Nutritional Periodization for the Serious Ironman…

Alan Couzens, MS (Sports Science)

Sept 18th, 2014

"Don't get set into one form, adapt it and build your own,
and let it grow, be like water"

- Bruce Lee

In a recent post I outlined the importance of a balanced energy system development for the endurance athlete with respect to aerobic glycolysis and lipolysis and the role that a balanced nutritional approach plays. I suggested that the athlete’s diet should match the demands of their training and that if they are training at intensities requiring aerobic glycolysis, by necessity, they need glycogen to power the training!

I really can't overstate the importance of getting this balance right and the importance of nutrition to the Ironman athlete. As I said in the last post, Ironman is fundamentally a game of a bunch of (similarly) very aerobically fit athletes testing their metabolic fitness head to head. I can attest from many hours spent testing these guys that, at the top level of the sport, both factors must be present for success - high levels of general aerobic fitness (VO2, lactate threshold) and high metabolic efficiency. If you want to race Ironman at the highest level, you had better have a VO2max in the range of 70ml/kg/min (~25kcal TOTAL oxidation per minute) AND you had better have max FAT oxidation rates at or above 7kcal/min. Nutrition plays a key role in the development of each.

Time and again, nutritional studies have clearly shown that you will burn what you eat. Beyond a theoretical construct, this is a thermogenic fact! If you take in 100% fat, the body is going to do everything it can to generate energy from that fat. Similarly, if you take in 100% CHO, the body will do what it can to supply energy needs from that CHO. This pattern scales linearly across the input->output spectrum as this chart from a study by Hurni et al. (1981) shows..

Hurni and colleagues altered the CHO intake of their subjects while living inside a metabolic chamber (basically a giant met cart that has continual collection and analysis of expired gases). What they found was that the CHO/Fat burn of the subjects changed remarkably in accordance with changes in diet. Put more simply – the subjects learned to burn whatever they ate.

The reason for this is simple, the body’s ability to store large amounts of CHO is limited and, unchecked, high levels of sugar (like alcohol) in the bloodstream is a ‘poison’ that the body wants to burn off. Even when it doesn’t need to, the body will preferentially use CHO as a fuel source in order to help to keep blood glucose numbers in check. However, this ‘defense mechanism’ is clearly limited & puts a great hormonal stress on the system (that it eventually gets tired of dealing with and voila you're a type 2 diabetic) Clearly, you don’t want to be forcing your body to ‘do something’ with excess CHO taken in above and beyond the needs of daily activity. In a movement-deprived sociey, this represents a far bigger problem than folks not having enough CHO in their body to outrun a rogue lion or tiger and I understand the push for this movement in the general population. However, for athletes, it’s a little different…

The flipside of the body (only) burning what you eat is that, in the absence of sufficient CHO, your body will simply not ‘go to’ those levels of intensity requiring large amounts of glycolysis to power them. This defense mechanism related to the body preserving the glucose levels needed to ‘power’ the brain. When glucose is low, the body will curtail muscle supply in favor of keeping the brain powered up. This is shown on the chart as the gap between CHO intake and oxidation. The body always 'holds back' some CHO to power the brain.

Put another way, if we flip this chart around, if an athlete has CHO oxidation needs of 50% of their daily expenditure, their diet will need to be ~70% CHO to keep up with this expenditure on an ongoing basis. This has been shown time and again in studies on elite athletes. At this level of output, diets less than 70% CHO consistently result in glycogen depletion and diminished performance, e.g. Bergstrom et al. (1966), Karlsson & Saltin, (1971), Johanssen et al. (1981), Helge et al., (1996). This brings us to the crux of this post:

"In an athlete's diet we want to give them the 'just right' amount of carbs - not so great that the body interprets them as a 'poison' that it needs to burn off (negatively affecting the athlete's metabolic fitness in the process) but also not so low that the brain is holding the intensity of training back out of fear that you're using its precious fuel(!)"

Obviously, for athletes, the sweet spot between these 2 points is going to change with…

  1. Fitness – fitter athlete = higher O2 demand = higher absolute energy supply required. In absolute terms, an athlete with an FTP of 300W will require twice the CHO of an athlete with an FTP of 150W to fuel the training on an ongoing basis (assuming both athletes have similar ‘metabolic fitness’).

  2. Duration – if more of your day is made up of aerobic intensity work vs resting, your glycogen needs will be higher. An athlete training 4hrs per day will require ~twice the CHO of an athlete training 2hrs per day (at the same relative intensity).

  3. Intensity – as we move up the intensity scale, by necessity, a significantly greater proportion of your energy needs will come from Carbohydrate. Therefore, athletes doing a lot of high intensity training within their week will need to take in more carbs. An athlete doing 1hr of threshold training per day will require 3-4x the CHO of an athlete doing an hour of easy-steady training per day(!)

When we look at these 3 factors together, it speaks to the needs of changing nutrition through the year as your training (volume/intensity) and your fitness changes. While protein needs will stay (relatively) constant, fat and carbohydrate demand will change quite a lot depending on the type and amount of training the athlete is doing.

For example, a low volume, low fitness athlete, say a guy training 1hr per day with an FTP of 200W may have CHO expenditure of ~300kcal per day, representing ~10% of his overall caloric needs. As the chart above shows, this can easily be met with a diet containing ~20% CHO. However, if we look at an elite endurance athlete, with an FTP of 350W, training 4hrs per day at a moderate intensity, CHO expenditure is likely to be ~3500kcal/day, representing more than half the athlete's energy needs and requiring an intake approaching 70% to keep up with daily oxidation 'burn'.

I’ve prepared a simple tool below to provide an example of this and to help you with tailoring your nutrition to your energy and CHO needs… Obviously, the specific food preferences and choices are highly individual, but hopefully the tool will give you a good starting point for the 'plate ratios' / macronutrient composition of your food selections at various points in your season.

Height cm:
Bodyweight (kg)
Age (yrs)
FTP (bike)
Volume (Hrs/day)

Weight loss?

CHOg% total kcal( g/kg)
PROg% total kcal
FATg% total kcal

If you play around a little bit with the numbers, you'll see that the recommended macronutrient breakdown (in order to match the respective amounts of each macronutrient 'burn') will differ markedly depending on the quantity and quality of training performed. For this reason, extreme nutrition approaches at either end of the spectrum are a little silly and have no place in performance sport. Are the low carb advocates 'right'? Sure if you're not including a whole lot of movement in your day (which represents a much larger problem). Are the folks in the 'athletes need really high carbohydrate diets' right? Sure if we're talking about a true high performance athlete in the 'meat' of their training prep. In 'the real world' athletes will oscillate somewhat between the range of those extremes and in order to keep both their general fitness and their metabolic fitness continually improving, must adapt and adjust their diet accordingly. Be like water, my friends.

Eat smart,



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