A ketogenic diet reduces metabolic syndrome-induced allodynia and promotes peripheral nerve growth in mice

Highlights

• A ketogenic diet can reverse high-fat-induced mechanical allodynia.

• Peripheral nerves are sensitive to metabolic change that may not mirror systemic metabolism.

• A ketogenic diet and ketones stimulate sensory axon growth in vitro and in vivo.

Abstract

Current experiments investigated whether a ketogenic diet impacts neuropathy associated with obesity and prediabetes. Mice challenged with a ketogenic diet were compared to mice fed a high-fat diet or a high-fat diet plus exercise. Additionally, an intervention switching to a ketogenic diet following 8 weeks of high-fat diet was performed to compare how a control diet, exercise, or a ketogenic diet affects metabolic syndrome-induced neural complications. When challenged with a ketogenic diet, mice had reduced bodyweight and fat mass compared to high-fat-fed mice, and were similar to exercised, high-fat-fed mice. High-fat-fed, exercised and ketogenic-fed mice had mildly elevated blood glucose; conversely, ketogenic diet-fed mice were unique in having reduced serum insulin levels. Ketogenic diet-fed mice never developed mechanical allodynia contrary to mice fed a high-fat diet. Ketogenic diet fed mice also had increased epidermal axon density compared all other groups. When a ketogenic diet was used as an intervention, a ketogenic diet was unable to reverse high-fat fed-induced metabolic changes but was able to significantly reverse a high-fat diet-induced mechanical allodynia. As an intervention, a ketogenic diet also increased epidermal axon density. In vitro studies revealed increased neurite outgrowth in sensory neurons from mice fed a ketogenic diet and in neurons from normal diet-fed mice given ketone bodies in the culture medium. These results suggest a ketogenic diet can prevent certain complications of prediabetes and provides significant benefits to peripheral axons and sensory dysfunction.

Introduction

The growing epidemic of obesity and diabetes has led to a dramatic increase in various pain syndromes and a personal as well as economic burden. A common complication associated with diabetes and metabolic syndrome is a loss of small fibers in the skin and increased pain (Callaghan and Feldman, 2013; Smith and Singleton, 2013). With the development of metabolic syndrome, some have proposed that there is a loss of axonal regenerative capacity leading to a loss of small fibers that can occur with diabetes (Singleton et al., 2015). Prediabetes can be modeled in rodents using a high-fat diets and consumption of high-fats and carbohydrates lead to metabolic alterations and changes in sensory function similar to changes human patients, including obesity, elevated blood glucose, insulin resistance, and mechanical allodynia (Groover et al., 2013; Guilford et al., 2011; Hoke, 2012; Obrosova et al., 2007). Physical activity can improve many of these symptomatic changes; including reversing mechanical allodynia induced by a high-fat diet (Groover et al., 2013). However, the mechanism(s) by which exercise leads to metabolic and sensory nerve benefits is poorly understood.

Previous research has demonstrated that exercise and a high-fat diet create distinctive metabolic phenotypes both systemically and in the peripheral nervous system (Cooper et al., 2016). Both exercise and a ketogenic diet intervention are attractive approaches as both can increase fat oxidation (Horowitz and Klein, 2000; Paoli, 2014). Additionally, both exercise and ketogenic diet can stimulate anti-inflammatory signaling cascades and reduce chronic inflammation that occurs in response to a high-fat diet (Ruskin et al., 2009).

High-fat, low carbohydrate “ketogenic” diets are a rapidly emerging intervention for a wide array of clinical diseases (Klein et al., 2014; Masino and Ruskin, 2013; Wheless, 2008). Historically, a ketogenic diet was popularized in the treatment of epilepsy after it was noted that patients who fasted observed reduced seizure occurrence (Wheless, 2008). Relevant to peripheral nerve function, only a limited number of investigations have examined how a ketogenic diet impacts mechanical and thermal sensation (Galdino et al., 2014; Ruskin et al., 2009, 2013; Ziegler et al., 2005). Caloric restriction increases ketone metabolism and can be a pro-growth signaling agent in the hippocampus and dentate gyrus (Lee et al., 2002a,b). However, the mechanism and function of increased neuronal growth is still under investigation.

Here, we examined several parameters of peripheral nervous system function from mice fed a ketogenic diet. We have previously reported that mice fed a high-fat and carbohydrate-rich diet develop negative symptoms related to peripheral nerve function similar to early peripheral neuropathy in prediabetes (Cooper et al., 2016; Groover et al., 2013; Guilford et al., 2011). In the current study, we provide evidence that mice fed a ketogenic diet fail to develop mechanical allodynia similar to mice fed a high-fat diet. Additionally, after 8 weeks of a high-fat diet, a ketogenic diet can reverse established mechanical allodynia. Lastly, we investigated the potential of a ketogenic diet to promote peripheral nerve outgrowth and noted that a ketogenic diet or ketone body supplementation increases axon growth of primary sensory neurons. Together, these results suggest that a ketogenic diet may be an attractive intervention for metabolic and diabetic peripheral neuropathy.