured a short-lived change in healthy young adults. It did not follow anyone long enough to link that change to any health outcome.
• It does not cover everyone. Sixteen people in their twenties cannot tell us how older adults, or people with prediabetes or diabetes, would respond.
• It did not test what to eat. Everyone got the same test meal. The study says nothing about whether a smaller or lower-carbohydrate first meal would produce a gentler response.
• It tested one fast length. The results are for 48 hours. Shorter daily fasts such as 16:8 were not part of it.

What this means if you do long fasts

If you fast for two days, expect your first meal to produce a higher blood sugar reading than usual, and expect the next day to look a little different too. If you wear a CGM, a high reading after breaking a 48-hour fast is the pattern this study describes, not necessarily a sign that something has gone wrong.

Do not count on a workout to cancel it out. In this study, two long cycling sessions made no measurable difference.

Treat refeeding as part of the fast, lasting at least a day, and not as a single meal. Many people choose to break a long fast with a modest meal instead of a large, sugary one. That is a reasonable, cautious habit, but be clear that this study did not test it. Our guide to structuring longer fasts covers the practical side of planning the days before and after.

Who should not try this without a doctor

Multi-day fasting is a bigger undertaking than skipping breakfast. This article is general information, not medical advice. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting. That matters most for fasts of 48 hours or more, where blood sugar and electrolytes both need attention.

Tracking the refeed with QoFast

QoFast is a fasting tracker app for iPhone and Android, for intermittent fasting and multi-day extended fasts. Extended fasts in the app can include an optional preparation phase and a refeeding phase, so the day after the fast is planned and not an afterthought. QoFast gives general information, not medical advice. It is free to download, with an optional Pro subscription: App Store and Google Play.

FAQ

Is a blood sugar spike after a 48-hour fast normal?

A higher than usual response to the first meal is what this study observed in all conditions, in healthy young adults. The abstract does not report how large the rise was, so it cannot tell you what number to expect.

How long does it last?

In this study the response was lower 24 hours after refeeding than at the end of the fast, but still above baseline. Measurements stopped there, so the study does not say when it returns to normal.

Will exercising during the fast prevent it?

Not in this trial. Daily cycling during the fast left the response unchanged.

Sources

• Bouck T, Marcotte-Chénard A, Barg D, et al. "Fasting-induced glucose intolerance is not impacted by the addition of daily exercise: A randomized crossover study." The Journal of Physiology, published online 5 July 2026. DOI: 10.1113/JP291000. PubMed 42402811.