CARBOHYDRATE METABOLISM IN TWO FORMS OF HYPERGLYCERIDEMIA

A causal role for uric acid in fructose-induced metabolic syndrome

The worldwide epidemic of metabolic syndrome correlates with an elevation in serum uric acid as well as a marked increase in total fructose intake (in the form of table sugar and high-fructose corn syrup). Fructose raises uric acid, and the latter inhibits nitric oxide bioavailability. Because insulin requires nitric oxide to stimulate glucose uptake, we hypothesized that fructose-induced hyperuricemia may have a pathogenic role in metabolic syndrome. Four sets of experiments were performed. First, pair-feeding studies showed that fructose, and not dextrose, induced features (hyperinsulinemia, hypertriglyceridemia, and hyperuricemia) of metabolic syndrome. Second, in rats receiving a high-fructose diet, the lowering of uric acid with either allopurinol (a xanthine oxidase inhibitor) or benzbromarone (a uricosuric agent) was able to prevent or reverse features of metabolic syndrome. In particular, the administration of allopurinol prophylactically prevented fructose-induced hyperinsulinemia (272.3 vs.160.8 pmol/l, P < 0.05), systolic hypertension (142 vs. 133 mmHg, P < 0.05), hypertriglyceridemia (233.7 vs. 65.4 mg/dl, P < 0.01), and weight gain (455 vs. 425 g, P < 0.05) at 8 wk. Neither allopurinol nor benzbromarone affected dietary intake of control diet in rats. Finally, uric acid dose dependently inhibited endothelial function as manifested by a reduced vasodilatory response of aortic artery rings to acetylcholine. These data provide the first evidence that uric acid may be a cause of metabolic syndrome, possibly due to its ability to inhibit endothelial function. Fructose may have a major role in the epidemic of metabolic syndrome and obesity due to its ability to raise uric acid.

Diet composition and the metabolic syndrome: what is the optimal fat intake?

Two cholesterol-raising fatty acids in the diet, saturated fatty acids and trans fatty acids, increase the serum low-density lipoprotein cholesterol concentration. This fact justifies the recommendation of a reduced intake of cholesterol-raising fatty acids. Emerging data suggest that diets higher in unsaturated fatty acids, particularly monounsaturated fatty acids, have several advantages over high-carbohydrate intakes. This advantage appears to hold, particularly for populations having a high prevalence of insulin resistance, such as the US population. If the US public were to modify its eating habits in the direction of better weight control and more exercise, higher intakes of carbohydrate might be better tolerated. At the same time, the experience with the Mediterranean population reveals that in healthier populations, diets relatively high in unsaturated fatty acids are well tolerated and are associated with a low prevalence of both coronary heart disease and type 2 diabetes.

Carbohydrate-induced hypertriacylglycerolemia: historical perspective and review of biological mechanisms

Current trends in health promotion emphasize the importance of reducing dietary fat intake. However, as dietary fat is reduced, the dietary carbohydrate content typically rises and the desired reduction in plasma cholesterol concentrations is frequently accompanied by an elevation of plasma triacylglycerol. We review the phenomenon of carbohydrate-induced hypertriacylglycerolemia, the health effects of which are among the most controversial and important issues in public health nutrition today. We first focus on how seminal observations made in the late 1950s and early 1960s became the basis for subsequent important research questions and areas of scientific study. The second focus of this paper is on the current knowledge of biological mechanisms that contribute to carbohydrate-induced hypertriacylglycerolemia. The clinical rationale behind mechanistic studies is this: if carbohydrate-induced hypertriacylglycerolemia shares a metabolic basis with endogenous hypertriacylglycerolemia (that observed in subjects consuming high-fat diets), then a similar atherogenic risk may be more likely than if the underlying metabolic mechanisms differ. The third focus of the paper is on both the positive metabolic changes that occur when high-carbohydrate diets are consumed and the potentially negative health effects of such diets. The review concludes with a summary of some important research questions that remain to be addressed. These issues include the level of dietary carbohydrate that induces carbohydrate-induced hypertriacylglycerolemia, whether the phenomenon is transient or can be avoided, whether de novo lipogenesis contributes to the phenomenon, and what magnitude of triacylglycerol elevation represents an increase in disease risk.

Effects of a low-fat, high-carbohydrate diet on VLDL-triglyceride assembly, production, and clearance

Low-fat, high-carbohydrate (LF/HC) diets commonly elevate plasma triglyceride (TG) concentrations, but the kinetic mechanisms responsible for this effect remain uncertain. Subjects with low TG (normolipidemic [NL]) and those with moderately elevated TG (hypertriglyceridemic [HTG]) were studied on both a control and an LF/HC diet. We measured VLDL particle and TG transport rates, plasma nonesterified fatty acid (NEFA) flux, and sources of fatty acids used for the assembly of VLDL-TG. The LF/HC diet resulted in a 60% elevation in TG, a 37% reduction in VLDL-TG clearance, and an 18% reduction in whole-body fat oxidation, but no significant change in VLDL-apo B or VLDL-TG secretion rates. Significant elevations in fasting apo B-48 concentrations were observed on the LF/HC in HTG subjects. In both groups, fasting de novo lipogenesis was low regardless of diet. The NEFA pool contributed the great majority of fatty acids to VLDL-TG in NL subjects on both diets, whereas in HTG subjects, the contribution of NEFA was somewhat lower overall and was reduced further in individuals on the LF/HC diet. Between 13% and 29% of VLDL-TG fatty acids remained unaccounted for by the sum of de novo lipogenesis and plasma NEFA input in HTG subjects. We conclude that (a) whole-food LF/HC diets reduce VLDL-TG clearance and do not increase VLDL-TG secretion or de novo lipogenesis; (b) sources of fatty acids for assembly of VLDL-TG differ between HTG and NL subjects and are further affected by diet composition; (c) the presence of chylomicron remnants in the fasting state on LF/HC diets may contribute to elevated TG levels by competing for VLDL-TG lipolysis and by providing a source of fatty acids for hepatic VLDL-TG synthesis; and (d) the assembly, production, and clearance of elevated plasma VLDL-TG in response to LF/HC diets therefore differ from those for elevated TG on higher-fat diets.

Hypercholesterolemia with cholesterol-enriched LDL and normal levels of LDL-apolipoprotein B. Effects of the step I diet and bile acid sequestrants on the cholesterol content of LDL

One form of hypercholesterolemia is characterized by high levels of LDL cholesterol and normal levels of LDL-apolipoprotein (apo) B. The reason for hypercholesterolemia, therefore, is enrichment of LDL particles with cholesterol. We have reported previously that about one third of patients with primary moderate hypercholesterolemia have this lipoprotein pattern and have no apparent abnormality in LDL-apo B metabolism. The current study was designed to determine whether the combination of the Step I Diet (30% of total calories as fat, <10% saturated fatty acids, and <300 mg per day cholesterol) with or without cholestyramine therapy will correct the hypercholesterolemia in patients of this type. Ten hypercholesterolemic men of this type were identified and recruited into the study. Their LDL cholesterol levels were > or = 160 mg/dL and LDL-apo B levels were <120 mg/dL (LDL cholesterol/apo B ratio >1.60). For patient selection, subjects were challenged with a high fat diet (40% of total calories as fat, 18% saturated fatty acids, and 400 mg per day cholesterol) for 6 weeks to confirm persistence of a high LDL cholesterol/apo B ratio. Thereafter, they were started on a Step I Diet, and lipoprotein analyses were repeated. Finally, cholestyramine (16 g per day) was added to the Step I Diet. The Step I Diet alone significantly reduced the LDL cholesterol/apo B ratios and produced a trend toward lowering LDL cholesterol levels. Cholestyramine therapy further reduced LDL cholesterol levels and maintained a normal LDL cholesterol/apo B ratio. The present investigation thus confirms the existence of a form of moderate hypercholesterolemia that arises from a defect in LDL composition. In addition, it demonstrates that the combination of Step I Diet and cholestyramine therapy corrects this defect and normalizes LDL levels and LDL composition.

Relationship of diet, abdominal obesity, and physical activity to plasma lipoprotein levels in Asian Indian physicians residing in the United States

OBJECTIVE

To examine the relationship of total fat, saturated fat, carbohydrate and energy intake, leisure-time activity, waist-to-hip ratio (WHR), and body mass index to the prevalence of atherogenic plasma triglyceride (TG) and high-density lipoprotein cholesterol (HDL-C) levels in Asian Indians living in the United States. High plasma TG and low HDL-C levels have been documented in Asian Indians worldwide.

DESIGN

This exploratory study examined medical history, demographics, anthropometrics, plasma lipids levels, food intake frequency, and self-reported leisure-time activity.

SUBJECTS

153 Asian Indian male physicians (mean age=47 years) were recruited at two annual national conferences of the American Association of Physicians from India, with free medical assessments as incentives.

STATISTICAL ANALYSES PERFORMED

Correlational and multiple regression analyses were performed.

RESULTS

Subjects' diets averaged 56% energy from carbohydrate, 32% from total fat, and 8% from saturated fat. High total fat intake was associated with high WHR (r=.21, P<.01) and high total carbohydrate intake (grams per day) was associated with high TG level (r=.24, P<.05). Leisure-time activity averaged 136 minutes/week and negatively correlated with total plasma cholesterol level (r=.22, P<.01) and low-density lipoprotein cholesterol level (r=.25, P<.001).

APPLICATIONS

Dietitians should recommend moderate dietary carbohydrate intake and meal patterns with energy distributed throughout the day, in addition to decreasing fat consumption, to Asian Indian clients at risk for coronary artery disease. High carbohydrate intake and uneven diurnal distribution (no breakfast and large evening meals) are associated with high TG and low HDL-C levels in this population.

Atherogenic dyslipidemia: lipoprotein abnormalities and implications for therapy

Atherogenic dyslipidemia is a lipoprotein profile combining 4 specific abnormalities: borderline-high total cholesterol levels; high triglyceride concentrations; small, dense, low-density lipoprotein (LDL) particles; and low high-density lipoprotein (HDL) concentrations. It is a predisposing factor to premature coronary artery disease (CAD), although separating and calculating the contribution of each abnormality to the risk of CAD is difficult, especially since the abnormalities often appear in this combination. The ratio of total cholesterol to HDL cholesterol is currently the most powerful single predictor of risk in dyslipidemic patients. Therapy for atherogenic dyslipidemia includes dietary changes aimed at decreasing intake of cholesterol-raising fatty acids and achieving weight reduction; exercise, which confers many of the benefits of weight reduction; and, when those measures fail to correct the lipid and lipoprotein profile, drug therapy. Nicotinic acid reduces triglyceride and cholesterol levels while raising HDL concentrations, but up to half of patients cannot tolerate its adverse effects. Fibric acids effectively lower triglyceride levels and are generally well tolerated but have little beneficial effect on the cholesterol profile. Statins offer marked reductions in total, LDL, and very low-density lipoprotein cholesterol levels and cause modest increases in HDL concentration. Combination therapy can enhance the efficacy of the individual drugs.

Comparison of monounsaturated fatty acids and carbohydrates for lowering plasma cholesterol

To examine the effects of dietary fatty acids and carbohydrate on plasma lipids and lipoproteins, 11 patients with a mean plasma total cholesterol level of 251 +/- 10 mg per deciliter were studied on a metabolic ward during three dietary periods, each lasting four weeks. A liquid diet rich in monounsaturated fatty acids ("High-Mono") and a diet low in fat ("Low-Fat") were compared with a diet high in saturated fatty acids ("High-Sat"). The High-Sat and High-Mono diets contained 40 percent of their total calories as fat and 43 percent as carbohydrate; the Low-Fat diet had 20 percent fat and 63 percent carbohydrate. Body weight was kept constant by adjusting total caloric intake. As compared with the High-Sat diet, both the High-Mono and Low-Fat diets lowered plasma total cholesterol (by 13 percent and 8 percent, respectively) and low-density lipoprotein cholesterol (by 21 percent and 15 percent, respectively). As compared with the High-Sat diet, the Low-Fat diet raised triglyceride levels and significantly reduced plasma high-density lipoprotein cholesterol. In contrast, the High-Mono diet had no effect on levels of triglycerides or high-density lipoprotein cholesterol. The ratio of low-density to high-density lipoprotein cholesterol was also significantly lower when the High-Mono diet rather than the Low-Fat diet was followed. Therefore, in short-term studies in which liquid diets are used and body weight is kept constant, a diet rich in monounsaturated fatty acids appears to be at least as effective in lowering plasma cholesterol as a diet low in fat and high in carbohydrate.

Primary lipoprotein lipase deficiency

The enzyme lipoprotein lipase plays a central role in the processing of energy in the form of calorically dense triglyceride. Classical LPL deficiency usually presents in childhood with the multiple manifestations related to chylomicronemia. Many patients with genetic variations have been noted who differ in one of many ways from the classical patients. With the development of techniques to measure enzyme mass and to study gene expression, the molecular defects in each of these families should become evident.

Comparison of clofibrate and caloric restriction on kinetics of very low density lipoprotein triglycerides

This study of 12 patients with mild-to-moderate hypertriglyceridemia compares the mechanisms of triglyceride (TG) lowering by caloric restriction and by clofibrate. Turnover rates of very low density lipoprotein triglycerides (VLDL-TG) were determined by using 3H-glycerol as a precursor. Radioactivity-time curves of VLDL-TG were analyzed with a multicompartmental model. Hypertriglyceridemia in these patients was due mainly to overproduction of VLDL-TG. Clofibrate therapy for 1 month had a variable effect on VLDL-TG levels. A group of relatively poor responders to the drug had a mild increase in the fractional catabolic rate (FCR) of VLDL-TG, but no change in production rates. The remaining patients were relatively good responders; they had increased FCR and modest reductions in synthetic rates of VLDL-TG. However, clofibrate largely failed to correct the primary defect in this group of patients, namely, the overproduction of VLDL-TG. Almost all patients responded to 1 month of caloric restriction (1000 cal/day) with marked reductions in VLDL-TG levels. The major response to reduced caloric intake was a decrease in production of VLDL-TG, although FCR was also increased to some extent. Despite these differences in mechanisms for VLDL-TG lowering, both regimens tended to raise levels of LDL and HDL.

Clofibrate, caloric restriction, supersaturation of bile, and cholesterol crystals

Lipid composition, cholesterol saturation, and cholesterol crystal formation of gallbladder bile were studied in seven type-IV hyperlipoproteinemic subjects who did not have gallstones. Thereafter, biliary cholesterol solubilization was overloaded, first by clofibrate and then by caloric restriction treatment. Initially increased cholesterol saturation was still increased by both clofibrate and caloric restriction treatment, but none of the subjects developed cholesterol crystals in bile, indicating that they had a mechanism to maintain cholesterol in solution in the bile despite remarkable supersaturation. This suggests that the patients who are at risk of developing gallstones can be better selected by cholesterol crystal analysis of bile samples than by analysis of lipid composition of bile.

Effects of interruption of the enterohepatic circulation of bile acids on the transport of very low density-lipoprotein triglycerides

An increase in plasma very low density lipoprotein-triglycerides (VLDL-TG) is seen frequently during treatment with bile acid-binding resins. The purpose of this study was to determine whether this increment in VLDL-TG is due mainly to an increase in synthesis of VLDL, or to an enhanced catabolism. Three types of patients were studied: (1) 7 normotriglyceridemic subjects. (2) 4 obese patients, and (3) 9 hypertriglyceridemic patients. Before treatment they underwent a study of VLDL-TG kinetics that employed multicompartmental analysis of specific activity curves following injection of 3H-glycerol. The patients were then treated with a bile acid-binding resin, either cholestyramine or colestipol, for several weeks to several months. At the end of the treatment period, they were readmitted to the hospital for a second study of VLDL-TG kinetics. The patients showed a variable response to resin therapy. Many had an increase in concentrations of VLDL-TG, but others had no change or even a slight decrease. However, analysis of the data showed a high correlation between change in production rates of VLDL-TG and change in concentration. Also, when the data for the 20 patients were combined, there was a statistically significant increase in both synthetic rates and concentrations of VLDL-TG; in contrast, the fractional catabolic rate (FCR) was unchanged by therapy. Therefore, our data show that when treatment with bile acid sequestrants causes an increase in plasma VLDL-TG, the increase is due to an increment in production and not to a decrease in catabolism.

Triglyceride and cholesterol metabolism in primary hypertriglyceridemia

To determine mechanisms of elevated plasma triglycerides (TG) in patients with primary hypertriglyceridemias, simultaneous studies were carried out on kinetics of very low density lipoprotein-triglycerides (VLDL-TG) and synthesis of cholesterol and bile acids. Sixteen hypertriglyceridemic patients with familial combined hyperlipidemia (FCHL) and 12 patients with poorly classified, primary hypertriglyceridemia were studied, and their results were compared to a series of normal and obese subjects previously studied in our laboratory. The mean value for transport (synthesis) of VLDL-TG in patients with FCHL was about twice normal. Although the upper normal synthesis rates overlapped with transport rates of some patients with FCHL, it appeared that the major cause of hypertriglyceridemia in FCHL was an elevated production of VLDL-TG. However, the height of the plasma TG in FCHL patients also was influenced by individual clearance capacities for VLDL-TG, and fractional clearance rates in several seemed particularly low. Synthesis rates for cholesterol and/or bile acids were high in several patients with FCHL, suggesting simultaneous overproduction of VLDL-TG and sterols; however, increased synthesis of both was not observed in all the patients. Most patients with poorly classified hypertriglyceridemia had over-production of VLDL-TG, but an apparent reduction in clearance was common. In these patients, increased synthesis of cholesterol and bile acids was infrequent. Our results indicate that abnormally high production of VLDL-TG seemed to be the major factor in causing primary hypertriglyceridemia, but that clearance capacity can play an important role in determining the the severity of the TG elevation.

Transport of very low density lipoprotein triglycerides in varying degrees of obesity and hypertriglyceridemia

Measurements of transport of triglycerides (TG) in very low density lipoproteins (VLDL) were carried out in 59 patients by injection of radioactive glycerol, determinations of specific activities of VLDL-TG for 48 h thereafter, and treatment of the data by multicompartmental analysis. The patients were divided into three groups: normal weight (89-120% ideal weight), mildly obese (120-135% ideal weight), and markedly obese (135% ideal weight). They had varying levels of VLDL-TG ranging from normal to markedly elevated. In many subjects, there was a positive correlation between concentrations and transport of VLDL indicating that overproduction of VLDL-TG contributed to hypertriglyceridemia. In others, and particularly in several markedly obese subjects, transport rates were greatly increased without significant hypertriglyceridemia, suggesting that they had enhanced capacity to clear TG. In all groups, however, there were patients whose degree of hypertriglyceridemia seemed out of proportion to their transport rates. This finding and the fact that many patients have increased secretion of VLDL-TG without elevated plasma TG suggests that both overproduction of VLDL-TG and insufficient enhancement of clearance contributed to the development of hypertriglyceridemia.The data showed a poor correlation between transport rates determined by our multicompartment analysis and single-exponential analysis used previously by other investigators (r = 0.46); this comparison was not improved by segregating patients according to their degree of obesity. Although two conversion pathways (fast and slow synthetic paths) were required to fit the data, there was no correlation between transport rates and the ratio of the two pathways. Also, despite the known pathway of conversion of VLDL to low density lipoprotein, no correlation was found between VLDL-TG transport rates and estimated low density lipoprotein-cholesterol concentrations.

Carbohydrate storage in man: speculations and some quantitative considerations

Recent information indicates that the capacity of man to store carbohydrate energy by transformation into fatty acids synthetized de novo is very limited in adipose tissue as well as in liver and intestine. This seems to be in contrast to other species such as the rat where de novo fatty acid synthesis can be induced to a high capacity of glucose removal. This leaves man with a limited capacity to store excess carbohydrate. The remaining possibilities are both the main glycogen stores in liver and in muscle. The latter is by far the largest. The capacity of muscle to assimilate glucose is dependent on its glycogen content that in turn is dependent on previous glycogen depletion to supply energy for muscle contraction. Man might, thus, be uniquely limited in the capacity to dispose of extra carbohydrate in the sedentary state. This might speculatively be thought to be an explanation for a carbohydrate excess syndrome in the sedentary state that may well increase the risk for obesity, hyperinsulinemia, and diabetes mellitus. The logical treatment for such a syndrome then is either a decreased intake of energy as carbohydrate or an increased disposal of carbohydrate energy by exercise. Exercise has, indeed, been shown to have such effects both after physical training programs and, perhaps more pertinent to the question, during a few days after a single exercise bout that has consumed a large amount of muscle glycogen.

Usefulness of serum lipid determination in diabetic practice

In order to evaluate the importance of measuring serum lipids in the current care of diabetics, blood triglycerides were measured in 155 diabetics and 59 controls. Comparison with a chemical method confirmed the usefulness of the nephelometric method for the diagnosis and control of hyperlipemia in current practice. The importance of measuring serum lipids was confirmed by a close correlation between lipemia and cardiovascular complications such as coronary insufficiency, high blood pressure, and peripheral arterial insufficiency. It appeared also that glycemia and cholesterol are not sufficient to assess the biological pattern and prognosis of diabetes. Thus, lipemia is an essential parameter in the evaluation of any diabetic because of its value regarding prognosis and control therapy.

Effects of polyunsaturated fats on lipid metabolism in patients with hypertriglyceridemia

Studies were carried out on the effects of polyunsaturated fats on lipid metabolism in 11 patients with hypertriglyceridemia. During cholesterol balance studies performed in eight patients, the feeding of polyunsaturated fats, as compared with saturated fats, caused an increased excretion of endogenous neutral steroids, acidic steroids, or both in most patients. Increases in steroid excretions were marked in some patients and generally exceeded the decrement of cholesterol in the plasma compartment. The finding of a greater excretion of fecal steroids on polyunsaturated fats in hypertriglyceridemic patients contrasts to the lack of change in sterol balance previously reported for patients with familial hypercholesterolemia; however, other workers have found that polyunsaturated fats also enhance steroid excretion in normal subjects. In most of the patients, simultaneous studies were carried out on biliary lipid composition, hourly outputs of biliary lipids, and pool sizes of bile acids. In several but not all patients, fasting gallbladder bile became more lithogenic after institution of polyunsaturated fats. This increased lithogenicity was not due to a decrease in bile acid pools; in no case was the pool decreased by polyunsaturated fats. On the other hand, two patients showed an increased output of biliary cholesterol, and frequently there was an increase in fecal neutral steroids that were derived from cholesterol; thus, polyunsaturated fats may increase bile lithogenicity in some patients through mobilization of cholesterol into bile. Reductions in plasma cholesterol during the feeding of polyunsaturated fats was seen in most patients, and these changes were usually associated with a decrease in concentration of plasma triglycerides. In fact, the degree of cholesterol lowering was closely correlated with the extent of triglyceride reduction. Therefore, in hypertriglyceridimec patients polyunsaturated fats may contribute to cholesterol reduction by changing the metabolism of triglycerides or very low density lipoproteins. The findings of changes in the metabolism of cholesterol, bile acids, and triglycerides in the patients of this study suggests that polyunsaturated fats may cause a lowering of cholesterol through multiple mechanisms, and it seems unlikely that a single action can explain all the effects of these fats on the plasma lipids.

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.

Very low density lipoprotein triglyceride transport in type IV hyperlipoproteinemia and the effects of carbohydrate-rich diets

Transport of plasma-free fatty acids (FFA) and of fatty acids in triglycerides of plasma very low density lipoproteins (VLDL-TGFA) was studied in two normal subjects, five patients with type IV hyperlipoproteinemia, and two patients with type I hyperlipoproteinemia. After intravenous pulse-labeling with albumin-bound 1-palmitate-(14)C, specific radioactivity of plasma FFA and VLDL-TGFA were determined at intervals up to 24 hr. The results were analyzed using several different multicompartmental models each compatible with the experimental data. Fractional transport of VLDL-TGFA was distinctly lower (no overlap) in the type IV patients than in the control subjects, both on a usual balanced diet (40% of calories from carbohydrate) and on a high-carbohydrate diet (80% of calories). However, net or total transport of VLDL-TGFA in the type IV patients was not clearly distinguishable from that in the control subjects, there being considerable overlap on either diet. The results suggest that in this group of type IV patients the underlying defect leading to the increased pool size of VLDL-TGFA is not overproduction but a relative defect in mechanisms for removal of VLDL-TGFA. Since some of these type IV patients had only a moderate degree of hypertriglyceridemia at the time they were studied, and since it is not established that patients with the type IV phenotype constitute a biochemically homogeneous population, the present results should not be generalized. Four studies were done (in two control subjects and two type IV patients) in which the kinetic parameters in the same individual were determined on the balanced diet and on the high-carbohydrate diet. All subjects showed an increase in VLDL-TGFA pool size. Using two of the models for analysis, all showed an increase in net transport of VLDL-TGFA; using the third model, three of the four studies showed an increase in VLDL-TGFA transport. The results are compatible with the interpretation that the carbohydrate-induced increase in VLDL-TGFA, both in controls and type IV patients, is at least in part due to an increased rate of production of VLDL-TGFA. The magnitude of the increase was approximately the same in controls and patients. Thus, metabolic adjustment to a high-carbohydrate regimen in these type IV patients may not be basically different from that in normal controls; the higher levels of VLDL-TGFA reached may simply be another reflection of a defective removal mechanism. An alternative interpretation, compatible with the data, would involve both a carbohydrate-induced increase in fractional rate of release of VLDL-TGFA from liver to plasma and a decrease in fractional removal of VLDL-TGFA from plasma without increase in net production rate. The simpler hypothesis of a single primary effect on net VLDL-TGFA production from FFA seems more likely.

Carbohydrate-induced hypertriglyceridaemia in a child

Carbohydrate-induced hypertriglyceridaemia is described in a 12-year-old boy, whose father died of premature coronary heart disease. Serum triglyceride and pre-fl-lipoprotein concentrations were high on a self-selected diet and were reduced to normal by a low carbohydrate diet. It proved difficult to maintain triglyceride levels by diet at home, but the addition of clofibrate improved the control. Clearing of lipid from the serum after an oral fat load was delayed when the fasting serum triglyceride was high, but became normal after the serum triglyceride had been reduced by diet. An oral glucose load showed impairment of glucose tolerance. Insulin resistance was suggested by a small decrease in plasma glucose, with a normal increase in serum insulin, after intravenous tolbutamide. Serum triglyceride and pre-f-lipoprotein concentrations increased after oral glucose. Observations in two other children with diabetic glucose tolerance curves suggested that the increase in serum triglyceride after oral glucose was related to the insulin response rather than to the degree of hyperglycaemia.

Disorders of the serum lipoproteins. II. Hyperlipoproteinaemic states

Images

Intramural and pericapillary distribution of lipids in gingival tissue of patients with carbohydrate-induced hyperglyceridemia

Gingival biopsy was performed on 16 patients with carbohydrate-induced hyperglyceridemia, in the attempt to discover histological abnormalities in the capillaries. For comparison, similar studies were also made on four patients with essential familial hypercholesteremia, on five whose hyperglyceridemia had been kept under control by a low carbohydrate diet for 6 to 26 months, and on eight healthy, normolipemic subjects. Variable amounts of Oil Red O-stainable lipid material were demonstrated both in and around the capillary wall in 13 of the 16 untreated hyperglyceridemic patients. Little or no such lipid-staining material was demonstrated in the biopsy material obtained from the remaining groups of subjects so studied. Microangiopathy of the pre-diabetic and diabetic type characterized by capillary basement membrane thickening was not demonstrated in any of the hyperlipemic patients in this series. Evidence is presented to support the concept that a sustained chylomicronemia is the primary factor in the production of the intramural and pericapillary collection of Oil Red O-staining lipids in these hyperglyceridemic patients.

Effects of diet on hepatic triglyceride synthesis

The effects of diet on the rate of triglyceride synthesis by rat liver homogenates was measured. Changes in triglyceride synthesis were correlated with the level of activity of L-alpha-glycerophosphate acyltransferase, the enzyme catalyzing the first specific reaction in hepatic glycerolipid synthesis. Fasting for 48-72 hr depressed the synthesis of triglyceride from L-alpha-glycerophosphate. High carbohydrate diets, fed to rats for 6 days, resulted in increased triglyceride synthesis. Diets high in starch were less effective than high glucose, sucrose, or fructose diets in increasing triglyceride synthesis. Diets high in corn oil did not alter triglyceride synthesis. These studies established the importance of dietary factors in the regulation of hepatic triglyceride synthesis.L-alpha-Glycerophosphate acyltransferase activity was measured after the same dietary changes. Both high carbohydrate and high fat diets resulted in increased enzyme specific activity. Fasting for 72 hr did not decrease activity. Thus, the specific activity of this enzyme did not correlate well with the measured rate of triglyceride synthesis indicating that other factors must participate in the regulation of triglyceride biosynthesis.