A Long Term High-Fat Diet Is Worse For Your Health Than You Think

A high-fat diet has been often linked to diabetes, heart disease, and certain types of cancers, but research has found that these issues are more accurately linked depending on the different types of fat a person consumes (per Harvard School of Public Health). According to a 2022 study in Metabolic Brain Disease, a high-fat diet might also play a role in developing Alzheimer's disease. The study compared the effects of a 30-week high-fat diet versus a standard diet in mice. The researchers found that the mice with an Alzheimer's-related gene who ate the high-fat diet began to experience brain decline associated with Alzheimer's disease. The mice also gained weight and developed symptoms of type 2 diabetes.

The researchers believed that the brain changes from the high-fat diet not only contributed to weight gain, but also caused the mice to behave differently, according to a press release about the study. "Obesity and diabetes impair the central nervous system, exacerbating psychiatric disorders and cognitive decline," said one of the study's authors, associate professor Larisa Bobrovskaya at the University of South Australia. Other studies have also looked at the connection between cognitive health and a high-fat diet.

The Metabolic Brain Disease study isn't the first to find a connection between a high-fat diet and our cognition. A 1997 study in The American Journal of Clinical Nutrition measured the diets of a sample of older adults for seven days. The adults with good cognitive functioning had lower fat and cholesterol, and ate more carbohydrates and fiber. Other studies found similar results with adults aged 20-59. A 2013 study in Nutritional Neuroscience had people remember what they ate the previous day. The higher the consumption of polyunsaturated fats and overall calories, the poorer the performance on learning and memory tests.

A 2014 review in Nutritional Neuroscience found that a diet high in saturated fats and trans fats — the typical Western diet — can contribute to declines in our thinking. The researchers said that because high-fat diets can increase insulin resistance, free radicals, and inflammation, they can change how our brain functions. A 2015 review in Physiology & Behavior found that mothers can pass on these cognitive changes to their children. While this is certainly cause for concern, that doesn't mean you should cut all fat out of your diet. 

Not all high-fat diets are bad. What matters is the kinds of fat you're eating. A 2014 study in Metabolism found that when the proportion of calories of fat was balanced equally between saturated, monounsaturated, and polyunsaturated, obese premenopausal women saw increases in fat oxidation and decreases in body composition and inflammation.

A 2020 study in Nutrition & Metabolism had people follow either a low-carb or a low-fat diet for eight weeks. The low-carb diet limited saturated fats (such as red meat) to 10% of the overall calories. The majority of the fats came from monounsaturated fats, coconut oil, and nuts. Compared to those following the low-fat diet, the low-carb dieters increased their good cholesterol levels and improved the body's ability to process blood glucose.

According to Healthline, very low-carb diets like the ketogenic diet will increase your caloric fat intake by up to 70%. These diets have been found to moderate blood sugar and delay the progression of Alzheimer's disease. However, the diet limits unhealthy, processed fats, opting instead for high-fat dairy and meats.

Harvard Health Publishing warns that because the results of high-fat, low-carb diets have been mixed, it's best to keep saturated fat levels to less than 7% of your daily calories. Harvard Health Publishing also warns of the effects of these diets for long-term use. Because the brain needs carbohydrates to function, reducing carbs to the point of ketosis can impair your thinking.