The Fat Max Test and relevance of keeping a food diary

Science around how to best prepare for an endurance even is evolving, and increasingly highlighting the importance of the right nutrition. Nutrition plays a key role in developing the body’s fat burning metabolism (discussed in the previous post) in support of running efficiency. What gets measured, improves, so first step is to assess the current crossover point, and assessing diet through keeping a food journal. In this article we delve into establishing this baseline. 

In preparation for my first endurance event in a long time, the Wild Coast 3-day trail run, I’m looking into how I should train and how I should plan my nutrition to be best prepared for the long hours on my legs. As it appears this type of long-distance events make the ability of using fat for fuel more important, my aim is to become a better runner on fat (read about why burning fat for fuel is important in the previous post). Following my meeting with Adrian Penzhorn, registered dietician and sports nutrition coach, the first step is to determine my baseline. This is done through a Fat Max Test, to establish where I can improve and which changes I should make in my training as well as my diet in order to maximise my running efficiency. Secondly, a food journal should give insights into current diet.

Keeping a food diary to reveal any patterns in your nutrition that aren’t necessarily good for your health or performance can be incredibly useful to make changes to your diet. I’d never done it though, and I realise now how valuable the exercise actually is. It’s a little bit intimidating to show someone what you eat throughout the day, however it’s like with any profession: a dietician has seen it all so don’t feel put off by that thought. I kept a food diary using the Food For Sport template so that Adrian Penzhorn, as a sports nutrition coach, would be able to give me recommendations on my diet to make it more conducive to my training programme.

Adrian also introduced me to David Leith, Sports Performance Biokineticis at the Sports Science Institute of South Africa, to assess my current fat burning capacity. The test lasts one hour, and starts with a basic body composition assessment measuring weight, length, and body fat percentage. Then the real work begins, and I have to wear a heart rate strap and a face mask that is linked to gas analysis equipment, which allows David to assess my breath-by-breath oxygen consumption, carbon dioxide production and heart rate. The test starts out slowly, running 6km/h for 3 minutes, and increasing the speed by 1km/hr every 3 minutes until my ratio of oxygen to carbon dioxide reached the point where only carbohydrate is theoretically being metabolised.

What follows is a test report that brings the theory of metabolic efficiency as explained to me by Adrian to life with graphs of my fat and carb oxidation, at different intensity levels.

The aim of this test was to establish at what intensity my fat oxidation is highest and for how long fat remained the primary fuel source. This is important information as this would indicate my crossover point that Adrian had explained to me.

Although the differences in athletes can be large, even between athletes with similar abilities, on average maximal fat oxidation rates in athletes have been found to be approximately 0.59g/min, at a relative intensity of 49% of VO2max. When athletes are more experienced, or better trained, better results can be achieved, and athletes with a well-developed aerobic system will achieve even higher maximal fat oxidation rates, up to 1.3-1.5g/min. This the result of both diet and training.

My test results showed that with 0.73 g/min I’m above the average athlete in terms of my maximal fat oxidation, with still some room for improvement to get to that top level of performance. To further improve my fat burning capacity, David Leith suggests to incorporate long slow runs into my training plan at a ceiling speed of 10km/hr, and ceiling heart rate of 134 bpm (based on the test results at this heart rate I’m still predominantly using fat). This type of training tied into what I learned about how to create more and better mitochondria to improve metabolic flexibility.

The good news is that my Fat Max occurred at 55% of my maximal oxygen consumption reached during the test, and it remained high up until 83% of my VO2peak and 88% of my peak heart rate recorded. That was good news for me, as it’s another indicator that my body is able to burn a relatively high ratio of fat at high relative intensities. This is an advantage, as it indicates that I should be able to derive a significant portion of my energy required from my body's fat stores, reducing my reliance on fuel intake during long runs. To further shift this level, David also recommends to incorporate training sessions during which limited glycogen is available, through fasted sessions, basically long runs without taking gels, sports drinks, etc, and by not consuming carbs immediately after a long run.

Even though the test results weren’t bad, I was keen to get insights from Adrian to get his view on the report, especially in light of the food log that he analysed. As mentioned before I only started realising now how valuable a food journal can be. You get so used to your ‘normal’ diet, that you easily overlook where in terms of macro nutrients you may miss the boat. Adrian’s focus was on these macro nutrients: carbs, protein and fat. Key in his observations to me were that my overall intake was a little too low, and where is was on par I wasn’t necessarily consuming enough carbs to meet my energy needs. At the same time, my protein intake also appeared on the low side.

I’ve become a more conscious meat eater, or call it flexitarian, a few years ago and haven’t been paying much attention as to what the impact of this change in lifestyle would be on my protein intake. As protein is often being associated with building muscle, and weight lifting, it wasn’t so important to me, right?

I couldn’t be further from the truth, making this nutritional analysis so much more valuable. Adrian explained that for a sedentary woman 0.8g of protein per kg body weight is the recommended daily allowance (RDA), and for an active woman, as part a training plan I should be aiming for 1.6g per kg. It’s been interesting to do the math, and look at protein intake throughout the day to make sure my intake meets the daily requirements to ensure good recovery after my training runs. Meat easily adds sufficient protein, but when I have my veggie stir-fry for lunch, what’s my protein intake? And my green smoothie packed with spinach, banana, apple, some soy milk, a few nuts: does it provide sufficient fuel?

Next: 5 best sources of protein and why they are important for everyone 

Previous posts:

What I didn't know about training for an endurance event

The role of nutrition in endurance sports