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How The Body Uses & Converts Food For Energy – Metabolism

Jaime Alnassim

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August 1, 2021

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If you follow the ‘awesome fitness guru on social media, you might see a few ‘metabolism hacks’ to speed up your fat-burning and get your summer body now. Biolayne had an interesting video on ‘Metabolic Adaptation Is An Illusion’ based on the study, ‘Metabolic adaptation is an illusion, only present when participants are in negative energy balance.’ The quick is, each subject went on a diet on a calorie deficit and then went on a maintenance phase to keep bodyweight the same. What this study showed was how in a way, a diet break would be very beneficial to helping control weight after a diet. The metabolism of the subjects did improve because as we go longer into a diet, our metabolic rate is likely going to decrease. As we diet for longer, our NEAT, the non-exercise activity thermogenesis goes down as we don’t move as much (Levine 2002), and exercising may not burn as many kcals (Almundarij 2017). So giving the body a chance to reset without a calorie deficit has been showing to bring positive adaptations, along with performance benefits in endurance performance. “a 1-week diet break improved muscle endurance in the legs and increased mental alertness, and reduced appetite and irritability” (Peos 2021). However, telling someone to take a break from their diet does not really sound like a ‘hack’ and is hard to make money off on social media. 

All these words sound cool, but how does the body uses and converts food for energy in the first place? Bioenergetics, the concept of energy flow through living systems. “Plants use photosynthesis to capture sunlight, and herbivores eat those plants to obtain energy. Carnivores eat the herbivores, and decomposers digest plant and animal matter” (Clark 2018). Metabolism is either categorized as catabolic (also, catabolism) pathways in which complex molecules break down into simpler ones or anabolic (also, anabolism) pathways that require energy input to synthesize complex molecules from simpler ones. Our bodies use three main nutrients to function. The carbohydrate (starches and sugars) are broken down into glucose and are used to supply energy to cells. Protein is broken down into amino acids and is mainly used to build muscle. What each is broken down to is called a monomer. Along with making other proteins that are essential for the body to function. Fat is broken down into fatty acids to make cell linings and hormones. Energy is transformed from food macronutrients into cellular energy, from there it is used to perform cellular work. 

Catabolism of food begins when food enters the mouth and we begin to crew. As enzymes, salivary amylase initiates the breakdown of carbohydrates, the process of digestion converts the large polymers in food to monomers that can be absorbed. Each monomer is then absorbed by the body into the bloodstream. Once in the body and absorbed, the blood transports the nutrients to cells. The cells then either use the nutrients in the catabolic or anabolic pathway.

Catabolic Pathways: Glycolysis – Glucose breakdown, Glycogenolysis – Glycogen breakdown, β-oxidation – Fatty-acid breakdown, & Proteolysis – Protein breakdown to amino acids.

Anabolic Pathways: Gluconeogenesis – Synthesize glucose, Glycogenesis – Synthesize glycogen, Lipogenesis – Synthesize triglycerides, & Amino-acid synthesis – Synthesize amino acids

Stage 1: Glycolysis for glucose, β-oxidation for fatty acids, or amino acid catabolism

Stage 2: Citric Acid Cycle (or Kreb cycle)

Stage 3: Electron Transport Chain and ATP synthesis

Stage 1: Glucose breakdown begins with glycolysis. This is a ten-step metabolic pathway yielding two ATP per glucose molecule. In addition to the ATP, there are also two three-carbon molecules, called pyruvate. Pyruvate that is transported into the mitochondria gets one of its carbons chopped off, yielding acetyl-CoA.

Stage 2: Acetyl-CoA, a two-carbon molecule common to glucose, lipid, and protein metabolism enters the second stage of energy metabolism, the citric acid cycle. This cannot be undone. The breakdown of fatty acids begins with the catabolic pathway (β-oxidation), which happens within the mitochondria. Within this process, four enzymatic steps sequentially remove two-carbon molecules from long chains of fatty acids and yielding acetyl-CoA molecules. For the amino acids, once the nitrogen is removed (called deamination) from the amino acid the remaining carbon skeleton can be enzymatically converted into acetyl-CoA (or some else if needed). Next within the citric acid, cycle acetyl-CoA is joined to a four-carbon molecule in a multi-step process. This energy is transferred into ≈2 more ATP molecules and high-energy electrons that are carried by the molecules. (nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2))

Step 3: NADH and FADH2 then is carried by the electrons (hydrogen) to the inner membrane of the mitochondria. The electron transport chain. In this metabolic pathway, a sequential transfer of electrons between multiple proteins occurs and ATP is synthesized with water also being formed. Heat is also a byproduct, as complete nutrient catabolism is between 30 and 40 percent efficient. 

All cells are in tune with their energy balance. When energy levels are high the cells will build molecules, and when energy levels are lower, catabolic pathways are initiated to make energy (break down stored Glucose, fatty acids, etc… and use as energy). This is what largest happens when we are in a true caloric deficit.  “A positive energy balance, in which energy intake exceeds expenditure causes weight gain, with 60–80 % of the resulting weight gain being attributable to body fat. In negative energy balance, when energy expenditure exceeds intake, the resulting loss in body mass is also accounted for by 60–80 % body fat” & “It has been estimated that relatively small changes in energy intake and expenditure totaling 100 kcal per day could arrest weight gain in most people.” (Hill 2013). It seems so crazy but the first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another” (2). Energy balance is key to weight loss and ‘metabolism hacks’ that promise to help you burn more fat without any lifestyle change don’t seem to add up. I lost weight without a diet, therefore diets are not needed is like saying I saved money without a budget, therefore no one needs a budget. You may not have tried but you were still in a caloric deficit.  


1. Almundarij TI, Gavini CK, Novak CM: Suppressed sympathetic outflow to skeletal muscle, muscle thermogenesis, and activity energy expenditure with calorie restriction. Physiol Rep. 2017;5(4)

2. Boundless. “Boundless Chemistry.” Lumen,

3. Catia Martins, Jessica Roekenes, Saideh Salamati, Barbara A Gower, Gary R Hunter, Metabolic adaptation is an illusion, only present when participants are in negative energy balance, The American Journal of Clinical Nutrition, Volume 112, Issue 5, November 2020, Pages 1212–1218,

4. Clark, Mary Ann, et al. “Energy and Metabolism.” Biology 2e, OpenStax, 5 Mar. 2018,

5. Hill, J. O., Wyatt, H. R., & Peters, J. C. (2013). The Importance of Energy Balance. European endocrinology, 9(2), 111–115.

6. Levine JA: Non-exercise activity thermogenesis (NEAT). Best Pract Res Clin Endocrinol Metab. 2002;16(4):679-702

7. Peos JJ, Helms ER, Fournier PA, Krieger J, Sainsbury A. A 1-week diet break improves muscle endurance during an intermittent dieting regime in adult athletes: A pre-specified secondary analysis of the ICECAP trial. PLoS One. 2021 Feb 25;16(2):e0247292. doi: 10.1371/journal.pone.0247292. PMID: 33630880; PMCID: PMC7906362.

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