Why does dietary fat cause weight gain?

It is important to include fat in the diet, however people eating a lot of high fat foods will gain weight. When looking at total consumption for the day, generally keeping fat intake to 30% will be a good balance for weight maintenance. When trying to lose weight staying between 20% & 25% fat intake will help keep the calories down. 

It is also important to choose most of your fats from the mono & poly-unsaturated fats. These "healthy fats" are the plant based fats, such as nuts, olive oil, avocado, peanut butter, etc. Saturated fats should be limited, these "unhealthy fats" are the animal-based fats that are found in meats, butter, cheese, & dairy that is not fat-free. Transfat should be avoided entirely.

Weight gain is a product of consuming more energy (calories) through food & beverages than you expend through the combination of what it takes to sustain the very basic functions of life (resting metabolism), metabolizing the food you ingest (thermic effect of food), and what you burn in daily activities (exercise + daily activities) also known as your exercise metabolic rate. A person can gain weight anytime he or she consumes more calories than needed to fuel these 3 things, (regardless of if they come from dietary fat, carbohydrates, protein, alcohol, or a mixture of these nutrients).

The reason fat is often blamed for weight gain is because every gram of fat contributes 9 kilocalories of energy, while sources of either carbohydrate or protein contribute only 4 kilocalories of energy per gram. Fat is more calorically dense, so it adds up quicker in the calorie department. 

Remember there are different types of fat and some is better than others. We need fat in our diets for many reasons; however, it is recommended that no more than 30% of your total daily caloric intake comes from dietary fat. Of that 30%, no more than 7-10% should be from saturated fat, as it has been associated with many negative health risks. Consuming too many carbohydrates or protein can also lead to weight gain because amounts beyond what the body needs will simply be stored as fat. 

The main reason that dietary fat may promote weight gain is because it is more Calorically-dense than protein or carbohydrates. Both protein and carbs have 4 Calories per gram, but fat has 9 Calories per gram (2.25 times more!). So, if you tend to eat a lot of high-fat foods, you are also eating a lot of Calories in a small amount (weight or volume) of food. If you were to replace the fat with a nutrient-dense food, such as vegetables, a broth-based soup, or fresh fruit, you could eat a much greater volume of food for the same or an even fewer amount of Calories.

You will gain weight whenever you are ingesting more calories through food than you expend in exercise. At 9 calories per gram, fat contributes more calories to your diet faster than either protein or carbohydrate. As a very practical example, 2 pats of butter contain more calories than the baked potato that you will put it on.

When you are looking for strategies to manage your weight, keeping a food diary and monitoring your fat intake is a good first step. But the fact remains that we need some fat daily to stay healthy. Schedule a visit with a Registered Dietitian to develop a meal plan that meets your specific needs.

Fat is higher in calories per gram than the other 2 macronutrients (Carbohydrates = 4 calories per gram, Protein = 4 calories per gram, Fat = 9 calories per gram). So, a smaller portion of a food high in fat generally packs more calories than the same size portion of a food that is primarily made up of carbohydrates or protein. Don't avoid fat completely, just watch portion sizes if you are watching your weight. Also, the kind of fat matters as well. Plant based fats (olive oil, nuts, avocado) generally provide more nutritional benefits than animal based fats (cheese, cream, beef).