Nutritional biochemistry review for dietitians

As dietitians, there was a time when nutritional biochemistry was fresh in the mind as part of the coursework studies. But in the professional life of applying specific nutrition principles, these details may have become fuzzy.  

Biochemistry is the foundation of nutrition, and every part of applying the science of nutrition is based on these principles. Whether this was something learned in depth or something crammed in before an exam, this review can bring the fundamentals to the top of mind and further connect daily work as a dietitian to the foundational biochemical principles. 

Why is nutritional biochemistry important for dietitians? 

People need food to live. It’s said that “food is fuel,” but what does that really mean? Food provides energy to the human body to maintain its cellular function. The mechanics of how it is found in biochemistry. 

People do not eat nutrients. They eat food and combinations of foods, like meals. Foods contain a combination of macronutrients and micronutrients. Macronutrients include carbohydrates, protein, and fat. The micronutrients are vitamins and minerals. 

These macronutrients are energy-yielding nutrients. During digestion, macronutrients are catabolized (broken down) by hydrolysis reactions (adding a water molecule: H2O) into basic forms that can be absorbed in the blood. 

What should dietitians understand about macronutrient metabolism? 

Carbohydrates are broken down into glucose and other monosaccharides (single-unit carbohydrates). The red blood cells and the brain depend upon glucose as their primary fuel. Fats or triglycerides (three fatty acid chains connected to a glycerol) are broken down into glycerol and fatty acids or stored as fat. 

Proteins are broken down into amino acids, which are mainly used to build protein in the body or nitrogen-containing compounds. However, some can be converted to glucose. Amino acids can enter directly into the tricarboxylic acid cycle (known as the TCA cycle, Krebs cycle, or citric acid cycle). 

Glucose, glycerol, fatty acids, and amino acids are chemical compounds made up of carbon, oxygen, and hydrogen atoms. Amino acids also contain nitrogen. The body primarily uses glucose and fatty acids for its energy source. For glucose, the body can use all carbohydrates and most amino acids. To make proteins, the body needs amino acids. 

In catabolism, the body separates the atoms in different pathways to either produce pyruvate (3 carbons) or acetyl-CoA. Glucose can be stored as glycogen or broken down to pyruvate and acetyl-CoA. Glucose is broken down to pyruvate in a process called glycolysis, where one molecule of glucose produces two molecules of pyruvate. Glycerol is a 3-carbon compound, like pyruvate, and is easily convertible to glucose or pyruvate. 

Fatty acids go through fatty acid oxidation, where, in a series of reactions, the fatty acid is broken down two carbons at a time, making acetyl-CoA. Hydrogens and their electrons are released during fatty acid oxidation and are carried to the electron transport chain. 

Amino acids are deaminated (losing their amino group, which contains nitrogen) and are catabolized. Some are converted to pyruvate, some to acetyl CoA, and others enter the TCA cycle directly as other compounds. Amino acids that are converted to pyruvate can also indirectly provide glucose. When energy is consumed beyond the body’s needs, all three macronutrients can contribute to the body’s fat stores. 

Why is the breakdown of nutrients into pyruvate and acetyl-CoA important?  

Pyruvate can provide the body with energy through an anaerobic (without oxygen) or an aerobic pathway (with oxygen). Anaerobically, pyruvate is converted to lactate for quick energy. Aerobically, pyruvate is broken down to acetyl-CoA. Acetyl-CoA can enter the TCA cycle and electron transport chain to provide more energy. 

Acetyl-CoA can synthesize fats or generate adenosine triphosphate (ATP). ATP is energy ready for cells’ use. If ATP is readily available, acetyl-CoA makes fat, which is how indirectly glucose, glycerol, fatty acids, and amino acids can make fat. If ATP is not available and cells need energy, acetyl-CoA proceeds through the TCA cycle and releases hydrogen with its electrons to the electron transport chain. Pyruvate can also be converted to glucose if the body is in need of glucose. 

All of the macronutrients can be broken down into acetyl-CoA, which can enter the TCA cycle. The reactions of the TCA cycle release hydrogen atoms with their electrons, which are carried by coenzymes to the electron transport chain. Acetyl CoA proceeds in the TCA cycle – a series of metabolic reactions that break down molecules of acetyl CoA to carbon dioxide and hydrogen atoms. 

The TCA cycle is a circular path triggered and replenished by oxaloacetate, which is made primarily by pyruvate. In the cycle through a series of reactions, acetyl CoA molecules break down to carbon dioxide and hydrogen atoms with their 8 electrons. Coenzymes from niacin and riboflavin transfer the electrons from the TCA cycle to the electron transport chain. 

It is in the electron transport chain that the majority of ATP is synthesized. At the end of the electron transport chain, energy is captured in ATP for use in the body. The TCA cycle fuels the electron transport chain – the final pathway that transports electrons from hydrogen and oxygen, and captures the energy released in the bonds of ATP. 

In the mitochondria, a series of proteins serve as carriers that pass the electrons down to each other until the electrons reach the oxygen at the end of the chain. Oxygen takes the electrons, combines with hydrogen, and forms water. During this process, energy is released and powers the synthesis of ATP, which captures the energy in its bonds. ATP then leaves the mitochondria and can be used for energy. 

Putting it all together 

This biochemical foundation is relevant to daily nutrition practice because fueling the body’s cells is a primary function of the food people consume. A diet that does not support the body’s metabolic functions on the biochemical level is not optimal for the body and can have negative consequences for health. Nutritional biochemistry informs the nutrition recommendations of evidence-based medical nutrition therapy. 

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References:  

Whitney E, Rolfes SR. Understanding Nutrition. 16th ed. Cengage Learning. 2022.  

Judge A, Dodd M. Metabolism. Essays Biochem. 2020;64:607-647. https://doi.org/10.1042/EBC20190041  

Patricia JJ, Dhamoon AS. Physiology, Digestion. In: StatPearls. StatPearls Publishing: 2025. Updated September 12, 2022. Accessed February 12, 2026. https://www.ncbi.nlm.nih.gov/books/NBK544242/  

Sanchez Lopez de Nava A, Raja A. Physiology, Metabolism. In: StatPearls. StatPearls Publishing: 2025. Updated September 12, 2022. Accessed February 11, 2026. https://www.ncbi.nlm.nih.gov/books/NBK546690/  

Alabduladhem TO, Bordoni B. Physiology, Krebs Cycle. In: StatPearls. StatPearls Publishing: 2025. Updated November 23, 2022. Accessed February 12, 2026. https://www.ncbi.nlm.nih.gov/books/NBK556032/  

Ahmad M, Wolberg A, Kahwaji CI. Biochemistry, Electron Transport Chain. In: StatPearls. StatPearls Publishing: 2025. Updated September 4, 2023. Accessed February 12, 2026. https://www.ncbi.nlm.nih.gov/books/NBK526105/