Comparative methodologies for measuring metabolizable energy of various types of resistant high amylose corn starch.

Energy values of high amylose corn starches high in resistant starch (RS) were determined in vivo by two different methodologies. In one study, energy values were determined according to growth relative to glucose-based diets in rats fed diets containing RS(2), heat-treated RS(2) (RS(2)-HT), RS(3), and amylase predigested versions to isolate the RS component. Net metabolizable energy values ranged from 2.68 to 3.06 kcal/g for the RS starches, and 1.91-2.53 kcal/g for the amylase predigested versions. In a second study, rats were fed a diet containing RS(2)-HT and the metabolizable energy value was determined by bomb calorimetry. The metabolizable energy value was 2.80 kcal/g, consistent with Study 1. Thus, high amylose corn based RS ingredients and their amylase predigested equivalents have energy values approximately 65-78% and 47-62% of available starch (Atwater factor), respectively, according to the RS type (Garcia, T. A.; McCutcheon, K. L.; Francis, A. R.; Keenan, M. J.; O'Neil, C. E.; Martin, R. J.; Hegsted, M. The effects of resistant starch on gastrointestinal organs and fecal output in rats. FASEB J. 2003, 17, A335).

Obesity - a perspective based on the biochemical interrelationship of lipids and carbohydrates.

Many factors affect the onset of obesity including satiety control, reduced levels of physical exercise as well as hormonal and genetic parameters which influence the metabolic pathways leading to the net accumulation of triacylglycerol (TAG). The predominant fatty acid of human adipose tissue TAGs is oleic acid, reflecting primarily the composition of the diet but also the product of de novo lipogenesis. Consequently, both carbohydrates and lipids are potential sources of these stored fats. Many studies have been carried out using a variety of differing experimental protocols on healthy, obese or diabetic humans and animals in positive or neutral energy balance to establish the underlying molecular basis for obesity particularly in humans. This short review discusses the interdependence and control of the metabolism of lipids and carbohydrates as it relates to lipogenesis and proposes a unified hypothesis for obesity which brings together a number of different approaches focusing on (i) the interaction of dietary fat and carbohydrate, which typically represent approximately 80% of the daily caloric intake, and their role in the synthesis of TAGs, (ii) the biochemical pathways which control the amount of TAG produced by controlling the composition of their fatty acids via the action of stearoyl-CoA desaturase (SCD), (iii) the control of lipogenesis and SCD by dietary polyunsaturated fatty acid (PUFA) and (iv) the interaction of PUFAs with the transcription factors, peroxisome proliferator activated receptors (PPAR) alpha and gamma, which maintain the balance between oxidation and storage of lipids. The hypothesis focuses on the central role of stearoyl-CoA desaturase (SCD) and its inhibition by polyunsaturated fatty acids (PUFAs) acting via transcription factors based upon data obtained from both animal and human studies. From these observations it should be possible to determine the relevance of the hypothesis to humans and to speculate how these aspects of metabolism may impact the risk of developing related diseases such as coronary heart disease, Type 2 diabetes and certain forms of cancer.

Postprandial carbohydrate metabolism in healthy subjects and those with type 2 diabetes fed starches with slow and rapid hydrolysis rates determined in vitro.

The objective of the present study was to investigate the effects of starches with differing rates of hydrolysis on exposure to pancreatin in vitro on postprandial carbohydrate metabolism in healthy subjects and in subjects with type 2 diabetes. Two test starches, prepared from uncooked native granular starch products, and naturally enriched with 13C, were consumed in a randomized crossover design by eight healthy and thirteen type 2 diabetic subjects. One starch was characterized in vitro as being rapidly hydrolysed (R, 94% after 180 min), and the other was more slowly hydrolysed (S, 51% after 180 min). Each subject consumed 50 g of each test starch. In addition, the type 2 diabetic subjects consumed 89.7 g of the S starch on a separate occasion. Blood samples were taken at 10 min intervals for 3 h, and at 20 min intervals for a further 3 h during a 6 h postprandial period. Breath 13CO2 enrichment was measured at the same time points, and indirect calorimetry was performed for seven 20 min sessions immediately before and during the 6 h postprandial period. With the R starch, plasma glucose concentrations and serum insulin concentrations rose faster and the maximum glucose change was approximately 1.8 times that for the S starch, averaged across both subject groups. The areas under the curves for glucose and insulin were, respectively, 1.7 and 1.8 times higher for the R starch compared with the S starch, averaged across both subject groups. The rate of 13CO2 output and the proportion of 13C recovered in breath after consumption of the R starch was similar for both subject groups. The results provide evidence that starches which have different rates of hydrolysis in vitro result in different patterns of glycaemia and insulinaemia in both healthy adults and in diet-controlled type 2 diabetic subjects. Data from the hydrolysis of novel starch products in vitro, therefore, are useful in predicting glycaemic responses in vivo.

Production of a freeze-thaw-stable potato starch by antisense inhibition of three starch synthase genes.

The use of unmodified starches in frozen foods is severely limited by the undesirable textural changes that occur after freezing and thawing. Retrogradation of glucan chains leads to syneresis, a separation of the starch gel and water phases. Stabilization of the starch structure is normally achieved by chemical modification to prevent these changes from occurring. We have now created a freeze-thaw-stable potato starch by alteration of starch composition and structure by genetic modification. An amylose-free starch with short-chain amylopectin was produced by simultaneous antisense downregulation of three starch synthase genes. This starch is extremely freeze-thaw-stable and shows no syneresis even after five freeze-thaw cycles. The use of this starch has potential for environmental and consumer benefits because its production requires no chemical modification.