Dr. Michele Sherwood Explains Exogenous Ketones: What They Can and Can’t Do

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Dr. Michele Sherwood Explains Exogenous Ketones: What They Can and Can’t Do

Ketones have become one of the most talked-about tools in metabolic health and biohacking. They appear in supplements, sports nutrition products and longevity conversations, often accompanied by claims about energy, mental clarity, fat burning and performance. But before we decide whether ketones deserve a place in our health strategy, we need to understand what they actually do.

Our bodies naturally produce ketones. When carbohydrate availability decreases during fasting, prolonged exercise or carbohydrate restriction, the liver converts fatty acids into ketone bodies that can be used as an alternative source of energy.

The three compounds commonly called ketone bodies are acetoacetate, beta-hydroxybutyrate, or BHB, and acetone. Of these, BHB is the predominant ketone circulating in the blood during nutritional ketosis and fasting, and it has become particularly interesting to researchers because it does more than provide fuel. BHB also functions as a signaling molecule, interacting with pathways involved in inflammatory signaling, oxidative stress, gene regulation and metabolism. That has generated considerable interest in its potential role in metabolic and cellular health. But an interesting biological mechanism is not the same thing as a proven clinical outcome. That distinction matters enormously.

Making Ketones and Taking Ketones Are Not the Same Thing

When your liver produces ketones during fasting or carbohydrate restriction, those ketones are part of a much broader metabolic response. Insulin generally falls. Glycogen stores are being utilized. Fat mobilization increases. Fuel selection changes, along with a variety of hormonal and cellular signals. Exogenous ketones allow us to do something different: consume ketones directly.

A ketone supplement can raise circulating BHB relatively quickly without requiring someone to fast for an extended period or significantly restrict carbohydrates. That can be useful in certain circumstances, but it does not mean the supplement has recreated the entire biology of fasting.

This is probably the single most important point I want consumers to understand. Having ketones in your bloodstream is not the same thing as being in a fasting state. You can consume exogenous ketones while also consuming carbohydrates. Your blood BHB may rise, but that doesn’t automatically mean you’re burning significant amounts of body fat or experiencing every metabolic adaptation associated with endogenous ketosis.

The ketone measurement tells us that ketones are available. It doesn’t tell us the whole metabolic story.

Why BHB Is So Interesting

BHB is particularly useful because it is relatively stable, water-soluble and readily transported through the circulation. It can cross the blood-brain barrier and be used by several mitochondria-rich tissues. The brain can use ketones when they become sufficiently available. The heart is highly oxidative and readily utilizes ketones alongside other fuels. Skeletal muscle can also oxidize ketones, with utilization changing depending on activity, nutritional state and fasting duration.

The kidney cortex can use ketones as well. Interestingly, the liver produces ketones but cannot use the ketones it makes because it lacks a key enzyme required for ketone oxidation. Red blood cells cannot use ketones either because they don’t contain mitochondria.

This is another example of how sophisticated human metabolism really is. Different tissues have different energy requirements and different capacities to use available fuels. It’s one of the reasons my husband, Dr. Mark Sherwood, and I spend so much time discussing metabolic flexibility and mitochondrial health in our work at the Functional Medical Institute. Health isn’t simply about having more of a particular molecule in the bloodstream. What matters is how effectively the body can produce, transport and utilize energy.

Not All Ketone Supplements Are the Same

This is where consumers need to look beyond the front of the package. “Ketones” is a broad category, and the products being sold aren’t necessarily interchangeable. Ketone esters generally raise circulating BHB more effectively than ketone salts. Ketone salts provide BHB bound to minerals such as sodium, potassium, calcium or magnesium, meaning larger doses can also introduce a significant mineral load.

MCT oil is frequently discussed alongside exogenous ketones, but it works differently. Rather than supplying BHB directly, MCTs provide fatty acids that the liver can convert into ketones.

The formulation matters. The dose matters. The individual’s health matters. Most importantly, the reason you’re taking it matters.

I’m always cautious when a legitimate area of metabolic science becomes a consumer trend because marketing tends to remove the context. The question shouldn’t simply be, “How high can I get my ketones?”

It should be, “What am I actually trying to accomplish?”

Can Exogenous Ketones Help During Fasting?

This is where the conversation becomes particularly nuanced. Some people experience fatigue, headache or mental fog when transitioning from frequent carbohydrate intake toward fasting. Because BHB can provide an immediately available energy source, exogenous ketones may theoretically make that transition more comfortable for selected individuals.

Research has also suggested that acute exogenous-ketone ingestion may temporarily lower blood glucose, although responses vary and this should never be used as a reason to alter diabetes medication without medical guidance. Because the brain and skeletal muscle can utilize ketones, there is also interest in whether supplementation can support perceived energy, cognition or physical function. The research is evolving, and results have been inconsistent depending on the product, dose and population studied.

This is where caution is needed with words like “boost,” “hack,” or “guarantee.” There may be interesting applications. But interesting is not the same as proven.

Do Exogenous Ketones Break a Fast?

Technically, yes. Exogenous ketones contain energy, so consuming them interrupts a true zero-calorie fast. They may also temporarily reduce the body’s need to manufacture its own ketones and can affect fat mobilization.

But whether that matters depends on why someone is fasting.

If you’re undertaking a strict water fast, fasting for religious reasons, preparing for a medical test or intentionally seeking a zero-calorie period, exogenous ketones generally don’t belong there. If the objective is different, perhaps metabolic flexibility or maintaining function during a compressed eating window, a clinician may view exogenous ketones as a targeted metabolic tool rather than worry about whether a fast remains technically “pure.”

This is why I don’t like universal health rules. The objective matters. The individual matters. The context matters.

Ketones Are a Tool, Not the Foundation

Perhaps my biggest concern with the growth of biohacking is that people can become so focused on advanced tools that they forget the basic biology that supports health. You cannot supplement your way out of poor nutrition, chronic sleep deprivation, inactivity, loss of muscle or significant metabolic dysfunction.

Before I’m concerned about someone’s ketone number, I’m interested in their metabolic health. Are they maintaining muscle? Are they moving? Are they sleeping? Are they eating nutrient-dense foods? How is their glucose regulation? What is happening with visceral fat, stress and recovery?

This philosophy is also reflected in approaches that emphasize overall metabolic health and multiple biological systems. Cellular energy and metabolic health don’t depend on one molecule or intervention. They reflect the interaction of multiple biological systems. Exogenous ketones may have a place within that larger picture.

They are not the picture.

What We Still Don’t Know

There is legitimate scientific interest in ketones and BHB, including research involving metabolism, cognition, inflammation, exercise and cellular signaling. What we don’t yet have is strong evidence that routinely consuming exogenous ketones will extend human lifespan or prevent major age-related diseases.

That’s an important distinction in a marketplace where longevity claims can quickly run ahead of the research.

I am excited by emerging science. I also believe part of being responsible in this field is acknowledging uncertainty.

The goal shouldn’t be to chase the highest BHB reading or purchase every new product associated with longevity. The goal is to understand our biology well enough to make intelligent decisions about which tools may be useful, and which are simply unnecessary.

This is a philosophy Mark, and I explore throughout our new book, Authentic Longevity: Unlocking Lifelong Vitality, The Sherwood Way. We deliberately begin with what people can control every day—nutrition, movement, sleep, stress and hydration, before moving into areas where additional guidance may be useful, including genetics, peptides, supplements, hormones and laboratory testing. That order is intentional.

Advanced tools should come after the foundation, not instead of it. Exogenous ketones are a perfect example. They may provide an interesting metabolic tool and give us another way to study the remarkable flexibility of human energy metabolism. But a supplement can raise BHB. It cannot bottle the entire biology of fasting. And it certainly can’t replace the everyday habits that build health in the first place.

About Dr. Michele Sherwood

Dr. Michele Sherwood, DO, is a physician, educator, author and speaker and co-founder of the Functional Medical Institute in Tulsa, Oklahoma. Together with her husband, Dr. Mark Sherwood, ND, a traditional naturopathic doctor and clinician, she focuses on personalized, whole-person health with an emphasis on prevention, metabolic health, healthy aging, performance and longevity. Through their clinical work, books, podcasts, speaking engagements, and educational outreach, the Sherwoods share information related to prevention, metabolic health, healthy aging, and longevity.

Their new book, Authentic Longevity: Unlocking Lifelong Vitality, The Sherwood Way, combines decades of clinical experience, real patient stories and actionable science to explore nutrition, movement, sleep, stress, hydration, genetics, peptides, supplements, hormones and laboratory testing alongside the often-overlooked role of emotional and spiritual health.