Good news: eating heart healthy fats also improves your cognitive health and prevent mental decline. Without healthy fats (polyunsaturated and monounsaturated) in your diet, your brain cannot work at its full potential and even a subtle decline in mental health can increase your risk for more serious health issues such as dementia and Alzheimer's in the future.
Of the 50% fat content in your brain, about one-third is omega-3 polyunsaturated fat, predominantly DHA which is found in fish oil and helps make EPA, another omega-3 polyunsaturated fat also found in fish oil. DHA and EPA are essential fatty acids—your body cannot synthesize them so you need to consume them, particularly DHA since it's needed to make EPA.
According to a 2013 study published in "The Journal of Nutritional Biochemistry," omega-3 fatty acids do this via several different pathways which makes sense seeing as they make up such as large portion of our brains. The polyunsaturated fats, especially DHA, may also help reduce developing depression. So a deficiency in these fats may actually affect your mood, memory and thinking skills now and as you age.
The best way to get DHA is by eating fish twice a week as recommended by the American Heart Association. DHA is present in smaller amounts in other foods including eggs, meats, nuts and seeds, whole grains and dark green leafy vegetables. You can also eat other foods that are high in healthy fats in order to get another fatty acid needed to make DHA, however, you would need a lot of this fatty acid to make a sufficient amount so your best bet is still to eat fish twice a week. Some of the best foods for making DHA include canola and cold-pressed flaxseed oil, chia seeds, walnuts, almonds, pumpkin seeds and avocados.
Studies are lacking on the effectiveness of taking fish oil supplements for improved cognitive health, dementia prevention and Alzheimer’s Disease. The National Institutes of Health has not made any recommendation for supplements in pill form but if you are concerned that you might be deficient in DHA and EPA, talk to your doctor about taking a fish oil supplement.
Wouldn't it be nice if you could improve your thinking skills and memory by means of your diet? Well, a study at Brigham and Women's Hospital in Boston that was published in 2012 suggests just that by replacing saturated and trans fats in your diet with unsaturated fats, particularly monounsaturated fats. Research was conducted over a course of 4 years that included 6,000 women aged over 65. The results showed that women who ate more monounsaturated fats such as olive oil, scored better on cognitive function tests including memory, than women who ate more saturated fats that are animal fats. Researchers also found that the amount of fat did not matter as much as the type of fat. Since declining mental function can increase your risk for dementia and Alzheimer's disease, prevention of these more serious health issues could be as simple as switching the saturated fats for unsaturated fats in your diet.
Some good sources of monounsaturated fats include natural nut butters and nuts especially macadamia, hazelnuts, almonds and pecans, fish such as herring and halibut, avocados, peanut oil, sesame seed oil, soy bean oil, olive oil, corn oil and canola oil. Try also switching your milk to unsweetened almond or soy milk and your butter to a non-hydrogenated vegetable oil spread with olive and canola oil.
 Brigham and Women's Hospital. With Fat: What’s Good or Bad for the Heart, May Be the Same for the Brain. (2012). Retrieved Feb 9, 2015 from: http://www.brighamandwomens.org/about_bwh/publicaffairs/news/pressreleases/PressRelease.aspx?PageId=1161
 Parletta N., Milte C.M., Meyer, B.J. (2013). Nutritional modulation of cognitive function and mental health. The Journal of Nutritional Biochemistry, 24(5), 725-743. Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/23517914
 The American Heart Association. Fish and Omega-3 Fatty Acids. Retrieved Feb 9, 2015 from: http://www.heart.org/HEARTORG/GettingHealthy/NutritionCenter/HealthyDietGoals/Fish-and-Omega-3-Fatty-Acids_UCM_303248_Article.jsp