We all need some fat in our diet. However, in more economically developed countries there is concern that people are eating too much fat, in particular eating too much saturated fat.
Eating too much fat can make us more likely to put on weight. This is because foods that are high in fat are high in energy too. Obesity develops if you combine eating too many energy-rich foods with a lack of exercise.
A lack of fat in the diet over time will result in weight loss. Omega 3 fatty acids provide protection from heart disease and this would be lost. There could also be deficiencies of the fat-soluble vitamins A, D, E and K.