Increased receptor binding of low-density lipoprotein from individuals consuming a high-carbohydrate, low-saturated-fat diet

Effect of carbohydrate-restricted dietary interventions on LDL particle size and number in adults in the context of weight loss or weight maintenance: a systematic review and meta-analysis

BACKGROUND

LDL particle size and number (LDL-P) are emerging lipid risk factors. Nonsystematic reviews have suggested that diets lower in carbohydrates and higher in fats may result in increased LDL particle size when compared with higher-carbohydrate diets.

OBJECTIVES

This study aimed to systematically review available evidence and conduct meta-analyses of studies addressing the association of carbohydrate restriction with LDL particle size and LDL-P.

METHODS

We searched 6 electronic databases on 4 January, 2021 for randomized trials of any length that reported on dietary carbohydrate restriction (intervention) compared with higher carbohydrate intake (control). We calculated standardized mean differences (SMDs) in LDL particle size and LDL-P between the intervention and control groups of eligible studies, and pooled effect sizes using random-effects models. We performed prespecified subgroup analyses and examined the effect of potential explanatory factors. Internal validity and publication bias were assessed using Cochrane's risk-of-bias tool and funnel plots, respectively. Studies that could not be meta-analyzed were summarized qualitatively.

RESULTS

This review summarizes findings from 38 randomized trials including a total of 1785 participants. Carbohydrate-restricted dietary interventions were associated with an increase in LDL peak particle size (SMD = 0.50; 95% CI: 0.15, 0.86; P < 0.01) and a reduction in LDL-P (SMD = -0.24; 95% CI: -0.43, -0.06; P = 0.02). The effect of carbohydrate-restricted dietary interventions on LDL peak particle size appeared to be partially explained by differences in weight loss between intervention groups and exploratory analysis revealed a shift from small dense to larger LDL subclasses. No statistically significant association was found between carbohydrate-restricted dietary interventions and mean LDL particle size (SMD = 0.20; 95% CI: -0.29, 0.69; P = 0.37).

CONCLUSIONS

The available evidence indicates that dietary interventions restricted in carbohydrates increase LDL peak particle size and decrease the numbers of total and small LDL particles.This review was registered at www.crd.york.ac.uk/prospero/ as CRD42020188745.

HMG-CoA reductase, cholesterol 7alpha-hydroxylase, LDL receptor, SR-B1, and ACAT in diet-induced syndrome X

BACKGROUND

Long-term consumption of Western diets can lead to acquired syndrome X, which presents with obesity, insulin resistance, hypertension, hyperlipidemia, and risk of atherosclerotic cardiovascular disease. While plasma lipid abnormalities in syndrome X have been well characterized, their molecular basis remains unclear. This study explored potential mechanisms of hypercholesterolemia in diet-induced syndrome X.

METHODS

Female Fischer rats were fed a high-fat, refined-carbohydrate (sucrose) diet (HFS) or standard rat chow (low-fat, complex carbohydrate, LFCC) for 20 months. Plasma lipids and hepatic tissue mRNA, protein, and/or activities of the key enzymes and receptors involved in cholesterol metabolism were determined.

RESULTS

The HFS group exhibited hypertension, hyperlipidemia, insulin resistance, obesity, significant down-regulation of hepatic cholesterol 7alpha-hydroxylase (the rate-limiting step in cholesterol catabolism) and low-density lipoprotein (LDL) receptor (LDL-R, the primary pathway of LDL clearance). In contrast, hepatic tissue acyl-coenzyme A:cholesterol acyltransferase (ACAT-2, the primary enzyme involved in intracellular esterification of cholesterol) and scavenger-receptor class B, type 1 (SR-B1 or HDL receptor) were up-regulated. While 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA expression was increased, its protein abundance and activity were unchanged, and HMG-CoA reductase-to-cholesterol 7alpha-hydroxylase ratio was increased in HFS-fed animals.

CONCLUSION

Hypercholesterolemia in diet-induced syndrome X is associated with depressed cholesterol 7alpha-hydroxylase, diminished LDL-R, elevated ACAT, and increased HMG-CoA reductase-to-cholesterol 7alpha-hydroxylase ratio. These findings point to impaired hepatic catabolism and uptake of cholesterol and inappropriate cholesterol production capacity as the underlying causes of hypercholesterolemia in rats with diet-induced syndrome X.

Effect of diet on adipose tissue and skeletal muscle VLDL receptor and LPL: implications for obesity and hyperlipidemia

This study was designed to examine the effect of a high-fat (primarily saturated), refined-carbohydrate (sucrose) diet (HFS), which is known to induce obesity and hyperlipidemia, on adipose tissue and skeletal muscle lipoprotein lipase (LPL) and very-low density lipoprotein receptor (VLDL-R) protein expressions. Female Fischer rats were placed on either a HFS or a low-fat, complex-carbohydrate (LFCC) diet for 22 months beginning at 2 months of age. After 20 months, a subgroup of the HFS rats were switched to the LFCC diet for 2 months (HFS/LFCC). Body weight, feed efficiency, plasma total cholesterol, VLDL-C, low-density lipoprotein cholesterol (LDL-C) and triglyceride (TG) concentrations and LDL-C to high-density lipoprotein cholesterol ratio were all significantly raised by the HFS diet and improved by conversion to the LFCC diet. Adipose tissue heparin-releasable, extractable and total LPL activity expressed per cell were significantly increased in the HFS-fed group. However, LPL protein abundance normalized against total cellular protein was unchanged in the HFS group. This observation is consistent with the presence of adipose tissue hypertrophy. Skeletal muscle LPL protein abundance and heparin-releasable activity were reduced by the HFS diet and improved after switching to the LFCC diet. Both adipose tissue and skeletal muscle VLDL-R protein levels were significantly reduced by the HFS diet and increased after conversion to the LFCC diet. We conclude that an HFS diet induces changes in LPL and VLDL-R in a manner which favors shunting of dietary fat from skeletal muscle to adipose tissue and decreases TG-rich lipoprotein clearance contributing to increased plasma lipids and obesity. Conversion to a LFCC diet can ameliorate the dyslipidemia and tissue changes induced by long-term HFS diet consumption.

Reversibility of Chronic Experimental Syndrome X by Diet Modification

Abstract —This study was designed to examine whether abnormalities that comprise the metabolic syndrome, including insulin resistance, hyperinsulinemia, hypertension, hyperlipidemia, and obesity, are reversible by diet. Female Fischer rats were placed on either a high-fat, refined-carbohydrate (HFS) diet or low-fat, complex-carbohydrate (LFCC) diet for a period of 20 months. After 20 months, a group of HFS rats were switched to the LFCC diet (HFS/LFCC) for a period of 2 months. Skeletal muscle glucose transport, plasma insulin, systolic blood pressure, and plasma lipids were measured in all groups after 22 months. Energy intake and body weight were measured weekly. In the HFS group, insulin-stimulated glucose transport was significantly reduced (67±4 versus 98±4 pmol · mg − 1 · 15 s − 1 ), whereas plasma insulin (300±49 versus 82±8 pmol/L), blood pressure (147±4 versus 123±4 mm Hg), plasma triglycerides (2.58±0.31 versus 0.39±0.04 mmol/L), LDL cholesterol (C) (3.45±0.40 versus 0.89±0.06 mmol/L), LDL-C to HDL-C ratio (2.9±0.1 versus 2.2±0.1), VLDL-C (1.53±0.23 versus 0.37±0.07 mmol/l), Total-C (5.56±0.58 versus 1.49±0.10 mmol/L), and body weight (360±11 versus 260±5 g) were all significantly elevated compared with the LFCC. Energy intake did not differ significantly; however, the LFCC had a much poorer feed efficiency. Conversion to a LFCC diet for 2 months led to normalization of glucose transport, blood pressure, plasma insulin, and VLDL-C and significant amelioration of obesity and other lipid abnormalities. These results demonstrate that syndrome X induced by an inappropriate diet is reversed with implementation of a low-fat, unrefined-carbohydrate diet without caloric restriction and suggest that diet may be a possible treatment for multiple simultaneous cardiovascular risk factors.

A very low-fat diet is not associated with improved lipoprotein profiles in men with a predominance of large, low-density lipoproteins

BACKGROUND

We found previously that men with a predominance of large LDL particles (phenotype A) consuming high-fat diets (40-46% fat) show less lipoprotein benefits of low-fat diets (20-24% fat) than do men with a high-risk lipoprotein profile characterized by a predominance of small LDL (phenotype B). Furthermore, one-third of men with phenotype A consuming a high-fat diet converted to phenotype B with a low-fat diet.

OBJECTIVE

We investigated effects of further reduction in dietary fat in men with persistence of LDL subclass phenotype A during both high- and low-fat diets.

DESIGN

Thirty-eight men who had shown phenotype A after 4-6 wk of both high- and low-fat diets consumed for 10 d a 10%-fat diet (2.7% saturates) with replacement of fat with carbohydrate and no change in cholesterol content or ratio of polyunsaturates to saturates.

RESULTS

In 26 men, phenotype A persisted (stable A group) whereas 12 converted to phenotype B (change group). LDL cholesterol did not differ from previous values for 20-24%-fat diets in either group, whereas in the change group there were higher concentrations of triacylglycerol and apolipoprotein B; greater mass of HDL, large LDL-I, small LDL-III and LDL-IV, and HDL3; lower concentrations of HDL cholesterol, apolipoprotein A-I; and lower mass of large LDL-I and HDL2.

CONCLUSIONS

There is no apparent lipoprotein benefit of reduction in dietary fat from 20-24% to 10% in men with large LDL particles: LDL-cholesterol concentration was not reduced, and in a subset of subjects there was a shift to small LDL along with increased triacylglycerol and reduced HDL-cholesterol concentrations.

High fat, high cholesterol diets alter low density lipoprotein size and binding affinity in monkeys

The purpose of this study was to examine the effects of various dietary fats on low density lipoprotein (LDL) binding in an in vitro system where receptor number is not regulated. Cynomolgus monkeys were fed diets containing 37% of energy from fat, with various degrees of saturation, and 0.4 mg/kcal cholesterol or low-fat (13% of energy), low cholesterol (0.03 mg/kcal) chow. Plasma LDL was isolated after 16 weeks. The fatty acid composition of LDL showed enrichment corresponding to the dietary fats consumed, and the high fat, high cholesterol diets produced marked hypercholesterolemia compared to chow feeding. Of those fed the high fat diets, monkeys fed the fish oil diet had the highest LDL cholesterol concentrations, 13.25 +/- 0.77 mmol/l, while those fed the safflower oil diet had the lowest, 7.51 +/- 3.31. LDL from chow fed monkeys had the lowest binding affinity; the Kd was 26.2 +/- 8.7 microg/ml, nearly twice that of the high fat diets (P = 0.003). No significant differences in binding were found between the different high fat diets, although there was a trend toward lower affinity in the diets enriched in polyunsaturated fat. LDL size was affected by diet with chow fed monkeys having the smallest average LDL, 259.3 +/- 1.7 A compared to the other groups (P = 0.03). Monkeys fed the fish oil diet tended to have smaller LDL, but this was not significantly different from the other high fat diets. Binding affinity was correlated with LDL size, r = 0.54, P < 0.01. LDL composition, as measured by apo B/cholesterol ratio, was altered by feeding a high fat, high cholesterol diet. The ratio was reduced in the LDL samples from monkeys fed the high fat diets compared to those fed chow, but this ratio was not significantly correlated with binding. Thus, it appears that increasing dietary fat and cholesterol intake increases LDL size and binding affinity, such that LDL metabolism may be altered independently from effects on receptor number; the type of dietary fat does not seem to influence this process when fat and cholesterol content is very high.