STARCH

Definition Starches are alpha-glucan polysaccharides. They comprise two major types of polymer: amylose, which is linear with a molecular size ranging between 110 and 60 K daltons and is virtually completely alpha 1-4 linked, and amylopectin, which contains alpha 1-6 and alpha 1-4 linkages and is branched.


Starches in the diet Starches are present in the diet in a number of physical states which confer physiological properties whose nutritional signifi- cance is still a matter for current research. In uncooked foods starches are present in granules with a highly ordered crystalline structure characteristic of the plant source. Heat in the presence of water causes the granules to swell and rupture so that the linear amylose molecules leach out and form a sol which retrogrades on cooling. The crystalline aggregated and retrograded structures formed partially resist enzymatic hydrolysis ("resistant starch"). A range of chemically modified starches are used as food ingredients or additives but their nutritional significance is small.


Intake and sources There are few data on intakes of different starches by UK populations because dietary surveys have almost always reported total carbohydrate consumption with no distinction between sugars and starches. The information available suggests that the amounts consumed in Britain provide about half the total carbohydrate. In the recent Dietary and Nutritional Survey of British Adults, starches provided 24 per cent of energy (1). No direct information is available concerning the intake of starches in children.


Physiology When foods containing the same amount of starch from different sources are fed to healthy subjects, the subsequent rises in blood glucose and insulin vary depending on the type of starch and its processing. By means of measurements of peripheral plasma glucose concentrations following standardised test meals, Jenkins and others have developed the concept of a glycaemic index" for foodS (2). Although the glycaemic index of a food is reproducible in groups of people, large variations are seen among individuals and it is difficult to apply the index to meals containing fat and protein, or when one meal closely follows another. Furthermore, whilst hyperinsulinaemia and insulin resistance are found in certain conditions (obesity, non-insulin dependent diabetes mellitus), evidence that they are primary pathogenetic rather than associated factors in diseases (eg hypertension, atherosclerosis) is conflicting. Moreover the changes seen are within normal physiological variance. The Panel considered that the ability to deal with short-term fluctuations of carbohydrate intake was a feature of normal function. The absence of this ability was a feature of disease, such as diabetes. For these reasons the Panel was unable to ascribe at this stage any definite practical health effect to the physiological differences in glucose absorption in response to starches from different foods.


Nutritional role Starches are a major component of most diets throughout the world and may provide up to 80 per cent of total energy. Some very high starch diets may be associated with shortages of some vitamins and minerals in circumstances where other sources of food energy are scarce. However this is not a direct, negative effect of starch, but a result of the consumption of diets low in foods providing essential protein and vitamins. The Panel was unable to find any evidence of harm resulting from high starch intakes in normal individuals, so long as energy balance and essential nutrient intake were maintained. Starchy foods contain many other nutrients although the variety and amount vary from food to food. Carbohydrate is a necessary part of the diet, to avoid ketoacidosis (see Metabolism of Glucose) and because there are substantial reasons why other sources of food energy, (fat, protein and alcohol) should not provide more than a certain proportion of total food energy (see Section 1). Although certain populations (eg Eskimo, Masai) survive on very low intakes of carbohydrate, the Panel agreed that carbohydrate should provide the major food energy requirement for UK populations. The Panel considered that starches together with intrinsic and milk sugars should provide the main source of carbohydrate food energy. The Panel did not find sufficient evidence to propose what proportion of this should be derived from starches.


Dietary Reference Values The Panel therefore proposed that starches and intrinsic and milk sugars should provide the balance of dietary energy not provided by alcohol, protein, fat and non-milk extrinsic sugars, that is on average 37 per cent of total dietary energy for the population. These values are based on data from adults. The Panel nevertheless recommended that the same principles be applied to children over 2 years old. The Panel endorsed the recommendation of the COMA Panel on Child Nutrition that breastfeeding provides the best infant nutrition6. Breastmilk does not contain starch.


References

1 Gregory J, Foster K, Tyler H, Wiseman M. The Dietary and Nutritional Survey ofbritish A dults. London: HMSO, 1990.

2 Jenkins D J A. Carbohydrates: dietary fiber. In: Shils M, Young V, eds. Modern Nutrition in Health and Disease. 7th Edn. Philadelphia: Lea and Febiger, 1988; 52-7 1.

3 Bingham S A, McNeil N I, Cummings J H. The diet of individuals. Br JNutr 1981; 45: 23-35, and unpublished.

4 Fehily A M, Yarnell J W G, Butland B K. Diet and ischaemic heart disease. Hum Nut: Appl Nut 1987; 41A: 319-326.

5 Hackett A F, Rugg-Gunn A J, Appleton D R, Eastoe J E, Jenkins G N. A 2-year longitudinal nutritional survey of 405 Northumberland children initially aged II. 5 years. Br J Nutr 1984;51: 67-75.

6 Department of Health and Social Security. Present Day Practice in Infant Feeding: Third Report. London: HMSO, 1988. (Report on health and social subjects; 32).