Oh this can be a contentious topic, so I'm hoping to present some facts here :) At the end of the day everyone is different, so it's important you find what's right for you with some experimentation!
The ideal intake of fat is 20-25% of your total calorie intake, the upper limit is 35%. If too much fat is eaten this reduces the amount of your daily calorie intake that’s available for carbs - which are the preferred source for energy and recovery. This fat intake should be enough to provide essential fatty acids and to transport fat soluble vitamins. You should aim for no more than 11% from saturated fat and 2% from trans fats, as these increase the risks of heart disease.
You can choose better fats;
Use low fat spreads and butter sparingly
Choose fats that contain olive oils and avoid hydrogenated vegetable oils
Avocado instead of fat spreads on bread
Cooking oils; walnut oil, rapeseed oil, groundnut oil, sunflower and corn oil
Add nuts and seeds
1-2 handful sized portions a day of mackerel, sardines, salmon and fresh tuna
Fat as a fuel for exercise
Carbohydrates and fats work together to provide energy to the body, depending on intensity and duration of activity, your aerobic capacity and fitness, overall diet, and how much of a carbohydrate store you have in your body (in your blood and liver) before and during exercise.
Generally the less intense and longer the exercise the more the body will move to burning fat. Fat use is not directly linked to immediate energy demands, unlike carbs, this is because there is a complex 6 stage process to break down fat into energy usable by muscles. If you have limited stores of carbohydrates it can limit how long you can sustain a certain intensity of activity
How can fat help?
If you are burning fat you can conserve muscle glycogen to perform for longer
You can use nutritional strategies to increase production of fatty acids during exercise and making it available to muscles for conversion to fuel that muscles can use. This would slow down glycogen store depletion and improve endurance
Can high fat, low carb diet improve endurance? Sadly it’s not quite that simple!
Exercise and fasting are similar, as they lead to a drop in insulin levels. So it might not be a good idea to combine the two and lead to a bigger insulin drop. Fasting means glycogen stores are low, so you have to exercise at lower intensity or feel fatigue earlier. It can compromise the outcomes of your training.
High fat low carb strategies;
Short term high fat, low carb works in one way, but fails in other
Over 4-5 days more fatty acids are available, but glycogen stores reduce, meaning more fatigue and reduction in performance
Fat loading with a 60-65% fat diet; studies showed fat use increased and glycogen use decreased, but performance measurements showed no benefit and potentially negative performance impacts
High fat diets for over 4 weeks, long term performance and adaptation to training are reduced. They can also feel uncomfortable on the digestion.