2019 Belfort Lecture

A tale of two cousins: starch and glycogen - complex carbohydrates whose structure is central to good nutrition and diabetes management

Lecturer: Prof Robert (Bob) G Gilbert | May 8, 2019

Starch and glycogen are both complex branched glucose polymers, starch being used by plants and glycogen by animals (and other organisms) as energy reservoirs. Both contain the same linear and branching linkages, and there are strong similarities between the enzymes involved in the biosynthesis of each. The main differences are that the branching structure in amylopectin (the main component in ordinary starch, the other being amylose) permits crystallization, while glycogen is always amorphous and also contains a small but significant amount of protein. The structures of both are important for prevention and management of diabetes. Thanks to pioneering work from the Whistler Center a decade ago, the structural features controlling the slow digestion of starch in foods, which is important for diabetes and obesity, are now known. High amylose content is important, but so are the relative amounts of long amylopectin chains and short amylose chains. A drawback is that foods with the right structural features tend to taste bad. The molecular reasons for this poor palatability will be discussed, as revealed by new methodologies. Methods will be discussed for developing cereals which combine acceptable palatability and slow digestion. The new combination of methods which enabled this discovery will also be discussed in detail, especially how to parameterize the structural features of starch by a small number of biologically meaningful parameters. These can be used in subsequent statistical analysis to reveal structure-property correlations and the underlying mechanisms.

The starch in foods is digested in the body to glucose, which is stored temporarily in glycogen, to be degraded back to glucose when needed. Diabetes is characterized by loss of blood-sugar (glucose) control. We have discovered that there is an important structural difference between glycogens from healthy and diabetic livers: the latter are molecularly fragile, and can readily fall apart to smaller particles which are more quickly degraded to glucose: a clear connection with the loss of blood-sugar control in diabetes. It will be shown that this molecular fragility can be reversed or prevented with diets containing slowly-digested starch, as well as by certain diabetes drugs. The mechanisms behind these discoveries will be discussed, and how they suggest new directions for diabetes drugs.

May 2019
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