Chain reaction: Nutritional keys to a healthier diet may lie in unraveling starch structures

By Tarun Sai LomteJun 6 2023Reviewed by Benedette Cuffari, M.Sc.

A recent study published in the journal Foods discusses the effects of starch structure on human health.

Study: Effects of the Molecular Structure of Starch in Foods on Human Health. Image Credit: ibreakstock / Shutterstock.com

What is starch?

Starch represents the most significant component of food energy for most of the global population. Ordinary starch is composed of amylose and amylopectin, whereas starches from some plants and mutants have intermediate structures.

A recent review described on the relationship between starch structure, digestibility, and thermal properties. Comparatively, the present study focused on the relationship between the chain length distribution (CLD) of starch and health-related effects.

Starch properties and biosynthesis

The location and rate of digestion of starch foods in the gastrointestinal (GI) tract correlate with human health. The rate of starch digestion rate also appears to impact mental health.

Rapidly digested starch (RDS) causes a spike in blood sugar levels that could strain the insulin system and could lead to diabetes if chronic. Slowly digested starch (SDS) decreases the glycemic load of the food product, thereby reducing the risk of breast or colon cancers and type 2 diabetes and increasing satiety.

Previous studies suggest a correlation between long-term SDS intake and improved metabolic profile. In fact, SDS intake can reduce postprandial glucose peaks, which is beneficial for diabetes management.

Resistant starch (RS) reaching the colon is a substrate for microbial fermentation, which yields carbon dioxide, hydrogen, short-chain fatty acids, and methane. This can attenuate postprandial glucose and insulin responses to ultimately benefit human health.

CLD is an essential determinant of the digestibility and pasting properties of starch. Moreover, CLD reflects the distribution of individual chains obtained following the cleavage of all (1→6)-α linkages by a debranching enzyme (DBE). CLD is derived by measuring the number or weight distribution as a function of the degree of polymerization (DP) of linear glucans.

The biosynthesis of starch primarily involves five enzymes, including starch synthase (SS), granule-bound SS I (GBSSI), starch branching (SBE) enzyme, DBE, and adenosine diphosphate (ADP)-glucose pyrophosphorylase (AGPase).

SS, DBE, and SBE are the main enzymes for amylopectin biosynthesis, whereas GBSSI, DBE, and SBE are involved in amylose biosynthesis. SBE cleaves (1→4)-α linkages and adds the short chain to a parent or other chain to create (1→6)-α linkages. By contrast, DBE directly hydrolyzes (1→6)-α linkages of polyglucans. AGPase catalyzes and generates

ADP-glucose is elongated through (1→4)-α linkages by SS, whereas GBSS elongates amylose chains and increases the number of amylopectin’s extra-long chains (ELCs). In contrast, SS isoforms (I-IV) are primarily involved in amylopectin synthesis.

Starch structure and health-related effects

The fine structure of starch can influence its digestibility, among other functional features. Various studies suggest that short A chains of amylopectin (DP 6 – 12) affect the quantity of SDS and RDS.

One study has reported a correlation between lower proportions of A chains of amylopectin and lower RDS, thus contributing to a higher SDS in long-grain rice. Furthermore, another study reported a negative correlation between short-chain proportion in cooked maize starch and digestion rate.

A higher proportion of long B chains of amylopectin (DP > 40) has the potential to impact the retrogradation of SDS on cooling and contribute to slow digestion. Amylopectin from cooked maize starch, with short chains, high branching density, and shortened non-reducing ends is slowly digested.

This might be due to the preferential cleavage of (1→4)-α linkages by α-amylase. Notably, the rate of (1→6)-α cleavage by amyloglucosidase is slower than (1→4)-α cleavage.

Amylose CLD can also significantly impact digestibility. The in vitro digestibility of cooked rice starch and native starch was affected by amylose content and the degree of branching of short or medium amylose chains (DP 100 – 500). Shorter amylose chains form double helices in debranching waxy starches aggregating into crystalline arrays upon cooling, which are slowly digested.

Intermediate chains (DP 500 – 1000) of amylose in non-waxy cooked rice starch and native starch prevent aggregation and form cross-linked networks, thereby slowing the digestion rate. The long chains of amylose (DP 1000 – 1500) can also have an effect. One previous study showed that long amylose chains correlated with rice starch digestibility but not as significantly as short and medium amylose chains.

Conclusions

While numerous studies have examined the relationship between the structural and functional features of starch, more research is needed to understand the effects of starch structure on human health. Although the impact of amylose content on digestibility is established, the effects of amylose CLD is relatively less studied.

Modifying the structural features of starch can have positive health benefits. Future studies may focus on the effects of intermediate or long amylopectin chains on digestibility.