Clinical research strategies for fructose metabolism

Maternal fructose boosts the effects of a Western-type diet increasing SARS-COV-2 cell entry factors in male offspring

Fructose-rich beverages and foods consumption correlates with the epidemic rise in cardiovascular disease, diabetes and obesity. Severity of COVID-19 has been related to these metabolic diseases. Fructose-rich foods could place people at an increased risk for severe COVID-19. We investigated whether maternal fructose intake in offspring affects hepatic and ileal gene expression of proteins that permit SARS-CoV2 entry to the cell. Carbohydrates were supplied to pregnant rats in drinking water. Adult and young male descendants subjected to water, liquid fructose alone or as a part of a Western diet, were studied. Maternal fructose reduced hepatic SARS-CoV2 entry factors expression in older offspring. On the contrary, maternal fructose boosted the Western diet-induced increase in viral entry factors expression in ileum of young descendants. Maternal fructose intake produced a fetal programming that increases hepatic viral protection and, in contrast, exacerbates fructose plus cholesterol-induced diminution in SARS-CoV2 protection in small intestine of progeny.

Taste Perception and Cerebral Activity in the Human Gustatory Cortex Induced by Glucose, Fructose, and Sucrose Solutions

Glucose, fructose, and sucrose are important carbohydrates in Western diets with particular sweetness intensity and metabolisms. No study has compared their cerebral detection and their taste perception. Gustatory evoked potentials (GEPs), taste detection thresholds, intensity perception, and pleasantness were compared in response to glucose, fructose, and sucrose solutions at similar sweetness intensities and at identical molar concentrations. Twenty-three healthy subjects were randomly stimulated with 3 solutions of similar sweetness intensity (0.75 M of glucose, 0.47 M of fructose and 0.29 M of sucrose - sit. A), and with an identical molar concentration (0.29 M - sit. B). GEPs were recorded at gustatory cortex areas. Intensity perception and hedonic values of each solution were evaluated as were gustatory thresholds of the solutions. No significant difference was observed concerning the GEP characteristics of the solutions according to their sweetness intensities (sit. A) or their molar concentration (sit. B). In sit. A, the 3 solutions were perceived to have similar intensities and induced similar hedonic sensations. In sit. B, the glucose solution was perceived to be less intense and pleasant than the fructose and the sucrose solutions (P < 0.001) and the fructose solution was perceived to be less intense and pleasant than the sucrose (P < 0.001). Since GEP recordings were similar for glucose, fructose, and sucrose solutions whatever the concentrations, activation of same taste receptor induces similar cortical activation, even when the solutions were perceived differently. Sweet taste perception seems to be encoded by a complex chemical cerebral neuronal network.

The extra-splanchnic fructose escape after ingestion of a fructose-glucose drink: An exploratory study in healthy humans using a dual fructose isotope method

BACKGROUND & AIMS

The presence of specific fructose transporters and fructose metabolizing enzymes has now been demonstrated in the skeletal muscle, brain, heart, adipose tissue and many other tissues. This suggests that fructose may be directly metabolized and play physiological or pathophysiological roles in extra-splanchnic tissues. Yet, the proportion of ingested fructose reaching the systemic circulation is generally not measured. This study aimed to assess the amount of oral fructose escaping first-pass splanchnic extraction after ingestion of a fructose-glucose drink using a dual oral-intravenous fructose isotope method.

METHODS

Nine healthy volunteers were studied over 2 h before and 4 h after ingestion of a drink containing 30.4 ± 1.0 g of glucose (mean ± SEM) and 30.4 ± 1.0 g of fructose labelled with 1% [U-¹³C₆]-fructose. A 75%-unlabeled fructose and 25%-[6,6-²H₂]-fructose solution was continuously infused (100 μg kg^-1 min^-1) over the 6 h period. Total systemic, oral and endogenous fructose fluxes were calculated from plasma fructose concentrations and isotopic enrichments. The fraction of fructose escaping first-pass splanchnic extraction was calculated assuming a complete intestinal absorption of the fructose drink.

RESULTS

Fasting plasma fructose concentration before tracer infusion was 17.9 ± 0.6 μmol.L^-1. Fasting endogenous fructose production detected by tracer dilution analysis was 55.3 ± 3.8 μg kg^-1min^-1. Over the 4 h post drink ingestion, 4.4 ± 0.2 g of ingested fructose (i.e. 14.5 ± 0.8%) escaped first-pass splanchnic extraction and reached the systemic circulation. Endogenous fructose production significantly increased to a maximum of 165.4 ± 10.7 μg kg^-1·min^-1 60 min after drink ingestion (p < 0.001).

CONCLUSIONS

These data indicate that a non-negligible fraction of fructose is able to escape splanchnic extraction and circulate in the periphery. The metabolic effects of direct fructose metabolism in extra-splanchnic tissues, and their relationship with metabolic diseases, remain to be evaluated. Our results also open new research perspectives regarding the physiological role of endogenous fructose production.

Are heterozygous carriers for hereditary fructose intolerance predisposed to metabolic disturbances when exposed to fructose?

Background

High fructose intake causes hepatic insulin resistance and increases postprandial blood glucose, lactate, triglyceride, and uric acid concentrations. Uric acid may contribute to insulin resistance and dyslipidemia in the general population. In patients with hereditary fructose intolerance, fructose consumption is associated with acute hypoglycemia, renal tubular acidosis, and hyperuricemia.

Objective

We investigated whether asymptomatic carriers for hereditary fructose intolerance (HFI) would have a higher sensitivity to adverse effects of fructose than would the general population.

Design

Eight subjects heterozygous for HFI (hHFI; 4 men, 4 women) and 8 control subjects received a low-fructose diet for 7 d and on the eighth day ingested a test meal, calculated to provide 25% of the basal energy requirement, containing 13C-labeled fructose (0.35 g/kg), glucose (0.35 g/kg), protein (0.21 g/kg), and lipid (0.22 g/kg). Glucose rate of appearance (GRa, calculated with [6,6-2H2]glucose), fructose, net carbohydrate, and lipid oxidation, and plasma triglyceride, uric acid, and lactate concentrations were monitored over 6 h postprandially.

Results

Postprandial GRa, fructose, net carbohydrate, and lipid oxidation, and plasma lactate and triglyceride concentrations were not significantly different between the 2 groups. Postprandial plasma uric acid increased by 7.2% compared with fasting values in hHFI subjects (P < 0.01), but not in control subjects (-1.1%, ns).

Conclusions

Heterozygous carriers of hereditary fructose intolerance had no significant alteration of postprandial fructose metabolism compared with control subjects. They did, however, show a postprandial increase in plasma uric acid concentration that was not observed in control subjects in responses to ingestion of a modest amount of fructose. This trial was registered at the US Clinical Trials Registry as NCT02979106.

Fructose metabolism and noncommunicable diseases: recent findings and new research perspectives

PURPOSE OF REVIEW

There is increasing concern that dietary fructose may contribute to the development of noncommunicable diseases. This review identifies major new findings related to fructose's physiological or adverse effects.

RECENT FINDINGS

Fructose is mainly processed in splanchnic organs (gut, liver, kidneys) to glucose, lactate, and fatty acids, which can then be oxidized in extrasplanchnic organs and tissues. There is growing evidence that splanchnic lactate production, linked to extrasplanchnic lactate metabolism, represents a major fructose disposal pathway during and after exercise. Chronic excess fructose intake can be directly responsible for an increase in intrahepatic fat concentration and for the development of hepatic, but not muscle insulin resistance. Although it has long been thought that fructose was exclusively metabolized in splanchnic organs, several recent reports provide indirect that some fructose may also be metabolized in extrasplanchnic cells, such as adipocytes, muscle, or brain cells; the quantity of fructose directly metabolized in extrasplanchnic cells, and its physiological consequences, remain however unknown. There is also growing evidence that endogenous fructose production from glucose occurs in humans and may have important physiological functions, but may also be associated with adverse health effects.

SUMMARY

Fructose is a physiological nutrient which, when consumed in excess, may have adverse metabolic effects, mainly in the liver (hepatic insulin resistance and fat storage). There is also concern that exogenous or endogenously produced fructose may be directly metabolized in extrasplanchnic cells in which it may exert adverse metabolic effects.

Adverse effects of fructose on cardiometabolic risk factors and hepatic lipid metabolism in subjects with abdominal obesity

BACKGROUND

Overconsumption of dietary sugars, fructose in particular, is linked to cardiovascular risk factors such as type 2 diabetes, obesity, dyslipidemia and nonalcoholic fatty liver disease. However, clinical studies have to date not clarified whether these adverse cardiometabolic effects are induced directly by dietary sugars, or whether they are secondary to weight gain.

OBJECTIVES

To assess the effects of fructose (75 g day^-1 ), served with their habitual diet over 12 weeks, on liver fat content and other cardiometabolic risk factors in a large cohort (n = 71) of abdominally obese men.

METHODS

We analysed changes in body composition, dietary intake, an extensive panel of cardiometabolic risk markers, hepatic de novo lipogenesis (DNL), liver fat content and postprandial lipid responses after a standardized oral fat tolerance test (OFTT).

RESULTS

Fructose consumption had modest adverse effects on cardiometabolic risk factors. However, fructose consumption significantly increased liver fat content and hepatic DNL and decreased β-hydroxybutyrate (a measure of β-oxidation). The individual changes in liver fat were highly variable in subjects matched for the same level of weight change. The increase in liver fat content was significantly more pronounced than the weight gain. The increase in DNL correlated positively with triglyceride area under the curve responses after an OFTT.

CONCLUSION

Our data demonstrated adverse effects of moderate fructose consumption for 12 weeks on multiple cardiometabolic risk factors in particular on liver fat content despite only relative low increases in weight and waist circumference. Our study also indicates that there are remarkable individual differences in susceptibility to visceral adiposity/liver fat after real-world daily consumption of fructose-sweetened beverages over 12 weeks.

Fructose intervention for 12 weeks does not impair glycemic control or incretin hormone responses during oral glucose or mixed meal tests in obese men

BACKGROUND AND AIMS

Incretin hormones glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP) are affected early on in the pathogenesis of metabolic syndrome and type 2 diabetes. Epidemiologic studies consistently link high fructose consumption to insulin resistance but whether fructose consumption impairs the incretin response remains unknown.

METHODS AND RESULTS

As many as 66 obese (BMI 26-40 kg/m²) male subjects consumed fructose-sweetened beverages containing 75 g fructose/day for 12 weeks while continuing their usual lifestyle. Glucose, insulin, GLP-1 and GIP were measured during oral glucose tolerance test (OGTT) and triglycerides (TG), GLP-1, GIP and PYY during a mixed meal test before and after fructose intervention. Fructose intervention did not worsen glucose and insulin responses during OGTT, and GLP-1 and GIP responses during OGTT and fat-rich meal were unchanged. Postprandial TG response increased significantly, p = 0.004, and we observed small but significant increases in weight and liver fat content, but not in visceral or subcutaneous fat depots. However, even the subgroups who gained weight or liver fat during fructose intervention did not worsen their glucose, insulin, GLP-1 or PYY responses. A minor increase in GIP response during OGTT occurred in subjects who gained liver fat (p = 0.049).

CONCLUSION

In obese males with features of metabolic syndrome, 12 weeks fructose intervention 75 g/day did not change glucose, insulin, GLP-1 or GIP responses during OGTT or GLP-1, GIP or PYY responses during a mixed meal. Therefore, fructose intake, even accompanied with mild weight gain, increases in liver fat and worsening of postprandial TG profile, does not impair glucose tolerance or gut incretin response to oral glucose or mixed meal challenge.

Fructose surges damage hepatic adenosyl-monophosphate-dependent kinase and lead to increased lipogenesis and hepatic insulin resistance

Fructose may be a key contributor to the biochemical alterations which promote the metabolic syndrome (MetS), non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes (T2DM): (a) its consumption in all forms but especially in liquid form has much increased alongside with incidence of MetS conditions; (b) it is metabolized almost exclusively in the liver, where it stimulates de novo lipogenesis to drive hepatic triglyceride (TG) synthesis which (c) contributes to hepatic insulin resistance and NAFLD (Lustig et al., 2015; Weiss et al., 2013; Lim et al., 2010; Schwarzet al., 2015; Stanhope et al., 2009, 2013) [1-6]. The specifics of fructose metabolism and its main location in the liver serve to explain many of the possible mechanisms involved. It also opens questions, as the consequences of large increases in fructose flux to the liver may wreak havoc with the regulation of metabolism and would produce two opposite effects (inhibition and activation of AMP dependent kinase-AMPK) that would tend to cancel each other. We posit that (1) surges of fructose in the portal vein lead to increased unregulated flux to trioses accompanied by unavoidable methylglyoxal (MG) production, (2) the new, sudden flux exerts carbonyl stress on the three arginines on the γ subunits AMP binding site of AMPK, irreversible blocking some of the enzyme molecules to allosteric modulation, (3) this explains why, even when fructose quick phosphorylation increases AMP and should therefore activate AMPK, the effects of fructose are compatible with inactivation of AMPK, which then solves the apparent metabolic paradox. We put forward the hypothesis that fructose loads, via the increase in MG flux worsens the fructose-driven metabolic disturbances that lead to unrestricted de novo lipogenesis, fatty liver and hepatic insulin resistance. It does so via the silencing of AMPK. Our hypothesis is testable and if proven correct will shed some further light on fructose metabolism in the liver. It will also open new roads in glycation research, as modulation of MG catabolism may be a way to dampen the damage. Research on this area may have important therapeutic potential, e.g., more momentum to find new and improved carbonyl quenchers, new insights on the action of metformin, more evidence for the role of GAPDH inactivation due to mitochondrial overload in diabetes complications. AMPK plays a central role in metabolism, and its function varies in different tissues. For that reason, synthetic activators will always stumble with unwanted or unpredictable effects. Preventing MG damage on the protein could be a safer therapeutic avenue.

Fructose Containing Sugars at Normal Levels of Consumption Do Not Effect Adversely Components of the Metabolic Syndrome and Risk Factors for Cardiovascular Disease

The objective of the current study was to explore our hypothesis that average consumption of fructose and fructose containing sugars would not increase risk factors for cardiovascular disease (CVD) and the metabolic syndrome (MetS). A randomized, double blind, parallel group study was conducted where 267 individuals with BMI between 23 and 35 kg/m² consumed low fat sugar sweetened milk, daily for ten weeks as part of usual weight-maintenance diet. One group consumed 18% of calories from high fructose corn syrup (HFCS), another group consumed 18% of calories from sucrose, a third group consumed 9% of calories from fructose, and the fourth group consumed 9% of calories from glucose. There was a small change in waist circumference (80.9 ± 9.5 vs. 81.5 ± 9.5 cm) in the entire cohort, as well as in total cholesterol (4.6 ± 1.0 vs. 4.7 ± 1.0 mmol/L, p < 0.01), triglycerides (TGs) (11.5 ± 6.4 vs. 12.6 ± 8.9 mmol/L, p < 0.01), and systolic (109.2 ± 10.2 vs. 106.1 ± 10.4 mmHg, p < 0.01) and diastolic blood pressure (69.8 ± 8.7 vs. 68.1 ± 9.7 mmHg, p < 0.01). The effects of commonly consumed sugars on components of the MetS and CVD risk factors are minimal, mixed and not clinically significant.

Fructose decreases physical activity and increases body fat without affecting hippocampal neurogenesis and learning relative to an isocaloric glucose diet

Recent evidence suggests that fructose consumption is associated with weight gain, fat deposition and impaired cognitive function. However it is unclear whether the detrimental effects are caused by fructose itself or by the concurrent increase in overall energy intake. In the present study we examine the impact of a fructose diet relative to an isocaloric glucose diet in the absence of overfeeding, using a mouse model that mimics fructose intake in the top percentile of the USA population (18% energy). Following 77 days of supplementation, changes in body weight (BW), body fat, physical activity, cognitive performance and adult hippocampal neurogenesis were assessed. Despite the fact that no differences in calorie intake were observed between groups, the fructose animals displayed significantly increased BW, liver mass and fat mass in comparison to the glucose group. This was further accompanied by a significant reduction in physical activity in the fructose animals. Conversely, no differences were detected in hippocampal neurogenesis and cognitive/motor performance as measured by object recognition, fear conditioning and rotorod tasks. The present study suggests that fructose per se, in the absence of excess energy intake, increases fat deposition and BW potentially by reducing physical activity, without impacting hippocampal neurogenesis or cognitive function.

The role of dietary sugars and de novo lipogenesis in non-alcoholic fatty liver disease

Dietary sugar consumption, in particular sugar-sweetened beverages and the monosaccharide fructose, has been linked to the incidence and severity of non-alcoholic fatty liver disease (NAFLD). Intervention studies in both animals and humans have shown large doses of fructose to be particularly lipogenic. While fructose does stimulate de novo lipogenesis (DNL), stable isotope tracer studies in humans demonstrate quantitatively that the lipogenic effect of fructose is not mediated exclusively by its provision of excess substrates for DNL. The deleterious metabolic effects of high fructose loads appear to be a consequence of altered transcriptional regulatory networks impacting intracellular macronutrient metabolism and altering signaling and inflammatory processes. Uric acid generated by fructose metabolism may also contribute to or exacerbate these effects. Here we review data from human and animal intervention and stable isotope tracer studies relevant to the role of dietary sugars on NAFLD development and progression, in the context of typical sugar consumption patterns and dietary recommendations worldwide. We conclude that the use of hypercaloric, supra-physiological doses in intervention trials has been a major confounding factor and whether or not dietary sugars, including fructose, at typically consumed population levels, effect hepatic lipogenesis and NAFLD pathogenesis in humans independently of excess energy remains unresolved.

IRE1 impairs insulin signaling transduction of fructose-fed mice via JNK independent of excess lipid

The unfolded protein response (UPR) pathways have been implicated in the development of hepatic insulin resistance during high fructose (HFru) feeding. The present study investigated their roles in initiating impaired insulin signaling transduction in the liver induced by HFru feeding in mice. HFru feeding resulted in hepatic steatosis, increased de novo lipogenesis and activation of two arms of the UPR pathways (IRE1/XBP1 and PERK/eIF2α) in similar patterns from 3days to 8weeks. In order to identify the earliest trigger of impaired insulin signaling in the liver, we fed mice a HFru diet for one day and revealed that only the IRE1 branch was activated (by 2-fold) and insulin-mediated Akt phosphorylation was blunted (~25%) in the liver. There were significant increases in phosphorylation of JNK (~50%) and IRS at serine site (~50%), protein content of ACC and FAS (up to 2.5-fold) and triglyceride level (2-fold) in liver (but not in muscle or fat). Blocking IRE1 activity abolished increases in JNK activity, IRS serine phosphorylation and protected insulin-stimulated Akt phosphorylation without altering hepatic steatosis or PKCε activity, a key link between lipids and insulin resistance. Our findings together suggest that activation of IRE1-JNK pathway is a key linker of impaired hepatic insulin signaling transduction induced by HFru feeding.