Different zones, different fuel.

I often explain to athletes that the fuel you need before and during a workout is dependent on how intense the workout is going to be.

In simple terms, if the workout is long, slow, distance (up to Zone 2) you’re fine heading out on the road in a fasted state and sipping on water throughout the first hour of the workout and then topping up with some energy drink. there’s no need for solid food on slow workouts under four hours long. Fat will be the fuel source. If the planned workout involves portions of Zone 3 or even higher you will need some [ideally low GI] breakfast similar to race day… oats, muesli, toast and honey, etc. In addition, you’ll need to top up by sipping energy drink throughout the workout because you’re working a different energy system and fat just won’t cut it… you need sugars to avoid a latent hangover.

The training programmes I prescribe are based upon ‘periodisation’ towards a key event – not an uncommon approach by any means. Each phase of the programme – as well as each workout within it – contains varying intensities and energy systems to prepare the athlete for a specific output on race day. Typically, the first period of [say] a 24-week programme is around 12 weeks and aims to build a ‘base’ fitness. Using the construction of a pyramid as a visual aid, your base fitness is the wide part at the bottom of the pyramid about one third the height… if your base isn’t good, the rest of the structure will find it pretty hard to stand up.

Although most programmes contain workouts in a variety of zones, the Base Phase is predominantly (80%) Zone 2 workouts… for very good reason, which you are about to find out. Each of the zones we train in (1-6 in increasing intensities for the sake of discussion) aims to elicit a certain physiological and metabolic response, but knowing what each of these responses is will help you understand what the different training zones are for.

In a 2014 article, Dr. Iñigo San Millán, Ph.D explained his wonderful findings not only on the importance of Zone 2 training, but also how different skeletal muscle fibers utilise different types of fuel.

If I can compare Inigo’s description to that of a motor vehicle: the efficiency of an athlete can be measured by how well they can transfer chemical energy into mechanical output. He goes on to describe a “molecular unit of currency” which is the chemical ATP or Adenosine Triphosphate. To continue my car analogy, if your breakfast is the petrol, ATP would be the spark plugs. Okay, I’m getting carried away and we haven’t even started talking about power meters!

“Unfortunately, many novice or young athletes barely train or are prescribed zone 2 training and therefore don’t develop a good “base”, thinking that the only way to get faster is by always training fast. By doing this they won’t improve nearly as much as if they trained zone 2 in large amounts.”

Dr. Iñigo San Millán, Ph.D

Basically, skeletal muscles need a constant supply of ATP to keep contracting during exercise which means humans have to make it. The two main methods for generating ATP are words you’ve probably heard before: aerobic and anaerobic, which correlate to the two main substrates of fats and carbohydrates… with just a little protein. Fat and carbs are stored in different places in the body, which means they are utilised at different times.

Most exercise intensities [zones] will use the aerobic system for generating ATP, using both fat and carbs; although up to around 75% of Vo2 Max only a small amount of carbs are used. Above this level, when ATP generation needs to be quicker – and fat just can’t be used quick enough – carbs are the predominant fuel source to get those spark plugs… well… erm.. sparking I suppose. Basically, going slow (but not TOO slow) lets your body use fat as a fuel source, and as the intensity increases so does the need for carbs.

“For the past 18 years working with professional and elite endurance athletes like cyclists, runners, triathletes, swimmers and rowers I have been able to see that zone 2 training is absolutely essential to improve performance.”

Dr. Iñigo San Millán, Ph.D

Put another way (still with me??) the body also has two types of muscle fibres: Slow Twitch and Fast Twitch, called Types I and II respectively. Type II also has some sub-types and the combinations thereof pretty much adds up to 6 levels of intensity… hence six zones. No surprise that the Slow Twitch fibres are the first to be used when we start exercising and guess what… the more intense the workout, the more the Fast Twitch fibres are used. Just as the two types of ATP generation (aerobic and anaerobic) these fibres are good at either burning fat or carbs. Type II fibres are great at holding and using glycogen for instant hard bursts, wheres Type I do well utilising fats.

So… to summarise… different exercise intensity levels will call on different methods for creating the mechanical output i.e. your limbs moving. For zone 1 you will be using almost all fats. Zones 2 and 3 will use combinations of fats and carbs but the notable point here is that Zone 2 will still only use Type I muscle fibres (remember the fat fuel??). Zone 4 and above will use carbs plain and simple.

“Carbohydrates are the most performance enhancing thing you can use”.

Tejay Van Garderen

By way of signing off… a reminder… if you have high intensity (Zone 3 +) workouts planned, you need to be taking on some carbs before, during and just after. If you are an athlete hell-bent on keto or paleo diets – or anything that is going to reduce the amount of sugars in your system – you MUST be addressing the fact that if you want to train hard you may well fall short, feel crappy and not perform or recover well.