Finding the Sweet Spot
Mitochondria are the 'powerhouse' of the cell, generating energy or ATP from a range of substrates, most commonly fatty acids and glucose. Strategies to maximise fat oxidation rates improves health, and later endurance performance with improved mitochondrial function. Maintaining endogenous carbohydrate stores improves high-intensity performance.
Mitochondrial function is essential to health and performance.
Carbohydrate and fat metabolism
The main function of adipose tissue (body fat) is to provide energy to the body, which is sufficient even for glycolytic-dependent tissues (Cahill, 1970). Adipose tissue is the largest endocrine organ in the body and exerts an impact on whole body metabolism, regulating appetite and fat mass through the hormones Leptin and Adiponectin among others; dysfunctional or ‘overfat’ (Maffetone and Laursen, 2016) adipose tissues are implicated in the development of inflammatory diseases, insulin resistance and cardiometabolic disorders (Coelho, 2013, Maffetone and Laursen, 2016, 2020).
Elevated blood glucose, or hyperglycemia, is a response to stress. Mitochondrial dysfunction is attributed to many chronic disease states. With high blood sugar levels, most mitochondrial Acetyl-CoA will be derived from glucose, and after a meal or in a ‘fasted’ state, once blood sugar levels return to normal, Acetyl-CoA is produced via β-oxidation of fatty acids (Alabduladhem and Bordoni, 2021). In the time after a meal, glycogen stores are spared for energy and are not used as the primary source of energy, unless for an acute emergency, such as high-intensity exercise, our in-built fight or flight response (Cahill, 1970).
(Purdom et al., 2018) The crossover concept. The relative decrease in energy derived from lipid (fat) as exercise intensity increases, our fight-or-flight response, corresponds with an increase in carbohydrate (CHO). The crossover point describes when the CHO contribution to substrate oxidation supersedes that of fat. MFO: maximal fat oxidation. Adapted from Brooks and Mercier, 1994.
Strategies to improve fat oxidation rates include training with low carbohydrate stores, modifying carbohydrate intake depending on the session, and training in the heat, which all improve cellular adaptations toward increased oxidation of lipid sources. (Maunder et al., 2021).
What are you burning?
Respiratory quotient (RQ) is a measure of gasses, oxygen and carbon dioxide absorbed and released during respiration. It can measure whether metabolism is burning fat, the normal resting and low intensity state, or sugar, the fight-or-flight response. Predominantly burning fat throughout the day leads to improved energy levels.
Children are better fat-burners than adults (Kostyak et al., 2007).
Reducing overall carbohydrate intake in conjunction with an increase in calories from fat can improve fat oxidation rates. This can improve low-intensity, steady state, sustained aerobic performance.
Take the test: hand-held respirometers are available to monitor fuel use in the body, whether you are predominantly burning fat or carbohydrate: Lumen: https://www.lumen.com/en-us/home.html.
Healthful carbohydrate metabolism is individual, and changes through lifespan.
Carbohydrate intake for performance is specific to the exercise context.
Alabduladhem T.O., Bordoni B. (2021). Physiology, Krebs Cycle. Feb 7. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing.
Cahill GF Jr. (1970). Starvation in man. N Engl J Med. 19;282(12).
Coelho M, Oliveira T, Fernandes R. (2013). Biochemistry of adipose tissue: an endocrine organ. Arch Med Sci. Apr 20;9(2):191-200.
Kostyak JC, Kris-Etherton P, Bagshaw D, DeLany JP, Farrell PA. Relative fat oxidation is higher in children than adults. Nutr J. 2007 Aug 16;6:19. doi: 10.1186/1475-2891-6-19. PMID: 17705825; PMCID: PMC2014754.
Maffetone PB, Rivera-Dominguez I, Laursen PB. Overfat and underfat: new terms and definitions long overdue. Front Public Health. (2016) 4:279.
Maffetone Philip B., Laursen Paul B. (2020). The Perfect Storm: Coronavirus (Covid-19) Pandemic Meets Overfat Pandemic. Frontiers in Public Health 8, 135.
Maunder, E., Plews, D.J., Wallis, G.A., Brick, M.J., Leigh, W.B., Chang, W.-L., Watkins, C.M. and Kilding, A.E. (2021), Temperate performance and metabolic adaptations following endurance training performed under environmental heat stress. Physiol Rep, 9: e14849.
Purdom, T., Kravitz, L., Dokladny, K. et al. Understanding the factors that effect maximal fat oxidation. J Int Soc Sports Nutr 15, 3 (2018). https://doi.org/10.1186/s12970-018-0207-1.