Our study of long-term use of high fat diet that lead to mild ketonemia in Wistar rats showed improvements in learning and memory, as well as larger hippocampi and higher concentrations of hippocampi tNAA (marker of neuronal viability), tCr (involved in cell energetics), and glutamate concentrations¹. Here, we report results of a DTI study to evaluate white matter structural integrity, as well as to ascertain that group differences in metabolite concentrations cannot be driven by group differences in mean diffusivity (MD).Our study of long-term use of high fat diet that lead to mild ketonemia in Wistar rats showed improvements in learning and memory, as well as larger hippocampi and higher concentrations of tNAA (marker of neuronal viability), tCr (involved in cell energetics) and glutamate (neurotransmitter) concentrations. To valuate, whether the changes in metabolite concentration are associated with potential changes in tissue hydration, we performed diffusion tensor imaging studies. Obesity worldwide has reached epidemic proportions, with more than 400 milion people affected. Human DTI studies have demonstrated lower fractional anisotrophy (FA) and higher mean diffusivity (MD) in obese humans². In animal models, high-fat diet (HFD) is commonly used to induce obesity³. We hypothesized that long-term HFD use in male Wistar rats leads to lower FA and higher MD than in control group. Furthermore, we compared MD within hippocampi between groups.Twenty five male Wistar rats were put on high fat diet (HFD) providing 60% of energy from fat (lard) and 28% from sugars on their 55^th day of life, while 25 control male rats (CON) remained on chow. The diet lead to increased levels of blood ketone bodies. At one year of age, all rats were scanned with DTI. MR measurements were performed on a 7T wide bore (30 cm) Bruker BioSpec at Mossakowski Medical Research Centre, Warsaw, Poland. Diffusion tensor was acquired with TE/TR=33/3750ms, along 72 directions, with resolution 0.156x0.172x0.7mm, no gap. Data were skull stripped and eddy-current corrected with FSL. Then, images were resized to 0.1x0.1x0.1 mm. FA images were than normalized to a study specific template using DARTEL in SPM8; Study-specific template was created by averaging all FA images. These transformations were applied to FA, MD, as well as perpendicular and parallel diffusivities. Images after normalization and smoothing with a smoothing kernel of 0.3mm at FWHM were compared between groups using two-sample t-tests (FWE<0.05, cluster size >27) with SPM8.Right cerebral peduncle contains a region of lower MD in in rats fed with HFD than in CON (p<0.05, FWE), accompanied by increased FA (p<0.001, uncorrected), contrary to our hypotheses (Figure 1). MD is elevated in corpus callosum and fimbria of HFD-fed rats than in CON (Figure 2), as well as in trigeminal nerve (Figure 3), consistent with our hypothesis. These changes were not accompanied by significant FA changes. No differences in parallel and radial diffusivities were noted. Furthermore, there were no differences in hippocampi MD between groups.The results are consistent with our results obtained with structural brain analyses, ¹H MRS, and behavioral tests. They likely reflect the effects of switching the animals to metabolize ketone bodies, as illustrated by increased concentrations of ketone bodies in the blood¹. There are reports that high fat diets in healthy humans are associated with improved lipid profiles and increased insulin sensitivity.^{4, 5} The results of this DTI study are partially consistent with our previous results. Unchanged mean diffusivity within hippocampi allows to imply that the differences in metabolite concentrations reported by us earlier are genuine.