Not all fats are bad. Diets high in monounsaturated fats or omega-3 fatty acids are associated with lower risks of heart disease and other health problems. The body needs certain types of fat to function. An ultra-low-fat diet (providing less than 10% of calories from fat) may cause a deficiency of essential fatty acids (EFAs), which are essential for the structure and function of the body’s cell membranes and many other other important functions. Low-fat diets, especially when most animal products are avoided, may lack good sources of vitamins E and B12 and zinc. With too little fat in the diet, the body may not properly absorb fat-soluble nutrients, such as vitamin A, vitamin D, vitamin E, and vitamin K, and carotenoids, which are essential for the health of the eyes, skin, immune system, bones and teeth. In addition, some low-fat diets are also extremely low in calories, which could lead to further nutrient deficiencies and other problems if followed for a long time.

A low-fat diet is usually high in carbohydrates. Some critics contend that the high amount of carbohydrates in typical low-fat diets is unnatural for humans, who evolved for hundreds of thousands of years while eating a low-carbohydrate diet. They say that the current overconsumption of carbohydrates has led to increasing problems with obesity, diabetes, and other health problems. The consumption of high-carbohydrate diets is presumed to result in insulin resistance and related metabolic disorders such as high triglycerides, low HDL-cholesterol levels, and high blood pressure. However, not all carbohydrate sources contribute equally to these problems, especially those from whole unprocessed foods and/or those with a low glycaemic index.