A low-carbohydrate high-fat diet increases weight gain and does not improve glucose tolerance, insulin secretion or β-cell mass in NZO mice

BACKGROUND/OBJECTIVES

Dietary guidelines for the past 20 years have recommended that dietary fat should be minimized. In contrast, recent studies have suggested that there could be some potential benefits for reducing carbohydrate intake in favor of increased fat. It has also been suggested that low-carbohydrate diets be recommended for people with type 2 diabetes. However, whether such diets can improve glycemic control will likely depend on their ability to improve β-cell function, which has not been studied. The objective of the study was to assess whether a low-carbohydrate and therefore high-fat diet (LCHFD) is beneficial for improving the endogenous insulin secretory response to glucose in prediabetic New Zealand Obese (NZO) mice.

METHODS

NZO mice were maintained on either standard rodent chow or an LCHFD from 6 to 15 weeks of age. Body weight, food intake and blood glucose were assessed weekly. Blood glucose and insulin levels were also assessed after fasting and re-feeding and during an oral glucose tolerance test. The capacity of pancreatic β-cells to secrete insulin was assessed in vivo with an intravenous glucose tolerance test. β-Cell mass was assessed in histological sections of pancreata collected at the end of the study.

RESULTS

In NZO mice, an LCHFD reduced plasma triglycerides (P=0.001) but increased weight gain (P<0.0001), adipose tissue mass (P=0.0015), high-density lipoprotein cholesterol (P=0.044) and exacerbated glucose intolerance (P=0.013). Although fasting insulin levels tended to be higher (P=0.08), insulin secretory function in LCHFD-fed mice was not improved (P=0.93) nor was β-cell mass (P=0.75).

CONCLUSIONS

An LCHFD is unlikely to be of benefit for preventing the decline in β-cell function associated with the progression of hyperglycemia in type 2 diabetes.

Metformin regulates the incretin receptor axis via a pathway dependent on peroxisome proliferator-activated receptor-α in mice

AIMS/HYPOTHESIS

Metformin is widely used for the treatment of type 2 diabetes. Although it reduces hepatic glucose production, clinical studies show that metformin may reduce plasma dipeptidyl peptidase-4 activity and increase circulating levels of glucagon-like peptide 1 (GLP-1). We examined whether metformin exerts glucoregulatory actions via modulation of the incretin axis.

METHODS

Metformin action was assessed in Glp1r(-/-), Gipr(-/-), Glp1r:Gipr(-/-), Pparα (also known as Ppara)(-/-) and hyperglycaemic obese wild-type mice with or without the GLP-1 receptor (GLP1R) antagonist exendin(9-39). Experimental endpoints included glucose tolerance, plasma insulin levels, gastric emptying and food intake. Incretin receptor expression was assessed in isolated islets from metformin-treated wild-type and Pparα(-/-) mice, and in INS-1 832/3 beta cells with or without peroxisome proliferator-activated receptor (PPAR)-α or AMP-activated protein kinase (AMPK) antagonists.

RESULTS

In wild-type mice, metformin acutely increased plasma levels of GLP-1, but not those of gastric inhibitory polypeptide or peptide YY; it also improved oral glucose tolerance and reduced gastric emptying. Metformin significantly improved oral glucose tolerance despite loss of incretin action in Glp1r(-/-), Gipr(-/-) and Glp1r(-/-) :Gipr(-/-) mice, and in wild-type mice fed a high-fat diet and treated with exendin(9-39). Levels of mRNA transcripts for Glp1r, Gipr and Pparα were significantly increased in islets from metformin-treated mice. Metformin directly increased Glp1r expression in INS-1 beta cells via a PPAR-α-dependent, AMPK-independent mechanism. Metformin failed to induce incretin receptor gene expression in islets from Pparα(-/-) mice.

CONCLUSIONS/INTERPRETATION

As metformin modulates multiple components of the incretin axis, and enhances expression of the Glp1r and related insulinotropic islet receptors through a mechanism requiring PPAR-α, metformin may be mechanistically well suited for combination with incretin-based therapies.

Peripheral insulin resistance develops in transgenic rats overexpressing phosphoenolpyruvate carboxykinase in the kidney

AIMS/HYPOTHESIS

To study the secondary consequences of impaired suppression of endogenous glucose production (EGP) we have created a transgenic rat overexpressing the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) in the kidney. The aim of this study was to determine whether peripheral insulin resistance develops in these transgenic rats.

METHODS

Whole body rate of glucose disappearance (R(d)) and endogenous glucose production were measured basally and during a euglycaemic/hyperinsulinaemic clamp in phosphoenolpyruvate carboxykinase transgenic and control rats using [6-(3)H]-glucose. Glucose uptake into individual tissues was measured in vivo using 2-[1-(14)C]-deoxyglucose.

RESULTS

Phosphoenolpyruvate carboxykinase transgenic rats were heavier and had increased gonadal and infrarenal fat pad weights. Under basal conditions, endogenous glucose production was similar in phosphoenolpyruvate carboxykinase transgenic and control rats (37.4+/-1.1 vs 34.6+/-2.6 micromol/kg/min). Moderate hyperinsulinaemia (810 pmol/l) completely suppressed EGP in control rats (-0.6+/-5.5 micromol/kg/min, p<0.05) while there was no suppression in phosphoenolpyruvate carboxykinase rats (45.2+/-7.9 micromol/kg/min). Basal R(d) was comparable between PEPCK transgenic and control rats (37.4+/-1.1 vs 34.6+/-2.6 micromol/kg/min) but under insulin-stimulated conditions the increase in R(d) was greater in control compared to phosphoenolpyruvate carboxykinase transgenic rats indicative of insulin resistance (73.4+/-11.2 vs 112.0+/-8.0 micromol/kg/min, p<0.05). Basal glucose uptake was reduced in white and brown adipose tissue, heart and soleus while insulin-stimulated transport was reduced in white and brown adipose tissue, white quadriceps, white gastrocnemius and soleus in phosphoenolpyruvate carboxykinase transgenic compared to control rats. The impairment in both white and brown adipose tissue glucose uptake in phosphoenolpyruvate carboxykinase transgenic rats was associated with a decrease in GLUT4 protein content. In contrast, muscle GLUT4 protein, triglyceride and long-chain acylCoA levels were comparable between PEPCK transgenic and control rats.

CONCLUSIONS/INTERPRETATION

A primary defect in suppression of EGP caused adipose tissue and muscle insulin resistance.

Transmission of human T-cell lymphotropic virus type III (HTLV-III) by artificial insemination by donor

Four of eight recipients of artificial insemination (AI) with cryopreserved semen from a symptomless carrier of human T-cell lymphotropic virus type III (HTLV-III) were found to have antibody to the virus. One has generalised, persistent lymphadenopathy while the other three remain symptom free 3 years after insemination. Three subsequently became pregnant more than a year after contact with the infected semen; the children, who are now over 1 year of age, are in good health and do not have HTLV-III antibodies. These observations emphasise the need for a rigorous screening programme for potential AI donors; they also suggest that fresh semen should not be used in AI. The findings confirm the role of semen in heterosexual transmission of the virus and suggest that in women with HTLV-III antibodies pregnancy and subsequent breast-feeding does not necessarily lead to infection of the infant.

Isoflurane in paediatric anaesthesia. Induction and recovery from anaesthesia

The characteristics of induction with and recovery from isoflurane anaesthesia were studied in 248 children. The mean time to loss of consciousness was 1.5 min (SD 0.5). Tracheal intubation, without interruption of spontaneous ventilation, was accomplished in a mean time of 4.2 min (SD 54 seconds). Movement and excitement, of 20-30 seconds duration, occurred in 23.9% children and 22 patients coughed during induction; 15 (12.6%) during the first 124 inductions; 7 (5.6%) subsequently. The mean half-times of reduction of alveolar isoflurane concentrations in 28 children whose lungs were ventilated with isoflurane and in 13 children who breathed isoflurane spontaneously during anaesthesia were: 45 sec after exposure for one hour, 70 sec after exposure of 2-3 hours and 110 seconds following exposures of 4-8 hours. The mean recovery times of the three groups were 6.5, 9.5 and 11.5 min respectively. In two further groups of nine children the mean half times of elimination of halothane and isoflurane were 220 seconds and 54 seconds respectively; recovery from isoflurane was markedly faster. Isoflurane is well accepted by children; induction is more rapid than with halothane, and the marked flexibility in the control of its effects are due to its relative insolubility. It has wide application in paediatric anaesthesia.