Revisiting Lipid Oxidation

In reply to canute1's question:


"I have a question.For marathons (or Tri events of Olympic distance or longer) a high capacity to burn lipids be expected to be very valuable as it should allow conservation of glycogen. However, as I understand it, lipid utilization is inhibited as lactic acid concentration rises, and hence lipid utilization tends to be switched off as lactate threshold is approached. Since a marathon is run a pace not far below lactate threshold pace, it seems to me that the crucial requirement is to be able to continue to utilise lipids almost up to lactate threshold. It might be expected that this capacity is best achieved by training near to the upper end of the aerobic range. Is there any evidence that enhancing the ability to use lipids at the lower end of the aerobic range (lets say 50-65% of VO2max) produces a substantial gain in the ability to utilize lipids in the upper part of the aerobic range?"

Yes, training at 50-65% of VO2max does increase the ability to utilise lipids generally, including in the so-called upper part of the aerobic range. As stated in my last post, the general adaptations of increased capillary density, mitochondria size and number, type I muscle fibre strengthening and hypertrophy, increased cardiac output and improved storage and access to stored lipids are fairly generic. I'll add that while training in the lower end will definitely improve the abilities at higher intensities (my training over this year has shown this for 10km racing), there is still a requirement to work at the higher aerobic intensities and even harder to gain improvement in all areas required for performance.

Enhanced lipid utilisation is very important for long distance races such as the marathon. Yes lactic acid does reduce the ability to use lipids. There are many factors affecting performance at these types of races including, lactate buffering/shunting in muscle and blood, utilisation of carbohydrate from stored muscle glycogen, liver glycogen and blood borne glucose, structural fatigue resistance, muscle recruitment and many others. I believe purely focusing on enhance the burning of fats can be a mistake that limits performance. For example eating a prolonged high-fat diet has been shown to increase the use of fats during moderate exercise. Unfortunately there is evidence that this is because the diet really induces a down-regulation of carbohydrate metabolism that is likely to impair performance.

For a recap on fat metabolism, read Training and Fat, where I believe I have addressed all of the above questions.

Comments

  1. Thanks for your answer to my question. I agree that we should not focus exclusively on lipid metabolism, but nonetheless, it makes sense to train in a way that will enhance lipid utilization at racing pace (especially at marathon pace). I agree that training in the lower aerobic zone will increase capillary and mitochondrial function and this will promote oxygen delivery and efficient glucose utilization and to some extent, also promote efficient lipid utilization at racing pace. However, to optimise lipid utilization at racing pace it seems to me that it is would be beneficial to shift the threshold acidity level at which lipid utilization is switched off, if that is possible..

    I have occasionally wondered whether including intermittent brief periods of upper aerobic zone running within a predominantly lower aerobic session (eg including bursts of a few minutes of near race pace within a lower aerobic session) might help achieve this by virtue of producing small surges of increased acidity during a period of lipid metabolism. I sometimes do sessions like this mainly because I enjoy that pattern of running – I suppose it might be described as low intensity fartlek – but I do not know if it is an efficient way to train.

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