Food quality and motivation: A refined low-fat diet induces obesity and impairs performance on a progressive ratio schedule of instrumental lever pressing in rats

Abstract

• High fat diets (HFDs) cause obesity and cognitive impairment in rodents.• HFDs are also highly refined obscuring the causal factors in their effects.• We fed rats a refined or unrefined low-fat diet (LFD).• The refined LFD induced significant weight gain and motivational impairment.• Therefore, diet quality, not fat, is a cause of obesity and cognitive impairment. Introduction: Purified high-fat diet (HFD) feeding causes deleterious metabolic and cognitive effects when compared with unrefined low-fat diets in rodent models. These effects are often attributed to the diet's high content of fat, while less attention has been paid to other mechanisms associated with the diet's highly refined state. Although the effects of HFD feeding on cognition have been explored, little is known about the impact of refined vs. unrefined food on cognition. We tested the hypothesis that a refined low-fat diet (LFD) increases body weight and adversely affects cognition relative to an unrefined diet. Materials and methods: Rats were allowed ad libitum access to unrefined rodent chow (CON, Lab Diets 5001) or a purified low-fat diet (REF, Research Diets D12450B) for 6 months, and body weight and performance on an instrumental lever pressing task were recorded. Results: After six months on their respective diets, group REF gained significantly more weight than group CON. REF rats made significantly fewer lever presses and exhibited dramatically lower breaking points than CON rats for sucrose and water reinforcement, indicating a chronic reduction of motivation for instrumental performance. Switching the rats' diet for 9 days had no effect on these measures. Conclusions: Diet-induced obesity produces a substantial deficit in motivated behavior in rats, independent of dietary fat content. This holds implications for an association between obesity and motivation. Specifically, behavioral traits comorbid with obesity, such as depression and fatigue [1], may be effects of obesity rather than contributing causes. To the degree that refined foods contribute to obesity, as demonstrated in our study, they may play a significant contributing role to other behavioral and cognitive disorders. a b s t r a c t a r t i c l e i n f o

Persistent effects of obesity: a neuroplasticity hypothesis

“…Female rats given a "junk-food" high-carbohydrate, low-protein diet had lower break points in an operant task for sucrose reward that persisted for 9 days after the diet was withdrawn. 193 Mice maintained on a HFD spent less time in the open arm of an elevated plus maze (interpreted as a high anxiety score), and this was exacerbated after withdrawal from a HFD. 11 However, rats put on a long-term monounsaturated or saturated fat diet did not show increased anxietylike behavior, 194 and these rats did not gain weight.…”

Concepts for further sustainable production of foods

“…Besides, refining alters bioavailability of macronutrients, affects taste and is suggested to stimulated food-addictive behaviours. Although not conclusive, human as well as rodent studies suggest adverse health effects of refined diets compared to comparable unrefined diets (Blaisdell et al, 2014;Kristensen et al, 2012). Supplementation of micronutrients through addition of chemically produced alternatives in many cases does not seem to be an efficient solution.…”

The role of a high-sugar refined diet on food and energy intake, body weight, exercise, and motivation in rats

“…We also assessed changes in motivation using the progressive ratio 5 (PR5) procedure of Blaisdell et al (2014) [3]. Prior to being placed in the running wheel cages, and while all 8 rats were consuming a REF diet on a restricted feed schedule as implemented in Blaisdell et al, rats received one session of operant lever pressing in which they had to make an increasingly greater number of lever presses to earn each subsequent sucrose reward.…”