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News from CRIS: Everyday Toxicology - Metabolism

October 6, 2025

Top Takeaways:

  • Metabolism is not about fat-burning or supplements. Metabolism in toxicology is about how the body processes and eliminates compounds.
  • Our bodies protect us daily. Enzymes in our liver and kidneys break down most compounds so they don’t build up.
  • Dose and exposure determine risk. The same compound can be safe at low levels, therapeutic at some doses, and harmful at higher ones.
  • Science uses metabolism to set safety limits. Toxicologists use metabolism data to establish thresholds that ensure safe real-world exposures.

Metabolism background.

Our bodies are constantly exposed to compounds like the nutrients we need, everyday chemicals in food and cosmetics, and sometimes substances that could cause harm. Whether a compound becomes beneficial, neutral, or harmful often depends on how our bodies process it through a system called metabolism.

In most cases, metabolism converts chemicals into less toxic and more water-soluble byproducts for rapid elimination, but in certain instances, the byproduct(s) of metabolism can be more toxic.

Metabolism isn’t just about how quickly you “burn calories.” In toxicology, it refers to the way the body absorbs, processes, and eliminates ingredients after exposure.

Understanding these steps helps us understand why dose, exposure, and individual differences all matter and are taken into the weight of evidence in determining safety.

What is metabolism?

Metabolism is the collection of chemical reactions inside our bodies that transform compounds into new forms so we can either use them or get rid of them.

  • For nutrients (like vitamins, fats, proteins, or sugars), metabolism turns them into energy or building blocks that keep our cells stay alive and functioning.
  • For medicines, toxins (naturally produced, such as by plants or fungi), and toxicants (human-made, such as industrial chemicals), metabolism modifies them, sometimes making them less harmful, sometimes briefly more reactive, before the body eliminates them through urine, feces, sweat, or breath.

In toxicology, metabolism is essential because it explains why the same compound can be safe at one dose, therapeutic at another, and harmful at much higher levels. It’s the body’s built-in processing system that connects exposure → dose → effect.

Is metabolism the same as fat burning?

Not in ingredient safety-focused toxicology. When people talk about “metabolism” in dieting, they usually mean how quickly the body uses calories or fat for energy.

In toxicology, metabolism is a much broader concept. It’s about how the body chemically transforms all compounds, including nutrients, medicines, toxins, and toxicants, into forms we can use or safely eliminate. It’s the body’s chemical processing and protection system.

How does metabolism function?

Before metabolism begins, a compound must enter the body. This happens through food, water, air, or skin contact. Our bodies absorb some ingredients readily, while others pass through with little effect.

For example, in our overview of exposure, we explain how much of a substance you’re exposed to and the way it enters the body, both of which shape potential risk.

There are two phases to the metabolism process.

Phase I metabolism

In this first stage of metabolism, enzymes (mainly found in the liver) introduce small chemical changes to compounds. These modifications, such as oxidation, reduction, or hydrolysis, can:

  • Make the compound more water-soluble, preparing it for elimination.
  • Activate a compound, as happens with some prodrugs in medicine.
  • Occasionally, it produces a more reactive or toxic intermediate that needs further processing.

Some compounds become less harmful, while others temporarily become more dangerous before Phase II detoxifies them.

Phase II metabolism

Phase II metabolism takes the products of Phase I and conjugates them by attaching them to molecules like sulfate, glucuronic acid, or glutathione. These additions:

  • Increase water solubility.
  • Mark the compound for elimination.
  • Reduce biological activity and potential toxicity.

This stage is one of the body’s primary natural detoxification strategies. It’s very different from commercial “detox” trends, which we explore in Detoxing Your Body.

Elimination

Once metabolized, compounds are eliminated primarily by:

  • Kidneys (urine) – the main pathway for water-soluble compounds.
  • Liver and bile (feces) – for larger, less soluble compounds.
  • Lungs (exhalation) – for gases or volatile substances.
  • Skin (sweat) – a minor route for some compounds.

As we discussed in Absorbing, Processing & Eliminating Ingredients, this elimination step is key to understanding why most everyday exposures don’t “build up” indefinitely in the body.

The efficiency of elimination helps determine half-life, which is how long a compound stays in the body.

A short half-life means quick elimination, while a long half-life suggests potential accumulation, which influences safety thresholds (Thresholds Overview).

How is metabolism used to understand ingredient safety?

Metabolism helps scientists answer critical questions that impact safety like:

  • Is the compound detoxified or bioactivated? Some compounds are neutralized, while others become more harmful intermediates before clearance.
  • Does it accumulate or is it eliminated quickly? Long-term buildup can increase risk, even at low exposures.
  • What dose matters? As explained in Reference Dose, toxicologists establish safe daily intake levels based on how compounds are metabolized and eliminated.
  • How do individual factors affect metabolism? Genetics, age, liver health, diet, and concurrent exposures (like medications) all can influence metabolic capacity.

Without answering those questions, researchers cannot effectively make safety recommendations.

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To continue reading the entire blog post, visit: https://cris.msu.edu/news/everyday-toxicology/everyday-toxicology-metabolism/.