Introduction
Type 2 diabetes has long been regarded as a chronic progressive condition, capable of amelioration but not cure. A steady rise in plasma glucose occurs irrespective of the degree of control or type of treatment [
1]. Beta cell function declines linearly with time, and after 10 years more than 50% of individuals require insulin therapy [
2]. The underlying changes in beta cell function have been well described [
3,
4], and beta-cell mass decreases steadily during the course of type 2 diabetes [
5,
6]. Overall, there is strong evidence that type 2 diabetes is inexorably progressive, with a high likelihood of insulin therapy being eventually required to maintain good glycaemic control.
However, type 2 diabetes is clearly reversible following bariatric surgery [
7]. The normalisation of plasma glucose concentration follows within days of surgery, long before major weight loss has occurred, and it has become widely assumed that the protective effects of gastrointestinal surgery are mediated by altered secretion of incretin hormones [
8,
9]. Improved control of blood glucose in type 2 diabetes by moderate energy restriction has been demonstrated by others [
10]. We have hypothesised that the profound effect of a sudden negative energy balance on the metabolism could explain the post-bariatric surgery effect [
11] and, specifically, that the decrease in the intracellular fatty acid concentrations in the liver would lead to a lower export of lipoprotein triacylglycerol to the pancreas, with the release of beta cells from the chronic inhibitory effects of excess fatty acid exposure.
This study was designed to test the hypothesis that acute negative energy balance alone reverses type 2 diabetes by normalising both beta cell function and insulin sensitivity. We examined the restoration of first-phase and total insulin response as well as hepatic and peripheral insulin sensitivity. Additionally, to examine the mechanistic basis of observed outcomes, we quantified the change in fat content of the pancreas and liver.
Discussion
This study demonstrates that the twin defects of beta cell failure and insulin resistance that underlie type 2 diabetes can be reversed by acute negative energy balance alone. A hierarchy of response was observed, with a very early change in hepatic insulin sensitivity and a slower change in beta cell function. In the first 7 days of the reduced energy intake, fasting blood glucose and hepatic insulin sensitivity fell to normal, and intrahepatic lipid decreased by 30%. Over the 8 weeks of dietary energy restriction, beta cell function increased towards normal and pancreatic fat decreased. Following the intervention, participants gained 3.1 ± 1.0 kg body weight over 12 weeks, but their HbA
1c remained steady while the fat content of both pancreas and liver did not increase. The data are consistent with the hypothesis that the abnormalities of insulin secretion and insulin resistance that underlie type 2 diabetes have a single, common aetiology, i.e. excess lipid accumulation in the liver and pancreas [
11]. This provides a unified hypothesis to explain a common disease that previously appeared to require separate disease processes affecting the pancreas and insulin-sensitive tissues.
Absence of rapid insulin secretion in response to a rise in plasma glucose is the hallmark of type 2 diabetes [
3,
21], and the decline in beta cell function determines the progression towards a need for insulin therapy [
2]. However, conventional therapy, even with sulfonylurea, fails to produce more than a small increase in the first-phase insulin response. As a consequence, the rapidity and extent of return of beta cell function in response to dietary energy restriction in the present study is striking. It supports the accumulating information on the inhibitory effect of fatty acids on insulin secretion in vitro and in vivo [
22‐
24] and is the first direct evidence in humans that the beta cell defect of type 2 diabetes is reversible by sustained negative energy balance. Prolonged elevation of plasma fatty acids in humans decreases insulin secretion [
25,
26], and it has previously been shown that there is an association between pancreatic fat content and type 2 diabetes [
27‐
29]. Prior to the onset of spontaneous diabetes in rodents, both total pancreatic fat and islet triacylglycerol content increase sharply [
30,
31]. In vitro, chronic saturated fatty acid exposure of beta cells inhibits the acute insulin response to glucose, and removal of fatty acids allows recovery of this response [
32].
The present data provide clear evidence that decreasing total pancreatic fat is associated with a return of beta cell function. However, it is probable that the negative effect on beta cell function is exerted by toxic intermediaries such as diacylglycerol and ceramides, which change rapidly in response to acute metabolic changes [
33], rather than by stored triacylglycerol per se, which acts as an index of fatty acid intermediary concentration. There was no correlation between indices of insulin secretion and pancreatic fat, which suggests that there are individual thresholds of tolerance for such toxic intermediaries rather than a simple dose–response relationship within the pancreas.
Fasting plasma glucose concentration is determined by the rate of hepatic glucose production, and hepatic insulin sensitivity is inversely proportional to intrahepatic lipid content [
13,
34‐
36]. Moderate weight loss has previously been shown to be associated with a fall in intrahepatic fat content [
10,
37]. Petersen et al. have previously reported improved liver and no significant change in muscle insulin sensitivity measured by euglycaemic hyperinsulinaemic clamps after 8 weeks of moderate energy restriction in type 2 diabetes [
10]. A very low energy intake has been observed to lower liver fat content in healthy obese individuals within days [
38]. The present study demonstrates for the first time the early time course with which both hepatic fat stores and hepatic glucose production fall in response to dietary restriction in type 2 diabetes. Change in peripheral insulin sensitivity played no part in the early return of normoglycaemia. It is possible that the sharp rise in plasma NEFA observed after 1 week of the hypocaloric diet could have prevented a change in peripheral insulin sensitivity even though this did not prevent the rapid improvement of hepatic insulin sensitivity.
We have previously hypothesised that establishing the pathophysiological changes that accompany the reversal of type 2 diabetes will illuminate the sequence of changes determining the onset of the disease [
11]. This twin-cycle hypothesis was informed by observations following bariatric surgery, especially the demonstration that fasting blood glucose concentrations fell within days after biliopancreatic diversion [
39]. As these changes occur before substantial weight loss, it has become widely accepted that bariatric surgery exerts such rapid effects by changes in incretin hormones. However, the extent of change in incretins is modest, not always present in type 2 diabetes, and absent after gastric banding [
40‐
43]. Little attention has been paid to the potential role of a sudden negative energy balance on glucose metabolism after bariatric surgery, although a recent small study suggested that the improvement in insulin resistance in the first week after surgery can be attributed to negative energy balance alone [
44]. Increased glucagon-like peptide-1 secretion following bariatric surgery compared with a hypocaloric diet has been reported, but the oral glucose load used in the presence of a gastroenterostomy is likely to explain the early rapid rise in both plasma glucose and the incretin, occasionally with the symptoms of early dumping [
45].
The limitations of this study must be considered. First, the sample size was necessarily small to allow the application of gold standard methods for metabolic investigation and examination by magnetic resonance techniques. However, the participants were drawn from the population with type 2 diabetes, and their clinical and anthropometric characteristics were typical for the condition.
Second, pancreatic fat measurements included intraorgan adipocyte fat content because current methodology precludes the assessment of the more mechanistically important islet intracellular fatty acid content. Animal data suggest that the two variables are linked [
31]. Although pancreatic fat content was 30% higher in the diabetic group, the study was powered to demonstrate responses to the dietary intervention rather than to test differences from weight-matched non-diabetic individuals. No correlation was observed between pancreatic fat and BMI within the restricted range of BMI examined in this study.
Third, the participants were selected to have had a relatively short duration of type 2 diabetes (up to 4 years), and further studies must establish the extent of reversibility with longer duration type 2 diabetes. In addition, further studies are required to determine the long-term outcome in respect of glucose regulation as the observations made after 12 weeks of return to a normal diet were necessarily limited.
Finally, the raised liver fat levels despite metabolic normality in some controls can be considered in the light of the recent description of the
PNPLA3 gene [
46]. The G allele of this gene determines high liver fat levels, but in a form that is not associated with metabolic abnormality. This provides a clear genetic basis for the observed individual variation in susceptibility to insulin resistance despite raised liver fat content, and offers a partial explanation of the overlapping hepatic fat levels in type 2 diabetic and control groups. It is likely that other genetic factors yet to be defined underlie the differing individual levels of susceptibility to pancreatic fat accumulation in terms of inhibition of glucose-dependent insulin secretion.
This study demonstrates for the first time the time course of a return of normal beta cell function and hepatic glucose output by acute restriction of dietary energy intake in individuals with type 2 diabetes. The changes occurred in association with decreases in pancreatic and liver triacylglycerol concentrations. This new insight allows an understanding of the causality of type 2 diabetes in individuals as well as in populations. It carries major implications for information to be given to newly diagnosed patients, who should know that they have a potentially reversible condition and not one that is inevitably progressive.
Acknowledgements
We thank L. Morris, C. Smith and T. Hodgson for magnetic resonance radiography, A. Lane for GC-MS analyses, J. Gerrard for research nurse support and K. Heron for dietary advice. We are most grateful to the volunteers for their enthusiastic participation. This study was funded wholly by project grant RD08/0003713 from Diabetes UK. Nestlé UK provided the Optifast on request but had no other input into the research.
E.L.L performed research, analysed data and wrote the manuscript. K.H. designed the research, analysed the data and edited the manuscript. B.A. analysed the data and edited the manuscript. M.J.C. performed research and contributed to the manuscript. J.M. designed the research and edited the manuscript. R.T. designed the research, analysed the data and reviewed/edited the manuscript. All authors approved the final version to be published.