Open any magazine, read any running blog or listen to any sports podcast and you’ll probably come across the term “fat adaptation”. However, how many of us really understand and know what this popular term means? And does becoming fat-adapted benefit or hinder our performance?
Simply put, “fat adaptation” describes the process of our bodies becoming better trained to use our fat stores even when we are working at a higher intensity. This, in turn, means that our glycogen stores and carbohydrate fuel source can be spared, resulting in us going faster for longer during endurance events. Remember that, for most of us, full glycogen stores will fuel high-intensity work for up to 90 minutes. The theory suggests that by becoming fat-adapted, you can spare these glycogen stores by using a higher percentage of fat as fuel, alongside carbohydrate, and prolong the decline in glycogen stores significantly.
Scientifically, though, what do we know? We know that our body actually uses a mix of both fat and carbohydrate for fuel; relying more on fat stored in the muscle in low to moderate intensity training. We also know from numerous studies over the past 20 years that, as exercise intensity goes above 65% of you maximum, the body needs carbohydrate in order to be able to maintain these faster paces. However, this doesn’t mean that we suddenly stop using fat for fuel. This will also continue but the body will rely on carbohydrate, which can be broken down into glucose and delivered to the working muscles quickly and efficiently.
Contrary to what you may have read, it is endurance training and SOME manipulation of your diet that actually improves your ability to oxidise fat for fuel, NOT just removing carbohydrate from your diet.
Ketogenic diets, where carbohydrate intakes are kept below 50g of carbohydrate a day and the diet is predominantly fat and protein based, have become very popular particularly in ultra running, with many high-profile athletes advocating its performance benefits. They are also popular from a weight-loss perspective.
Professionally I do not advocate the use of Ketogenic diets and feel very strongly that such a regime should not be sustained for long periods of time. It is still a relatively new area of sports nutrition and so long-term studies and the effects on the body are not understood. However, we do know that insufficient carbohydrate intakes in athletes can lead to a depressed immune system, over-reaching and over-training syndrome, as well as changes to hormone and bone health. From a weight-loss perspective, looking at the literature, it is apparent that this type of diet was rarely complied with for more than 12 months. Similarly, it was also noted that it was not the composition of the diet that caused weight loss but the overall energy consumption – it is still possible to achieve weight loss while eating carbohydrate as long as energy intake is lower than energy expenditure!
‘Train low, compete high’
I tend to recommend a more periodised approach of ‘training low and competing high’ in the elite ultra athletes I work with. This means that they still choose to make high-carbohydrate choices around high-intensity training sessions and races, to ensure that they hit target paces necessary for progression. However, during long endurance training sessions, at a low to moderate intensity, they avoid carbohydrate before and/or during to ensure that they use fat stores to fuel that session. Remember we know the body will already use a higher proportion of fat for fuel at these intensities; by training at these paces in a carbohydrate-depleted state, the body will be even more reliable on this fat as fuel. With time, training in this way means that the body gets better at using a higher percentage of fat as fuel. So even in high-intensity sessions or races, although the body will still rely on carbs for fuel, by being able to burn a high percentage of fat too, you can make your carbohydrate stores last a lot longer.
Practically, this means aiming to do these low-intensity/moderate-intensity runs in a fasted state or avoiding carbs for up to four hours prior. During the session, fluid should be drunk and possibly electrolytes if it is very warm; energy will be in the form of fat or protein – good options include salted peanuts and beef jerky.
However, it is important to stress here that firstly these studies have been done on well-trained athletes; I would not recommend trying this if you are new to endurance sport. Secondly, even in highly trained athletes, I would recommend NO MORE THAN 3 fasted or depleted sessions a week in order to prevent any issues with a depressed immune system or increased injury risk.
If this is something that you want to try but have never done before, remember it has to be a low to moderate intensity run. Start short – maybe 30-40 minutes – and build up the time running fasted or depleted slowly.
Renee McGregor is a sports nutritionist the author of Training Food