Energy restriction does not prevent insulin resistance but does prevent liver steatosis in aging rats on a Western-style diet

Highlights

• A Western-style diet contributes to aging by increasing insulin resistance and lipid accumulation.

• Energy restriction did not prevent insulin resistance in aged rats on a Western-style diet.

• Energy restriction on a Western-style diet did not prevent lipid accumulation in plasma and skeletal muscle.

• Energy restriction prevented hepatic steatosis despite the Western-style diet.

• The nature of the diet is important when studying energy restriction.

Abstract

Objective

The aim of this study was to evaluate the effects of long-term energy restriction (ER) on plasma, liver, and skeletal muscle metabolite profiles in aging rats fed a Western-style diet.

Methods

Three groups of male Sprague-Dawley rats were studied. Group 1 consisted of 2 mo old rats fed ad libitum; group 2 were 19 mo old rats also fed ad libitum; and group 3 were 19 mo old rats subjected to 40% ER for the last 11.5 mo. To imitate a Western-style diet, all rats were given a high-sucrose, very low ω-3 polyunsaturated fatty acid (PUFA) diet. High-resolution magic angle spinning-¹H nuclear magnetic resonance spectroscopy was used for hepatic and skeletal muscle metabolite determination, and fatty acid profiles were measured by capillary gas chromatography on plasma, liver, and skeletal muscle.

Results

ER coupled with a Western-style diet did not prevent age-induced insulin resistance or the increase in triacylglycerol content in plasma and skeletal muscle associated with aging. However, in the liver, ER did prevent steatosis and increased the percent of saturated and monounsaturated fatty acids relative to ω-6 and ω-3 PUFA.

Conclusions

Although steatosis was reduced, the beneficial effects of ER on systemic insulin resistance and plasma and skeletal muscle metabolites observed elsewhere with a balanced diet seem to be compromised by high-sucrose and low ω-3 PUFA intake.

Introduction

Energy restriction (ER) is reported to extend life span by up to 50% in rodents [1], [2]. Although the mechanisms implicated are not fully understood, some hypotheses have emerged [3], [4], [5], [6]: ER may prevent or delay metabolic changes occurring during aging, especially in lipids. Several age-induced metabolic changes such as impaired mitochondrial energy metabolism [7] and increased hepatic de novo lipogenesis [8] lead to tissue fat accumulation that could contribute to age-induced insulin resistance (IR) [9]. By activating a serine kinase cascade, fat accumulation could impair insulin signaling and therefore lead to decreased glucose transport in skeletal muscle and impaired glucose production by the liver [10], [11], [12]. ER improves insulin sensitivity during aging [13], [14], which may be linked to lower age-dependent fat accumulation [15], [16], [17]. ER also up-regulates some metabolic pathways in the liver, including gluconeogenesis, fatty acid β-oxidation, and ketogenesis [13], [18].

Over the past few decades, dietary habits in Western countries have evolved toward higher sugar and lower ω-3 polyunsaturated fatty acid (PUFA) intakes both of which promote IR and lipogenesis and may contribute to the development of obesity, diabetes, and fatty liver, as described both in humans and rodent models [19], [20], [21]. In rats consuming a Western-style diet, we questioned whether ER might mitigate changes in metabolites and lipids. Rats are a widely studied animal model and have the advantage of exhibiting some of the same physiological responses to ER as observed in humans. Our focus was on metabolites in liver and skeletal muscle, two insulin-sensitive organs that are important in lipid and glucose metabolism. Some liver and skeletal muscle metabolites were assessed on tissue biopsies by high-resolution magic angle spinning ¹H-nuclear magnetic resonance (HRMAS ¹H-NMR) spectroscopy. Plasma and tissue fatty acid profiles were analyzed by capillary gas chromatography (GC). We aimed to evaluate the effects of aging on a Western-style diet by comparing metabolite and fatty acid profiles in 2 and 19 mo old rats fed ad libitum; and to determine if ER mitigates the effect of a lifelong Western-style diet by comparing metabolite and fatty acid profiles of 19 mo old rats either fed ad libitum or subjected to a 40% ER from 7.5 to 19 mo of age.