Understanding the Glycaemic Behaviour of Rice: Insights from In-Vitro Gastrointestinal Digestion

Rice is a daily staple for millions of people, particularly across Asia, and a major contributor to dietary carbohydrate intake. While it is an important source of energy, rice consumption is often associated with rapid post-meal rises in blood glucose, an increasing concern in the context of diabetes and metabolic health. Importantly, all rice does not behave the same way in the body. Differences in variety, processing history, and starch structure strongly influence how quickly sugars are released during digestion. This variability is captured through the glycaemic index (GI), which reflects how rapidly a carbohydrate-rich food raises blood glucose after consumption. Foods that digest quickly tend to produce sharper glucose spikes, while slower-digesting foods result in a more gradual response. For rice, simple indicators such as total carbohydrate content or grain colour are insufficient predictors of GI; instead, the rate and extent of starch breakdown during digestion are the key determinants.

Why In-Vitro GI Models Matter

GI is conventionally measured through in-vivo human studies, where blood glucose responses are monitored following food intake. Although scientifically robust, such studies are expensive, time-consuming, and influenced by inter-individual variability in metabolism, age, and health status. These limitations make them impractical for routine screening, comparative assessment, or early-stage product development. As a result, food scientists increasingly rely on in-vitro gastrointestinal digestion models, which simulate the oral, gastric, and intestinal phases of human digestion under controlled laboratory conditions. Rather than measuring blood glucose directly, these models assess the rate and extent of starch hydrolysis into sugars, parameters that are strongly linked to post-prandial glycaemic response. Crucially, in-vitro methods allow digestion to be tracked over time, enabling kinetic analysis rather than reliance on single endpoint measurements.

Scientific Rationale and MSK’s In-Vitro GI Framework

At MitraSK (MSK), a physiologically relevant and internally validated in-vitro GI assessment framework was developed to evaluate the glycaemic behaviour of selected rice varieties. The digestion protocol was adapted from internationally recognised GI and INFOGEST principles, with controlled modifications to enzyme concentrations, digestion volumes, and analytical endpoints to enable robust, laboratory-scale screening while preserving physiological relevance.
This approach focuses on intestinal digestion kinetics, from which the Hydrolysis Index (HI) and predicted Glycaemic Index (pGI) are derived using white bread as a reference. Unlike clinical GI values, HI and pGI are model-based, comparative indicators that reflect enzymatic digestibility and starch breakdown behaviour under standardised conditions. These indices are widely used for screening, formulation optimisation, and comparative evaluation of carbohydrate-rich foods prior to in-vivo validation.

Relevance to Industry and Real-World Outcomes

In-vitro GI screening provides a practical and scalable tool for rice producers, food manufacturers, quality-testing laboratories, and regulators. It enables objective comparison of rice varieties, assessment of processing interventions, early-stage screening before costly human trials, and evidence-based product positioning for health-focused markets. While in-vitro GI does not replace clinical GI testing, extensive global evidence and international GI tables demonstrate strong alignment between in-vitro starch digestion behaviour and in-vivo GI classifications. Foods that exhibit rapid starch hydrolysis in vitro consistently correspond to high-GI foods in human studies, whereas slower digestion profiles align with lower-GI categories. In this context, the MSK framework serves as a predictive screening tool, bridging laboratory digestion behaviour with real-world metabolic relevance and supporting informed decision-making across the rice value chain.

Experimental Approach: From Mouth to Intestine

A standardised three-phase in-vitro digestion model was employed, comprising of simulated salivary buffer (SSF), simulated gastric buffer (SGF) and simulated intestinal buffer (SIF). Rice samples (white bread as reference, hydrothermally modified rice, brown rice, red rice, and black rice) were subjected to sequential digestion under controlled pH, temperature, enzyme activity, and ionic conditions. Reducing sugar release was quantified using a validated DNSA-based assay, and glucose equivalents were calculated to track starch hydrolysis. This framework allows comparison of both: Extent of digestion (total reducing sugars released), and Rate of digestion (kinetics over time).

Oral Phase (SSF)- Limited Contribution to Starch Hydrolysis: Across all rice varieties, the oral phase resulted in minimal release of reducing sugars, reflecting the inherently limited hydrolytic capacity of salivary α-amylase under short oral residence times. Only minor differences were observed between rice types, indicating that oral digestion contributes negligibly to varietal discrimination in glycaemic behaviour. These findings confirm that salivary digestion plays a marginal role in overall starch hydrolysis, and that meaningful differentiation in glycaemic potential arises predominantly during subsequent gastrointestinal phases.

Gastric Phase (SGF)-Structural Effects Begin to Emerge: During the gastric phase, reducing sugar concentrations increased slightly, largely due to continued enzyme action initiated earlier and matrix softening under acidic conditions. However, starch hydrolysis remained limited, as gastric enzymes do not actively digest starch. Notably, pigmented and whole rice varieties (brown, red, black) consistently released lower reducing sugar than white bread, suggesting greater structural resistance to enzymatic penetration even after gastric processing.

Intestinal Phase (SIF)- Where Glycaemic Differences Become Clear: The intestinal phase represents the most critical stage for glycaemic response, driven by pancreatic α-amylase–mediated starch hydrolysis. Clear differentiation among rice varieties was observed:

•   White bread showed the highest reducing sugar release, consistent with its highly gelatinised, enzyme-accessible starch structure.
•   Hydrothermally modified (HMT) rice exhibited lower overall reducing sugar release relative to bread, indicating restricted enzymatic hydrolysis.
•   Pigmented rice varieties displayed intermediate behaviour. While some showed measurable reducing sugar release at later stages, their overall digestion remained slower than bread.

It is important to note that the DNSA assay quantifies total reducing ends, including dextrins and oligosaccharides, and not glucose alone. Therefore, intestinal reducing sugar levels should be interpreted as indicators of digestive potential, rather than direct predictors of blood glucose response.

Digestion Kinetics: Rate as a Determinant of Glycaemic Index

To capture digestion dynamics and enable prediction of glycaemic index, reducing sugar release was monitored over time during the intestinal phase. White bread exhibited rapid early-phase starch hydrolysis, with steep increases in reducing sugar concentration within the first 30-60 minutes, a kinetic pattern characteristic of high-GI foods. In contrast, hydrothermally modified (HMT) rice showed a slower rate of hydrolysis with a partial plateau at later time points, indicating delayed enzymatic accessibility. This behaviour is consistent with starch retrogradation, and the formation of enzyme-resistant crystalline regions induced by hydrothermal treatment. Such kinetic differences are physiologically significant, as the rate of early-phase starch digestion strongly influences post-prandial glycaemic spikes, highlighting the importance of kinetic profiling beyond endpoint measurements alone.

From Digestion Curves to Glycaemic Indices

Hydrolysis Index (HI) and predicted Glycaemic Index (pGI) were calculated using the incremental area under the curve (iAUC) of intestinal digestion, with white bread used as the reference (HI = 100). This approach aligns with internationally accepted in-vitro GI methodologies. While some rice samples exhibited HI values approaching or exceeding the reference, it is critical to emphasise that:

• HI is a relative index, and
• pGI is a model-based estimate, not a clinical GI value.

Key Takeaways for Industry and Regulators

•   Rice glycaemic behaviour is not uniform; it is strongly influenced by processing and varietal characteristics.
•   Hydrothermal modification can effectively attenuate early-phase starch digestion, reducing glycaemic potential.
•   Pigmented and whole rice varieties exhibit slower digestion kinetics despite measurably reducing sugar release at later stages.
•   Kinetics-based evaluation provides deeper insight than endpoint measurements alone.
•   In-vitro GI screening is a valuable preclinical tool for product development, reformulation, and quality benchmarking.

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