Lipid responses to dietary carbohydrates

Lipogenesis in a wing-polymorphic cricket: Canalization versus morph-specific plasticity as a function of nutritional heterogeneity

The influence of variable nutritional input on life history adaptation is a central, but incompletely understood aspect of life history physiology. The wing-polymorphic cricket, Gryllus firmus, has been extensively studied with respect to the biochemical basis of life history adaptation, in particular, modification of lipid metabolism that underlies the enhanced accumulation of lipid flight fuel in the dispersing morph [LW(f)=long wings with functional flight muscles] relative to the flightless (SW=short-winged) morph. To date, biochemical studies have been undertaken almost exclusively using a single laboratory diet. Thus, the extent to which nutritional heterogeneity, likely experienced in the field, influences this key morph adaptation is unknown. We used the experimental approach of the Geometric Framework for Nutrition and employed 13 diets that differed in the amounts and ratios of protein and carbohydrate to assess how nutrient amount and balance affects morph-specific lipid biosynthesis. Greater lipid biosynthesis and allocation to the soma in the LW(f) compared with the SW morph (1) occurred across the entire protein-carbohydrate landscape and (2) is likely an important contributor to elevated somatic lipid in the LW(f) morph across the entire protein-carbohydrate landscape. Nevertheless, dietary carbohydrate strongly affected lipid biosynthesis in a morph-specific manner (to a greater degree in the LW(f) morph). Lipogenesis in the SW morph may be constrained due to its more limited lipid storage capacity compared to the LW(f) morph. Elevated activity of NADP⁺-isocitrate dehydrogenase (NADP⁺-IDH), an enzyme that produces reducing equivalents for lipid biosynthesis, was correlated with and may be an important cause of the increased lipogenesis in the LW(f) morph across most, but not all regions of the protein-carbohydrate landscape. By contrast, ATP-citrate lyase (ACL), an enzyme that catalyzes the first step in the pathway of fatty acid biosynthesis, showed complex morph-specific patterns of activity that were strongly contingent upon diet. Morph-specific patterns of NADP⁺-IDH and ACL activities across the nutrient landscape were much more complex than expected from previous studies on a single diet. Collectively, our results indicate that the biochemical basis of an important life history adaptation, morph-specific lipogenesis, can be canalized in the face of substantial nutritional heterogeneity. However, in some regions of the protein-carbohydrate landscape, it is strongly modulated in a morph-specific manner.

Role of protein synthesis in the carbohydrate-induced changes in the activities of acetyl-CoA carboxylase and hydroxymethylglutaryl-CoA reductase in cultured rat hepatocytes

Changes in the activities of acetyl-CoA carboxylase and HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase were studied in primary cultures of adult-rat hepatocytes after exposure of the cells to insulin and/or carbohydrates. To determine the contribution of protein synthesis to changes in enzyme activity, the relative rate of synthesis of each enzyme was measured and the amount of translatable mRNA coding for the enzymes was determined by translation in vitro and immunoprecipitation. Addition of insulin to the culture medium increased the activities of acetyl-CoA carboxylase and HMG-CoA reductase by approx. 4- and 3-fold respectively. Although similar increases in the relative rate of synthesis of each protein and template activity were noted, initial increases in the activity of each enzyme occurred before any changes in protein synthesis were observed, suggesting the involvement of post-translational modification of enzyme activity in addition to changes in protein synthesis. The addition of fructose to the culture medium, in the absence of insulin, increased the activity of the carboxylase and the reductase approx. 3-fold, similar to the effects of insulin. However, the effect of fructose was to increase the rate of synthesis and the amount of translatable mRNA coding for acetyl-CoA carboxylase, whereas the increase in the activity of HMG-CoA reductase was not accompanied by any changes in the rate of synthesis or template activity. The effects of fructose could not be mimicked by glucose unless insulin was also present in the culture medium. Similar to observations in vitro, the injection of insulin or the feeding of a high-fructose diet to rats made diabetic by the injection of streptozotocin produced an increase in the activities of acetyl-CoA carboxylase and HMG-CoA reductase, and only the increase in the activity of the carboxylase was accompanied by an increase in the amount of translatable mRNA coding for the enzyme. The results are discussed in terms of the effects of fructose on the synthesis of enzymes involved in lipogenesis.

Free sugars in foods

The values for the free sugars analysed in the samples collected for the fourth edition of McCance and Widdowson's 'The composition of foods' have been combined with values from the literature to provide some basic information on the distribution of free sugars in a range of foods.

A biochemical and morphologic study of very low density lipoproteins in carbohydrate-induced hypertriglyceridemia

On a high carbohydrate, fat-free diet, control and hypertriglyceridemic subject had a three-fold increase in d < 1.006, very low density lipoprotein (VLDL) triglyceride, and somewhat lesser increases in VLDL cholesterol and protein. Cholesterol and protein in 1.006 < d < 1.21 lipoprotein decreased in a reciprocal fashion, suggesting that these components might have been utilized in VLDL production. Electron microscope studies demonstrated a significant increase in the size of lipoprotein particles of the VLDL class and, in three of four subjects, an apparent increase in particle number. The change in particle size correlated with an increase in the triglyceride/protein ratio of the d < 1.006 lipoprotein. Hypertriglyceridemic individuals differed from the control subjects in that they had greater absolute increases in VLDL triglyceride, cholesterol, and protein, and greater decreases in 1.006 < d < 1.21 cholesterol and protein. In addition, they had larger VLDL particles with a higher triglyceride/protein ratio, both before the study and at the peak of the carbohydrate effect. The data suggest that the increase in plasma triglycerides induced by a high carbohydrate diet is usually due to the appearance in plasma of both greater numbers of VLDL particles and larger particles that are relatively richer in triglyceride content than those isolated during the basal state.