Is there evidence for converting bread and rice into resistant starch?

Converting Bread and Rice into Resistant Starch

Yes, there is strong evidence that cooling cooked rice and bread after cooking significantly increases their resistant starch content through a process called starch retrogradation, which can lower postprandial glycemic responses.

Evidence for Rice

The most compelling evidence comes from controlled studies on rice preparation:

• Cooling cooked white rice for 24 hours at 4°C (refrigerator temperature) then reheating increases resistant starch content from 0.64 g/100g to 1.65 g/100g—a 2.6-fold increase 1
• This cooling and reheating method significantly lowered glycemic response compared with freshly cooked rice (125±50.1 vs 152±48.3 mmol·min/L, p=0.047) in healthy adults 1
• Even cooling at room temperature for 10 hours increases resistant starch to 1.30 g/100g, though refrigeration is more effective 1
• Multiple heating/cooling cycles further enhance resistant starch formation—three cycles increased resistant starch in cereals from 1.86% to 3.25% (a 75% increase) 2

Evidence for Bread and Other Starches

The American Diabetes Association guidelines acknowledge that:

• Gelatinized and retrograded starches (RS type 3) form when cooked starchy foods are cooled, creating resistant starch 3
• Cooling boiled potato overnight at 4°C produced a 2.8-fold increase in resistant starch content 4
• The same retrogradation mechanism applies to bread and other cooked starchy foods 3

Practical Clinical Application

To maximize resistant starch formation in rice and bread:

1. Cook the starch-containing food completely (full gelatinization is necessary) 1, 2
2. Cool in refrigerator (4°C) for at least 10-24 hours—longer cooling times and colder temperatures produce more resistant starch 1
3. Reheating is acceptable and does not eliminate the resistant starch that formed during cooling 1
4. Multiple heating/cooling cycles can further increase resistant starch, though this may not be practical for daily use 2

Metabolic Benefits

The American Diabetes Association notes that resistant starch:

• Produces lesser increases in postprandial glucose than digestible starch with correspondingly lower insulin levels 3
• Is fermented in the colon and produces approximately 2 kcal/g of energy (versus 4 kcal/g for regular starch) 3
• May modify postprandial glycemic response and reduce hyperglycemia 3

Important Caveats

The evidence has significant limitations:

• Long-term studies have not consistently confirmed metabolic benefits—while short-term one-meal studies show reduced postprandial glucose and insulin responses, second-meal effects are mixed, and long-term studies (beyond a few weeks) show inconsistent results 3
• The absolute increase in resistant starch content remains modest (typically 1-3% of total starch) 1, 2
• The total amount of carbohydrate in meals remains more important than the source or type for glycemic control 3
• Individual food processing techniques and chewing can affect the amount of resistant starch 4

Clinical Bottom Line

For patients seeking to reduce glycemic impact of rice and bread, recommend cooking then cooling in the refrigerator for 24 hours before consumption (reheating is acceptable). This simple intervention increases resistant starch 2-3 fold and demonstrably lowers postprandial glucose responses 1. However, emphasize that this is an adjunctive strategy—total carbohydrate counting and portion control remain the primary determinants of glycemic control 3.