What Is Resistant Starch? Cooled Potatoes, Rice and the Register Claim

Resistant starch is the part of starch your small intestine cannot digest, so it reaches the colon and is fermented there, like a fibre. A food in which at least 14% of the starch is resistant may say that replacing digestible starch with it "contributes to a reduction in the blood glucose rise after that meal".¹ Chilling cooked potatoes raises it and reheating lowers it again; chilled then reheated rice still held more than fresh.^{2, 3} This page is part of the Wellgard fibre guide.

Evidence at a glance

• Trials: 19 randomised trials pooled in 2021; 36 randomised trials of 982 people with type 2 diabetes or prediabetes reviewed in 2023.
• Largest single trial cited: 45 young adults, single meals of hot, cold and reheated pasta.
• Strongest measured outcome: a smaller blood glucose rise after a meal with type 2 resistant starch in type 2 diabetes or prediabetes (standardised mean difference -0.96).
• Not tested in the trials cited here: cooled leftovers eaten regularly for weeks.

Sources checked on 13 October 2026.

What is resistant starch, and is it a fibre?

Most starch is digested in the small intestine and absorbed as sugar. Resistant starch escapes digestion, reaches the colon and is fermented by bacteria into short-chain fatty acids such as butyrate.⁴ So it behaves like a fibre; how much fibre is in food gives everyday amounts. It forms no gel; of the three jobs in soluble vs insoluble fibre, it ferments. There are five types.⁴

What is resistant starch allowed to say in Great Britain?

One wording is authorised for use in Great Britain: "Replacing digestible starches with resistant starch in a meal contributes to a reduction in the blood glucose rise after that meal." Its condition, as the register states it: "The claim may be used only for food in which digestible starch has been replaced by resistant starch so that the final content of resistant starch is at least 14 % of total starch."¹ It describes a reformulated food, not leftovers or a scoop stirred into a meal.

The register rejected "RS promotes a healthy digestive system. RS delivers prebiotic activities" and a wording that resistant starch "helps favour a normal colon metabolism".¹ "Prebiotic" and "gut health" are not authorised claims for it; what is inulin explains the word prebiotic.

Does resistant starch lower blood sugar?

After a meal, yes, in pooled trials; between meals, by a small amount. In type 2 diabetes or prediabetes, a 2023 review of 36 randomised trials (982 people) found types 1 and 2 lowered the glucose rise after a single meal (standardised mean differences -0.54 and -0.96). Over weeks, both lowered post-meal glucose, and type 2 also lowered fasting glucose and insulin.⁴

Across 19 randomised trials comparing resistant starch with digestible starch, fasting blood glucose was 0.09 mmol/L lower; above 28 g a day it was 0.16 mmol/L lower, and in trials longer than 8 weeks 0.12 mmol/L lower. Insulin resistance (HOMA-IR) improved by 0.33; fasting insulin did not differ.⁶

Does resistant starch help cholesterol or weight?

Across 19 trials total cholesterol was 0.19 mmol/L lower, and across 16 LDL cholesterol was 0.09 mmol/L lower, with clearer effects after 4 weeks; more than 20 g a day also lowered triglycerides.⁷

A 2019 pooling of 22 trials (670 people) of type 2 resistant starch at 8 g a day or more found triglycerides 0.10 mmol/L lower in 269 healthy people and body weight 1.29 kg lower in 90 people with type 2 diabetes. A few small studies drove both results, and nothing else changed.⁸ Resistant starch has no authorised weight or cholesterol wording in Great Britain.¹

Does resistant starch help you poo, or feed your gut bacteria?

It adds some bulk without making you go more often. Pooled randomised trials in healthy adults found that resistant starch raised daily stool weight by about 35.5 g, raised faecal butyrate and lowered faecal pH, while stool frequency did not change significantly.⁹ No authorised wording attaches a benefit to that butyrate.¹ The fibres with constipation trials are in the constipation guide.

Does cooling potatoes, rice or pasta make resistant starch, and does reheating destroy it?

Cooling makes more; reheating does not undo all of it. In a laboratory study of three potato varieties, chilled cooked potatoes held more resistant starch than hot or reheated ones, baked held more than boiled, and the variety made no difference.² Rice chilled for 24 hours then reheated held 1.65 g per 100 g against 0.64 g freshly cooked, and gave 15 healthy adults a lower glucose response than fresh rice.³ In 45 young adults, reheated pasta gave a smaller glucose response than freshly cooked pasta, and cold pasta did not differ significantly from reheated.¹⁰

In two trials of 32 adults with type 1 diabetes on insulin pumps, cooled and reheated rice lowered the peak glucose rise from 3.9 to 2.7 mmol/L but brought more low-glucose episodes on a standard insulin dose (12 against 3).¹¹ Cooled and reheated pasta lowered it from 4.7 to 2.8 mmol/L, with no difference in low-glucose episodes.¹² If you use insulin, talk to your diabetes team first.

For rice, the NHS says to "refrigerate it within 1 hour and eat it within 24 hours" and that rice "should not be reheated more than once".¹³

Which foods are high in resistant starch?

Wholegrains and pulses with their structure intact (type 1), high-amylose grains and raw green banana flour (type 2), and cooked starchy foods once cooled (type 3) contain it.^{4, 5} No verified UK table gives grams per food as eaten, and methods differ: one trial's pasta measured 8.03 g per 100 g fresh and 12.88 g cooled.¹²

Resistant starch supplements: what should you read on the label?

Look for grams of resistant starch per serving, not grams of powder or flour; the type and source, such as type 2 from green banana or high-amylose maize; and the claim wording, since neither "prebiotic" nor "gut health" is authorised for it.¹ If you take glucose-lowering medicines, tell your diabetes team before adding large amounts.

What we don't know

• Every leftovers trial cited here was a single meal.
• No verified UK table gives grams of resistant starch for everyday foods as eaten.
• The 2023 review called for more trials of types 3, 4 and 5 in diabetes and prediabetes.

Frequently asked questions

Does toasting bread make it resistant starch?

Not on the evidence we found. In a study of 17 people with type 2 diabetes, hard toasted bread held no more resistant starch than white bread.¹⁴ Toasting did change the glucose response: in 10 healthy adults, toasting lowered it for white bread, and so did freezing and defrosting a homemade loaf.¹⁵

How much resistant starch should you eat a day?

There is no UK target for it; the NHS fibre target for adults is 30 g a day.¹⁶ Pooled trials used at least 8 g a day of type 2, and fasting glucose fell more above 28 g.^{8, 6}

Is resistant starch good for people with diabetes?

In type 2 diabetes and prediabetes, types 1 and 2 lowered the glucose rise after meals in trials.⁴ In type 1 diabetes, cooled rice lowered the rise but brought more low-glucose episodes on a standard insulin dose.¹¹ Talk to your diabetes team before making changes.

Does green banana flour count as resistant starch?

Raw green banana flour contains type 2 resistant starch. In 10 people with an ileostomy, 30 g a day of the raw flour left more undigested material at the end of the small bowel than the cooked flour.⁵

Can too much resistant starch cause bloating?

It is fermented in the colon, and fermentation makes gas, so a sudden large increase can bring wind and bloating. NICE advises people with IBS to reduce resistant starch, "often found in processed or re-cooked foods".¹⁷ Why fibre makes you bloated covers building up slowly.

Sources

1. Department of Health and Social Care. Great Britain nutrition and health claims register, dated 19 May 2026, entries 681, 682, 783. gov.uk
2. Raatz SK et al. Food Chemistry 2016. Laboratory analysis, three potato varieties; one author at a pet-food manufacturer, part-funded by potato industry groups. PubMed 27132853
3. Sonia S et al. Asia Pacific Journal of Clinical Nutrition 2015. Laboratory analysis and randomised crossover trial, 15 healthy adults, single meals; publication grant from a food company's foundation. PubMed 26693746
4. Pugh JE et al. Frontiers in Nutrition 2023. Systematic review of 36 randomised trials (31 pooled), 982 people with type 2 diabetes or prediabetes; publicly funded. PubMed 37051127
5. Langkilde AM et al. American Journal of Clinical Nutrition 2002. Randomised crossover study, 10 people with an ileostomy; funding not checked. PubMed 11756067
6. Xiong K et al. British Journal of Nutrition 2021. Pooled analysis of 19 randomised trials. PubMed 32959735
7. Yuan HC et al. Nutrition Research 2018. Pooled analysis of 20 trials from 14 articles. PubMed 29914662
8. Snelson M et al. Nutrients 2019. Pooled analysis of 22 randomised trials, 670 people, 1 to 12 weeks; no external funding. PubMed 31398841
9. Shen D et al. International Journal of Food Sciences and Nutrition 2017. Pooled analysis of randomised trials in healthy adults; trial count not given in the abstract. PubMed 27593182
10. Hodges C et al. Foods 2019. Randomised crossover trial, 45 adults aged 20 to 24, single meals; no external funding. PubMed 31881647
11. Strozyk S et al. Nutrition and Diabetes 2022. Crossover trial, 32 adults with type 1 diabetes, single meals; funding not stated. PubMed 35429987
12. Rogowicz-Frontczak A et al. Nutrients 2026. Randomised crossover trial, 32 adults with type 1 diabetes, single meals; no external funding. PubMed 41978202
13. NHS. Starchy foods and carbohydrates. Page last reviewed 15 March 2023 (review overdue). nhs.uk
14. Giacco R et al. British Journal of Nutrition 2001. Randomised trial, 17 adults with type 2 diabetes, single meals; no funding statement. PubMed 11227031
15. Burton P, Lightowler HJ. European Journal of Clinical Nutrition 2008. Randomised repeated-measures trial, 10 healthy adults, single meals; funding not checked. PubMed 17426743
16. NHS. How to get more fibre into your diet. Page last reviewed 2 March 2026. nhs.uk
17. NICE CG61. Irritable bowel syndrome in adults, recommendation 1.2.1.4. Guideline; last updated 4 April 2017. nice.org.uk

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