Does Cooling Rice and Potatoes Actually Lower Blood Sugar?
Does Cooling Rice and Potatoes Actually Lower Blood Sugar?
By Jaden Choi — a plain-English look at what cooling (and reheating) your starches really does to your blood sugar, and where the viral version of the hack goes wrong.
You've probably seen the clip: cook your rice or potatoes, stick them in the fridge overnight, and — the voiceover promises — you've magically slashed the carbs, the calories, and the blood sugar spike. It's one of those food hacks that sounds too tidy to be true, which is usually a sign that part of it isn't.
Here's the honest version up front. Cooling cooked rice and potatoes does change the starch in a measurable way, and in controlled human studies the cooled version really did produce a smaller blood sugar rise than the same food eaten hot. The effect is real. But it's modest — a gentler peak, not a disappearing one — and the popular claim that chilling your rice "removes the calories" is mostly wishful. Let's walk through what actually happens, what the trials measured, and how much this is worth to you.
- Cooling cooked starch creates resistant starch — a form your small intestine can't fully digest — so it reaches your bloodstream more slowly and gently.
- In human trials, cooled rice and chilled potatoes produced a significantly lower glucose spike than the same food served hot. A type 1 diabetes study saw a lower peak; a chilled-potato study saw glucose drop 4.8–9.2% and insulin ~23–26% in the first half hour.
- Reheating does not undo it. In one rice study the cooled-and-reheated rice had the most resistant starch of all — and still lowered the glucose response.
- The effect is small, trials are short, and the "far fewer calories" claim is overstated — the resistant-starch gain is only about 1 gram per 100 grams. This isn't a license to eat unlimited rice.
What Cooling Actually Does to Starch
When you cook rice or potatoes, heat and water make the tightly packed starch granules swell and burst open — a process called gelatinization. That's what turns hard, chalky starch into something soft and digestible. The downside, for blood sugar, is that this open, gelatinized starch is easy for your digestive enzymes to break down fast, which is exactly why a hot bowl of white rice sends glucose up quickly.
Something interesting happens as the food cools. The loosened starch chains — mostly a type called amylose — slowly re-organize and re-crystallize into a tighter structure. This is retrogradation, and the new structure is called retrograded, or RS3, resistant starch. Your amylase enzymes can no longer fully unzip it, so that fraction slips past your small intestine undigested instead of flooding your bloodstream with glucose.
In other words, cooling doesn't add anything or remove the food's substance — it rearranges the starch into a form that behaves more like fiber than like sugar. That single change is the whole basis of the hack.
What the Rice Studies Found
Mechanisms are just hypotheses until someone feeds real people real rice. Two studies are worth knowing.
In a 2015 crossover study in the Asia Pacific Journal of Clinical Nutrition, researchers measured the resistant starch in white rice prepared three ways. Freshly cooked rice had about 0.64 grams of resistant starch per 100 grams. Rice cooled for 10 hours at room temperature had 1.30 grams. And rice cooled for 24 hours in the fridge and then reheated had the most: 1.65 grams. When 15 healthy adults ate the cooled-and-reheated rice, their glucose response was significantly lower than after fresh rice (125 vs. 152 mmol·min/L, p = 0.047).
A 2022 study in Nutrition & Diabetes tested the same idea in 32 adults with type 1 diabetes, who are especially sensitive to carbohydrate timing. Each person ate long-grain white rice on two occasions — once fresh, once cooled for 24 hours at 4°C and reheated. The cooled rice produced a significantly lower maximum glucose and a smaller glucose-curve area.
But that same study carries the single most important caveat in this whole topic, and we'll come back to it: the cooled rice also triggered more low-blood-sugar (hypoglycemic) episodes — 12 versus 3. For someone dosing insulin to their meal, a quietly gentler carbohydrate can become a problem, not a perk.
What the Potato Study Found
Potatoes retrograde the same way. In a 2019 randomized crossover trial published in Nutrients, 30 women with elevated fasting glucose and insulin each ate 250 grams of russet potato on two occasions — once boiled and hot, once baked then chilled.
The chilled potato lowered the early response measurably. In the first 15 and 30 minutes, glucose was about 4.8% and 9.2% lower, insulin about 25.8% and 22.6% lower, and an incretin hormone called GIP (which helps drive insulin release and fat storage) dropped by roughly 41% and 37%. The area under the curve for insulin and GIP was lower overall.
Two details keep this honest, though. First, the total glucose area under the curve wasn't significantly different — the chilled potato blunted the early surge more than it changed the whole picture. Second, people reported the same fullness after both versions, and in the 48 hours afterward they actually ate more carbohydrate and less fat. The cooled potato improved the hormonal response to that one meal; it didn't magically make people eat less overall.
The trials at a glance
| Study | Food & prep | Who | Main result (cooled vs. hot) |
|---|---|---|---|
| Sonia 2015 (APJCN) | White rice, cooled 24h + reheated | 15 healthy adults | Glucose response 125 vs. 152 mmol·min/L; most resistant starch of all three preparations |
| Strozyk 2022 (Nutrition & Diabetes) | White rice, cooled 24h + reheated | 32 adults, type 1 diabetes | Lower peak glucose & smaller curve — but more hypoglycemia (12 vs. 3 episodes) |
| Patterson 2019 (Nutrients) | Russet potato, baked then chilled | 30 women, high fasting glucose/insulin | Glucose −4.8%/−9.2% and insulin ~−23–26% at 15/30 min; total glucose AUC unchanged |
Does Reheating Ruin the Benefit?
This is the question that trips most people up, and the answer is reassuring: no, reheating does not erase the resistant starch. Once those starch chains have re-crystallized during cooling, a lot of that structure survives a normal reheat. That's not a guess — in the 2015 rice study, the preparation with the most resistant starch was the one that had been cooled for 24 hours and then reheated, and it still produced the lowest glucose response.
You don't have to eat your rice or potatoes cold. The resistant starch forms during the cooling step and largely persists through reheating — so cook, chill thoroughly (ideally overnight in the fridge), then warm it back up and eat it like normal. One practical note that has nothing to do with blood sugar: cooked rice left at room temperature can grow Bacillus cereus, a bacterium that causes food poisoning, so cool it in the fridge promptly rather than leaving it on the counter.
The "Fewer Calories" Claim — What It Gets Wrong
Here's where the viral version oversells. The headlines usually jump from "resistant starch isn't fully digested" to "so you've cut the calories in half." The arithmetic doesn't support that.
Look again at the numbers: cooling pushed resistant starch in rice from about 0.64 to 1.65 grams per 100 grams. That's a gain of roughly one gram of resistant starch per 100 grams of rice. Resistant starch does yield fewer calories than regular starch (it behaves like fiber, delivering roughly 2 instead of 4 calories per gram), but one extra gram of it is a rounding error against a 130-calorie cup of rice. You are not meaningfully "removing calories" by chilling your dinner.
What the studies actually measured — and found — was a gentler blood sugar and insulin response, not a calorie deficit. Those are different claims. A smaller glucose and insulin spike is a genuine, worthwhile effect, especially if you're managing blood sugar. But it won't do the job of eating less, and in the potato study people even nudged their later eating in the other direction. Treat cooled starch as a way to make a carbohydrate meal a little kinder to your blood sugar — not as a weight-loss trick.
So Is It Worth Doing?
Zoom out from the single meals and the picture holds, but stays modest. A 2021 systematic review and meta-analysis in the British Journal of Nutrition pooled 19 trials and found that resistant starch produced a small but statistically significant drop in fasting glucose — on the order of 0.09 mmol/L — with a clearer effect at higher doses (above ~28 g/day) or over longer periods (more than 8 weeks), plus an improvement in insulin resistance. A 2023 review in Frontiers in Nutrition reached a similar place for people with prediabetes or type 2 diabetes: retrograded resistant starch helped the glucose response, but the effects were small and depended heavily on the type and dose.
So the realistic verdict: cooling and reheating your rice and potatoes is a free, zero-downside tweak that genuinely nudges your glucose response in the right direction. The people most likely to notice are those with impaired glucose control. For everyone else, it's a nice bonus on top of the things that move blood sugar far more — portion size, what you pair the carbs with, and whether you move after you eat.
This is the one real caution. In the type 1 diabetes study, the cooled rice didn't just lower the peak — it caused more hypoglycemic (low blood sugar) episodes, because the same insulin dose was now matched to a slower-absorbing meal. If you dose insulin or take medication that lowers blood sugar, quietly changing how your carbs behave can push you too low. Don't adjust your carbs or your medication on your own — talk to your doctor or diabetes care team first.
Frequently Asked Questions
How long do I need to cool the rice or potatoes?
In the studies that saw the biggest resistant-starch gain, the food was cooled for about 24 hours in the refrigerator (around 4°C). Cooling for a shorter time at room temperature produced less resistant starch. Overnight in the fridge is the practical target — and cooling promptly also keeps the food safe to eat.
Does this work with pasta, oats, or bread too?
The same retrogradation process happens in other cooked starches like pasta and potatoes, and cooled pasta does form resistant starch. The best-documented human glucose trials, though, are in rice and potatoes. Expect the effect to be in the same modest ballpark rather than dramatically larger in other foods.
Does cooling rice really cut the calories?
Not in any meaningful way. Cooling converts roughly one extra gram of starch per 100 grams into resistant starch, which yields slightly fewer calories — but that's a trivial amount against the total. The measured benefit in studies was a gentler blood sugar and insulin response, not a calorie reduction.
Is the blood sugar effect large enough to matter?
It's real but modest — a smaller early spike, not a flat line. Across pooled trials the fasting-glucose change was small, with the clearest effect at higher doses and in people with impaired glucose control. It's a helpful nudge, not a substitute for portion control, meal composition, or prescribed treatment.
Is cooled rice safe to eat?
Yes, as long as you handle it safely. Cooked rice left at room temperature can grow Bacillus cereus bacteria that cause food poisoning. Refrigerate cooked rice within about an hour, keep it chilled, reheat it thoroughly, and don't reheat the same batch more than once.
Bottom Line — What to Actually Do
If you want to try it, the approach that matches the evidence is simple:
- Cook as usual, then chill thoroughly — ideally overnight in the fridge, not just a few minutes on the counter.
- Reheat and eat it normally — the resistant starch survives reheating, so you don't have to eat it cold.
- Keep your expectations modest — you're softening a blood sugar spike, not deleting calories or carbs.
- If you take insulin or glucose-lowering medication, talk to your doctor before changing how your carbs behave.
It's a genuinely nice, free tweak with a real (if small) payoff — best thought of as one more gentle lever alongside the bigger ones: portion size, pairing carbs with protein, fiber, or fat, and taking a short walk after meals.
Sources
- Strozyk S, Rogowicz-Frontczak A, Pilaciński S, et al. "Influence of resistant starch resulting from the cooling of rice on postprandial glycemia in type 1 diabetes." Nutrition & Diabetes, 2022;12:21. doi.org
- Sonia S, Witjaksono F, Ridwan R. "Effect of cooling of cooked white rice on resistant starch content and glycemic response." Asia Pacific Journal of Clinical Nutrition, 2015;24(4):620–625. PubMed
- Patterson MA, Fong JN, Maiya M, et al. "Chilled Potatoes Decrease Postprandial Glucose, Insulin, and Glucose-dependent Insulinotropic Peptide Compared to Boiled Potatoes in Females with Elevated Fasting Glucose and Insulin." Nutrients, 2019;11(9):2066. doi.org
- Systematic review and meta-analysis of resistant starch and glycaemic control (19 randomized trials). British Journal of Nutrition, 2021. PubMed
- Systematic review of resistant starch in type 2 diabetes and prediabetes. Frontiers in Nutrition, 2023. doi.org
This article summarizes published research for general educational purposes and is not medical advice. It does not diagnose, treat, or recommend any specific diet, carbohydrate target, or medication change. Resistant starch affects blood sugar modestly and individually, and people who take insulin or other glucose-lowering medication should be especially careful — talk to your doctor or a licensed healthcare provider before making any change to your diet or treatment. Research evolves and findings can be revised. Information is current as of October 2026.