The Big Fat Chat – Part 1

Fat has been blamed for a variety of health issues over the years and unfortunately the low-fat myth still persists. This is a misguided and outdated view, mostly based on the fact that fat is more calorie dense and easily eaten to excess. However, if you want to optimise your health and performance it’s essential to include enough good quality fat in your diet.

As this is going to be a big chat about fat, I suggest you grab a snack and get comfy in your favourite chair so you can last the two-part distance.

You need fat for two main reasons:

  1. Body fat is essential for critical functions such as providing protection for your organs, forming building blocks for cell membranes and nerve sheaths, producing a range of hormones and assisting with internal temperature regulation. Your body fat is active, living tissue – even excess body fat is active and grows its own blood supply (that’s one of the reasons it can contribute to chronic health ailments such as heart disease and cancer).
  2. Dietary fat provides a source of high-density energy and specific nutrients that your body can’t make for itself, it delays gastric emptying (ie. how quickly food moves through your stomach), provides a feeling of fullness after eating, and it enhances the absorption of some key nutrients (eg. vitamins A, D, E, K and beta carotene).

Athletes can become obsessed with being lean – however being too lean can compromise both your performance and your health. A very low fat diet providing less than 20% of energy from dietary fats reduces the intake of essential vitamins and fatty acids. If you have low body fat, and also restrict your dietary fat intake, you won’t produce enough of the key steroid hormones that govern fertility and muscle building (testosterone and oestrogen). For females, this can lead to loss of menstrual cycle and increased risk of stress fractures and poor bone health in later life. For men, this can impact muscle gains, bone health and reduce fertility. Steroids are also important for other essential processes such as blood pressure, the stress response and the inflammatory healing response.

Fat as Fuel

Fat is stored in various places throughout the body. As an athlete, intramuscular fat is of specific importance – this is the fat stored within muscle tissue and used for energy during exercise. However, this energy source isn’t accessed until you’ve done at least 60 minutes of aerobic intensity exercise. Endurance training teaches your muscles to become more efficient at utilising fat for energy, and your muscles will begin to store more fat to meet these energy demands over time. A secondary source of fat for fuel is the fat tissue stored around your body. Some of this fat is circulating in your blood stream all the time as free fatty acids, whether you’re active or sitting on the couch bingeing on your favourite Netflix series.

Your body can only use fat for fuel in the presence of oxygen – for that reason the preferred fat burning zone is the aerobic (low to moderate) intensity exercise zone. Your body is skilled at being efficient, so initially it will choose to burn the free roaming fatty acids in your blood stream. As the duration of your exercise increases, the fat fuel source will shift from free fatty acids to intramuscular fats. During longer exercise bouts your body will eventually switch back to accessing free fatty acids.

Fat burning is reliant on several factors:

  • First, the digestion and assimilation of fats from the foods we eat is slow, unlike carbohydrates which can be eaten and utilised during an exercise session.
  • Second, different muscle types have different fuel preferences. Low to moderate exercise relies on slow twitch muscle fibres (type I) – these contain more fat burning enzymes. Conversely high intensity and power exercise relies on fast twitch muscle fibres (type II) – these contain mostly carbohydrate burning enzymes and less fat burning enzymes.
  • Third, fatty acids undergo a complex process to be turned into usable energy and this process requires co-factors such as carnitine.
  • Fourth, if your exercise intensity goes above the aerobic threshold, your body will use less fat for fuel, and it will instead begin seeking more glycogen from carbohydrates to maintain pace. This can be one reason for ‘bonking’ on a long training ride with mixed intensities or insufficient carbohydrate intake.

Fat and Performance

Quite simply, the more you train at aerobic intensities, the better your body will become at burning fat for fuel. The conversion of fuel into energy occurs within cells called the mitochondria. As you adapt to larger training loads, your body produces more mitochondria and fat burning enzymes – this improves your ability to burn fat and conserve glycogen. Highly trained athletes can teach their bodies to burn fat more efficiently, and at higher intensities, for longer.

There’s an interesting difference in the fat burning capacities between men and women. Both men and women burn fat for fuel better at intensities higher than 55% VO2 max for running. But here’s the kicker – women burn fat better than men at higher intensities for cycling as well. By comparison, men tend to burn carbohydrates better than women for fuel. One reason for this could be the differences in muscle fibres between the sexes – men have more type II fibres and women have more type I, and as I mentioned above, type I muscle fibres have more fat-burning enzymes than type II.

From the research I’ve looked at, the optimal fat burning zone for both sexes appears to be 65-75% of VO2 max.

So now I guess you’re wondering:

  1. Is it better to exercise in a carbohydrate depleted state to improve my fat burning efficiency?
  2. Will a higher fat diet improve my endurance performance?
  3. What are the best fats to eat?
  4. When is the best time to eat fats?

Training on empty

Training in a fasted state has performance drawbacks, and doesn’t improve your fat burning capacity. The most common situation would be doing a workout session early morning before breakfast. Training on an empty stomach without any pre-exercise fuel will inevitably lead to reduced power, stamina and early fatigue, as your stored glycogen reserves will quickly run out. For any training sessions lasting longer than 60 minutes, especially with higher intensities, you should have a pre-exercise carbohydrate snack if you haven’t had a meal within the previous 2 hours.

There is research to suggest that endurance training on low carbohydrate stores will promote adaptations in your body such as fat burning – however there is the potential for adverse adaptations as well, such as increased stress hormones and reduced immunity.

If your primary aim is to lose excess weight and to be able to complete endurance events, then training at lower intensities and teaching your body to become fat adapted with a high fat – low carbohydrate diet can produce good results. For the athlete seeking high performance training and racing improvements, this kind of diet has drawbacks such as reduced ability to train at higher intensities and produce explosive power.

Eating fat to go faster for further

There’s been a lot of hype recently about the low carbohydrate-high fat diet. This is sometimes referred to as the ‘keto diet’, because the body changes the way it produces energy for fuel by way of a process called ketosis. Glucose is the primary energy source for the cells in your body, but when this isn’t readily available you can make energy from fat and protein. The body can turn fats into ketones and via the process of ketosis these become fuel for energy. This an extreme situation for the body – you are effectively putting your body into a fasting state. As a clinical tool, the ketogenic diet is used with some success as a treatment tool for some conditions such as epilepsy, brain tumours and neurodegenerative diseases, as it can help to protect brain cells.

But will eating more fat improve your ability to burn fat – and if so, will it allow you to go faster for further?

Sports scientists have been studying fat loading since the 1990’s yet the results from studies done on athletes is still mixed. There are some noticeable health benefits, such as enhanced body composition and overall fat burning capacity, which is good if fat loss is a significant health goal for an athlete. However, when it comes to actual performance measures such as training, recovery and time trials, the evidence I’ve seen is conflicted. While the body’s ability to burn fat for fuel improves on the diet, this doesn’t seem to translate to consistently improved endurance capacity. Also, there is a decrease in power, especially for short sprint intervals, as glycogen isn’t readily available on this diet, and the body’s ability to burn carbohydrates for fuel is affected. These studies are short term, only 4 – 12 weeks in duration, so there is no data on the long-term performance pros and cons of this diet.

The keto diet is also very restrictive. If you think it’s a free licence to eat bacon and eggs for breakfast and cheese-topped mince pies for lunch, you’ll be in for a rude shock. To get the benefits of ketosis, you have to follow a very strict and limited eating plan. If you don’t, your body won’t adapt and you’ll be putting your energy systems under a lot of undue stress.

Join me in Part 2 where I discuss the best types of fats to include in your diet with some tips on timing.


As this isn’t a thesis, I won’t list all my resources. Here are some key references that may be of interest to those of you who want more information:

Nutrient Timing for Peak Performance, H. Skolnik & A. Chernus, Human Kinetics, 2010

Clinical Nutrition: a functional approach (2nd ed), The Institute for Functional Medicine, 2004

Fat Oxidation in Men and Women Endurance Athletes in Running and Cycling, Int J Sports Med 2004; 25(1): 38-44

Maximal Fat Oxidation is Related to Performance in an Ironman Triathlon, Int J Sports Med. 2017 Nov;38(13):975-982.

Understanding the factors that effect maximal fat oxidation, J Int Soc Sports Nutr., 2018 Jan 12;15:3.

Carbohydrate Availability and Physical Performance: Physiological Overview and Practical Recommendations, Nutrients, 2019 May; 11(5): 1084.

Nutritional Ketosis Alters Fuel Preference and Thereby Endurance Performance in Athletes, Cell Metab. 2016 Aug 9;24(2):256-68.

Ketone Bodies and Exercise Performance: The Next Magic Bullet or Merely Hype? Sports Med. 2017 Mar;47(3):383-391.

Effects of Ketone Bodies on Endurance Exercise, Curr Sports Med Rep. 2018 Dec;17(12):444-453.

Keto-Adaptation and Endurance Exercise Capacity, Fatigue Recovery, and Exercise-Induced Muscle and Organ Damage Prevention: A Narrative Review, Sihui Ma and Katsuhiko Suzuki, Sports (Basel), 2019 Feb; 7(2): 40

Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations, Kenneth Vitale and Andrew Getzin, Nutrients. 2019 Jun: 11(6): 1289

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