If you practice intermittent fasting or have done any research on the science behind it, you are probably familiar with ketone bodies. They are “small lipid-derived molecules that serve as a circulating energy source for tissues in times of fasting or prolonged exercise.” Their increased appearance in your bloodstream is an indicator that you’ve entered the metabolic state of ketosis or fat burning. The LIFE Fasting Tracker even has a “red zone” to help you visualize this ketone level shift. This is because your liver actually creates ketone bodies from fatty acids that are released from your fat cells during fasting. When glucose goes down, ketones go up and vice versa.

One important ketone body produced during fasting is called beta-hydroxybutyrate or BHB. You might have heard of it within the context of fasting or even fasting and performance enhancing supplements.

But you might not know (because this is really hot science) that ketone bodies appear to also work as signaling molecules. They act like other important hormones in your body, carrying messages to various organs, tissues and cells in response to changes in your environment. What could ketone bodies be signaling? They tell your body how to better regulate its metabolism in a stressful environment, because evolutionarily speaking fasting suggests that you are in a stressful, resource-limited environment. They may also tell your body to ramp up anti-inflammatory processes and to repair damage caused by stress.

Ketones: The Brain Food

“I think it’s really important to understand the mechanisms of ketones in the brain if we are going to use them therapeutically to improve the aging brain in people.” – John Newman.

John Newman is a researcher at the Buck Institute for Research on Aging. In the lab, Newman is using various exogenous (made outside of the human body) ketones as well as ketogenic diet interventions to study the impact of ketones on fat metabolism and the overall regulation of metabolism in animal models. He is particularly interested in how ketone bodies impact the brain and how exactly they improve memory in the brains of aging mice (because they do). Do they act to reduce inflammation levels? Could they even be regulating gene expression? More on that in a bit!

Newman is also a geriatrician. He works as a physician in the field of inpatient medicine and inpatient geriatrics, with a keen interest in helping to prevent and treat delirium and mobility decline. These are the two most important syndromes that can leave an older adult who goes into the hospital a different person when they leave. An originally healthy and self-sufficient patient who leaves the hospital with delirium and reduced mobility may no longer be capable of living on their own or taking care of themselves.

“I’ve heard delirium described as acute brain failure,” Newman said. “It’s not dementia; it occurs very quickly and can affect someone with a normal brain. It’s often triggered by being in the hospital and being sick. It’s an acute confusional state in which people are disoriented, they don’t know where they are or what is happening. They may be hallucinating, paranoid and suffering memory loss. It can pass if you treat it correctly, but sometimes it lingers even if you do everything right.”

Delirium happens to up to half of older adults in the hospital. Infection is a common cause, but it isn’t necessary to bringing on the syndrome. Medications and disruption of circadian rhythms (caused by disrupted sleep and eating patterns) may also trigger delirium.

When we look closely at patients with delirium, we see that their brains have high levels of inflammation, suggesting that inflammation may be a trigger for delirium. Another trigger may be metabolic failure in the brain, where the brain is not able to produce the energy required to sustain its normal function. Inflammation and metabolic failure are also linked, so the interaction between them may be a key component of delirium.

“You could imagine that the state of someone’s brain determines how resilient it is to the triggers of delirium,” Newman said. “For someone with Alzheimer’s disease, it may not take a whole lot to push them into delirium. The state of the art in preventing and treating delirium is trying to identify and fix the key precipitants, such as an infection or an electrolyte problem, but also to systematically do everything possible to prevent other precipitants. This can include making sure people in the hospital can sleep through the night, that they are getting up to walk and are staying hydrated.”

Newman hopes to better understand the biology of delirium through both his work with patients on the geriatric ward and his work in the lab researching the impacts of ketone bodies in the brains of animal models. Molecules that provide efficient energy and also dampen inflammation are tempting in a situation like delirium, Newman says. If ketone bodies make the aging brain more resilient and metabolically efficient, they could perhaps protect people in the hospital from delirium, or help to treat this syndrome of the older, vulnerable brain?

Ketones: More Than a Fuel Source?

The big new idea in ketone biology is that ketones aren’t just an alternative fuel source for your brain and other organs when glucose is in short supply. A slew of recent studies suggest that they are actors. They aren’t just passive carriers of energy but rather perform important biological activities. They are signaling molecules and may even alter gene expression changes that last beyond their lifetime in your bloodstream.

“Ketone bodies are doing things that are part of the biology of fasting, exercise and dietary restriction and how these things affect our health,” Newman said.

But the science on ketone bodies as signaling molecules is very new. It has been difficult for researchers to identify just how much the presence of ketone bodies contributes to the observed benefits of fasting and other interventions that put the body into ketosis. Newman refers to exercise and nutrient-related interventions like calorie restriction diets and intermittent fasting as biological sledgehammers. They can have dramatic but sometimes uneven impacts on healthspan and longevity in animal models. We don’t yet precisely and fully understand their mechanisms.

“For almost a hundred years, we’ve known that manipulating the way we eat, the time that we eat and the nutrients that we eat, along with physical activity, have major effects on aging and longevity, at least in animal models. They are great tools for understanding the possibilities of what we could do to affect the problems of aging in humans.” – John Newman

But the more Newman studies exercise and nutrient restriction interventions, the more he is convinced that ketone bodies are one of the important means by these interventions work their magic on health and longevity.

“Calorie restriction, intermittent fasting, ketogenic diets… in all of these interventions, your body produces ketone bodies,” Newman said. “These ketones are absolutely necessary for your body to survive these states; they are produced as a way for your body to use the energy stored in your fat and provide it to your heart, your brain and other organs. This is the ‘old-fashioned, boring’ activity of ketone bodies that we’ve known about for a long time. But we’ve come to understand over the last decade that while ketone bodies are serving that critical energy function, they are doing all kinds of other stuff too.”

Ketone bodies are acting like drugs in our bodies. They are binding directly to proteins and modifying cellular processes that affect our biology as well as our aging.

Ketone Body Magic

Every year new research studies are published that add to the list of the exciting things that ketone bodies can do. One of the biggest surprises was the finding that the ketone body is a ligand for certain receptors in the body. It binds to receptors on the surfaces of cells and activates processes within those cells as a result. This is similar to how neurotransmitters and hormones work.

There are two receptors that bind the ketone body beta-hydroxybutyrate, Newman explains. Both of these receptors bind to other ligands as well. For example, one of these receptors also binds nicotinic acid, which is a form of a B-vitamin related to NAD. (You might have heard of NAD as a popular DNA repair and “longevity” supplement). Another receptor that binds beta-hydroxybutyrate also binds short-chain fatty acids such as butyrate. But what do these receptors do? They help to control metabolism. For example, they regulate the function of sympathetic neurons that control our body’s metabolic rate.

“This is probably why your heart rate drops when you are in ketosis,” Newman said. “Beta-hydroxybutyrate likely binds these receptors on nerve cells in your sympathetic ganglia to lower your sympathetic tone. The outcome is a lowered heart rate. This has been shown elegantly in rodents in the laboratory, and it’s probably how it works in people, too.”

The receptors that bind beta-hydroxybutyrate also regulate lipid release from fat cells. By doing this, ketone bodies can actually regulate their own production through negative feedback. Elevated levels of ketone bodies slow down the rate at which lipid is removed from fat cells, likely so that this fatty acid release doesn’t spiral out of control.

These functions of the ketone body beta-hydroxybutyrate have been known for over a decade. But Newman and colleagues including Eric Verdin, current CEO of the Buck Institute for Research on Aging, have more recently discovered other exciting functions and possibilities of beta-hydroxybutyrate as a potent signaling molecule in the body.

Tadahiro Shimazu was a postdoctoral researcher in Eric Verdin’s lab investigating ketone biology when he realized that the physical structure of beta-hydroxybutyrate looks a lot like that of butyrate. Butyrate is a short chain fatty acid that was the first discovered inhibitor of deacetylase enzymes that are critical for controlling gene expression. Butyrate, formed by microbes in our gut as they metabolize dietary fiber, has been shown to regulate the expression of genes that lower inflammation and protect our cells and mitochondria (the cell’s energy producers) from stress.

“Deacetylase enzymes are everywhere,” Newman said. “They are in all of our cells, in all of our cells’ nuclei. They exist in complexes with many other proteins that tell these enzymes where to go and what genes to interact with and turn up or down.”

Shimazu, in collaboration Newman and others, demonstrated that beta-hydroxybutyrate could, similar to butyrate, also inhibit some deacetylase enzymes. Even more exciting, this ketone body appeared to have a measurable impact on deacetylase enzyme activity at concentrations that ketones actually reach in the body of a person who is fasting. In other words, we wouldn’t need to take exogenous beta-hydroxybutyrate supplements to reach levels of ketone bodies that could theoretically change gene expression through deacetylase enzymes. Fasting or dietary ketosis would be enough.

Of course, the research on ketone bodies as gene expression regulators is so new that there still some debate over when, where and in what contexts they work. However there is mounting evidence that at least in brain tissue ketone bodies can indeed act as histone deacetylase (HDAC) inhibitors that activate anti-inflammatory and oxidative stress reducing gene products. In animal models, ketones have even been found to increase the production of BDNF, a growth factor that promotes nerve growth and brain plasticity, through their action as HDAC inhibitors!

DNA packaging. Image credit: National Human Genome Research Institute.

The Story of The Silenced Genes

*We are about to get really nerdy on you, but we promise it will be insightful in the end!* To understand what histone deacetylase enzymes do, we have to step down to the microscopic view of the DNA double helix.

If you pulled out the DNA from one of your cells and extended it like a string, it would be over three feet long. But because biology is so elegant, it has found a way to wrap DNA around histone proteins that serve as protective packaging to keep your DNA coiled tightly and neatly within your cells. This can happen because opposites attract. DNA has an overall negative charge from phosphates on its backbone, while histone proteins have little tails that are studded with positively charged lysine residues. The catch is that when your DNA is coiled tightly around histone proteins, it can’t easily be transcribed and “read” to create new proteins.

This is where acetylases come in. Lysines on histones can become acetylated, at which point they no longer have a net positive charge. This loosens the interaction between the “packaging” histones and DNA, allowing enzymes to enter the scene and transcribe genes from sections of unprotected DNA.

Acetylation generally activates genes, while deacetylation silences them. By inhibiting certain deacetylases, butyrate and ketone bodies thus keep targeted genes accessible and active. And this is only one piece of the puzzle that we are slowly putting together on ketone bodies’ signaling functions.

Yingming Zhao, a researcher at the University of Chicago, recently showed that not only does beta-hydroxy-butyrate inhibit deacetylase enzymes, but it actually also binds directly to histones themselves. This means that histone tails can be beta-hydroxybutyrylated.

“This is super interesting to us,” Newman said. “We aren’t sure exactly what effect this has. But as part of this study Dr. Zhao’s group fasted mice and looked to see at what genes he could find histones with beta-hydroxybutyrate residues on their tails. He found these residues in the location of genes turned on by fasting, especially genes involved in fat metabolism. Does that mean that ketone bodies are helping to turn these genes on? Maybe.”

All this points to beta-hydroxybutyrate being a signaling molecule that regulates our gene expression to help us activate new genes in response to changes in our environment, or in response to scarce resources in the case of fasting. These new genes include genes responsible for metabolizing fat (not so surprising) as well as genes involved in repairing damage and responding to stress (more surprising!)

“In our lab, we found beta-hydroxybutyrate activity at the site of genes involved in the repair of oxidative damage,” Newman said. “Just giving mice beta-hydroxybutyrate changed the histone acetylation at promoter regions of genes involved in repairing oxidative damage, increasing expression of these genes. It makes sense, too. It’s all about responding to stresses and responding to new environments. If you are ketotic, especially as a result of fasting, in an evolutionary sense you could imagine that things aren’t going that great for you. You probably need to adapt to a new environment to get the calories you need, maybe by learning new things or adapting to use your available energy more efficiently.”

Is the Ketone the King of Fasting?

How much of the benefits of fasting are due to the alternative fuel and signaling functions of ketone bodies? While we don’t exactly know the answer, Newman guesses that ketones play only one part, but a very important part.

Fasting, dietary restriction and exercise are big changes that turn on and modify lots of things. But one of the great breakthroughs in studying aging is the realization that these lifestyle interventions are all mediated by very specific pathways and that we can identify what these pathways are, Newman says.

“It’s not that eating less or fasting has health benefits in and of itself,” Newman said. “It’s that the state of nutrient deprivation is detected by signaling molecules in our cells and turns on specific downstream pathways that change gene expression and enzyme function. These are what provide the health benefits of fasting, not the not eating itself.”

In recent years, the discovery of specific cellular pathways modified by lifestyle factors such as exercise and fasting have even led to some new drugs. The mTOR pathway is an example; it senses protein levels and when activated turns off autophagy (cellular “self-eating”) and recycling of damaged proteins. By studying mTOR and its downstream impacts, researchers have been able to develop drugs like rapamycin and other mTOR inhibitors that are now being used clinically in older adults in clinical trials that target aging.

“Ketone signaling is just one of the pathways activated by fasting,” Newman said. “But I think we are going to find out that ketones are up there with the big players of lifestyle interventions that target aging. These players include IGF, insulin, AMPK and mTOR, and their activities along with ketone signaling all intersect. The cutting edge of science right now in the aging field is to try to define and quantify the effects of these different players. What are the impacts of mTOR inhibitors alone, or ketone bodies alone? This is what I’m doing in my own lab now, trying to answer the question of what ketones by themselves do.”

It turns out to be rather difficult to isolate the impacts of ketones, especially in human studies. For instance, for your liver to start making ketone bodies, your insulin levels must also be low. Fasting thus hits two birds with one stone: You lower your insulin levels (and insulin signaling pathways that promote cell growth and inhibit autophagy) while at the same time increasing your ketone body concentrations. However, animal model studies may allow us to investigate the impacts of ketones in isolation.

Make the Switch: The Benefits of Off-and-On Ketosis

We’ve talked a lot about how ketone bodies are important as both an alternative energy source to glucose as well as signaling molecules that may prompt stress-busing gene expression changes. But there’s a relatively new idea that staying in ketosis or a state of fat-burning and high ketone levels long term isn’t necessary for health and longevity. It might be enough to intermittently raise your ketone levels.

Mark Mattson at the NIH and others have suggested that it’s the switch back and forth between fat burning and sugar burning, or the fasted and the fed state, that is critical for some of the observed health benefits of intermittent fasting. These observed benefits include improved cognitive function and cellular recycling and rejuvenation.

“I really like this idea of metabolic switching,” Newman said. “It makes sense from an evolutionary perspective. We probably weren’t meant to be in an overfed metabolic state for our whole lives. I suspect that the switch from being fed to fasting and back again is the important part. Whatever you do to trip that switch and change your metabolism on a regular basis is going to more or less work. That’s why there are so many different ways now to practice fasting, from time-restricted eating to fasting mimicking diets to daily dietary restriction. These interventions in animal studies all have pretty much the same effects on health and longevity.”

Healthier, Smarter, Younger: Mice on a Cycling Ketogenic Diet for a Lifetime

Newman was recently obsessed with this question: Could exposure to ketone bodies over the entire lifespan of a mouse improve its health and longevity as it ages? He set off to answer this question by putting mice on a ketogenic diet and planning to observe their health and cognitive function over a mouse lifetime. Simple, right?

But he ran into a problem. His mice started to get fat!

“Mice are a convenient little model of mammals’ metabolism, but mice are not people. Unlike people, they love eating ketogenic diets,” Newman said. “Humans don’t usually gain weight on a ketogenic diet, because we usually don’t find them very easy to eat. It’s difficult for us to overeat on a ketogenic diet, but it’s not for a mouse!”

Newman had to find a way to prevent his mice from getting obese on their ketogenic diet. Obese mice on a keto diet are likely to have other health issues that would interfere with an evaluation of the impact of ketones on metabolism, health and aging.

Newmans’s colleagues in other labs have addressed this problem in mouse model studies of ketogenic diets by individually restricting the amount of ketogenic diet chow, to keep each mouse at a stable weight. But Newman had the idea of cycling the diet. He fed his mice a ketogenic diet for one week and then switched them to a normal diet for one week, repeating this process over the course of their lifetime. The mice always gained a little weight on their ketogenic diet weeks, but then conveniently lost the exact same amount of weight during their normal diet weeks.

Newman’s approach allowed him to answer another interesting question. Does an animal have to be ketotic all of the time to reap significant health benefits?

Newman tested various aspects of health and symptoms of aging in his mice, including memory, endurance and balance. (If you are thinking he must have used a tiny mouse balance beam, you’d be correct!) He always tested his mice during their normal diet weeks. He found that mice on a cycling ketogenic diet had improved memory, lived longer and had improvements on other markers of aging as compared to mice that were always on a normal diet.

“This tells us that you likely don’t have to be on a ketogenic diet all of the time, or even at the time that you are testing memory, to see changes,” Newman said. “It changes the body in a way that’s more persistent, that lasts for a while. I think, and I’m trying to test this in the lab now, that this is because it’s not just about ketone bodies being used for energy by the brain, but rather about ketone bodies changing epigenetic patterns and gene expression that in turn affects inflammation and maybe even cellular senescence in ways that are more persistent over time.”

The memory impact in mice on a cycling ketogenic was quite dramatic. As they aged, these mice performed just as well or even better on several different memory tests than they did when they were younger mice. This memory improvement was even observed at the end of the study, two months after aged mice had eaten any ketogenic diet chow. Improving memory with age? We will take that!

Newman also found that a diet low in carbohydrates (15% of calories from carbohydrates), but not quite ketogenic (zero carbohydrates), had an intermediate effect on lifespan. It was little better than a normal diet, but not quite as good as a ketogenic diet.

“I think that’s a sign that while carbohydrate restriction alone is enough to improve health and lifespan, there’s something extra that happens when the carbohydrate intake is low enough to prompt ketosis,” Newman said. “Part of this something extra is insulin and mTOR activity, but we think that a big part of it is ketone activity.”

As mentioned above, some of Newman’s colleagues have studied the impacts of a calorie-restricted continuous ketogenic diet as opposed to a cycling ad libitum ketogenic diet in mice. They observed improved memory, strength and endurance in aging mice on a ketogenic diet. However, Newman’s cycling approach did something different than the calorie-restricted continuous ketogenic diet. Cycling the diet prevented Newman’s mice from becoming fully keto-adapted.

“This could be a good thing or a bad thing, depending on what you are looking for,” Newman said. “Both people and mice definitely adapt to a ketogenic diet over time in terms of getting more efficient at utilizing ketone bodies for fuel. They develop more transporters on the blood brain barrier to get ketones across, and our tissues get more efficient at using ketone bodies quickly for fuel. But this also means that levels of ketone bodies in the blood drop.”

In other words, when we first start a ketogenic diet, our blood ketone levels are as high as during an extended fast. But over the course of several weeks, these levels come down as your tissues adapt to using ketones as fuel. Mice on a ketogenic diet go from having blood ketone levels 20x higher than normal at the beginning to more like 2-3x higher than normal within 8 weeks of starting the diet.

“If you want to use ketones as fuel, this is a good thing,” Newman said. “If you think they are acting like drugs, though, maybe you want ketones to hang around in the blood more and not be as efficiently used up as fuel.”

When Newman cycled his mice on and off a ketogenic diet, their blood ketones always remained high. They never had a chance to adapt to the metabolic state of ketosis by using up more of these ketones as fuel. This might be a good thing if higher ketone levels in the bloodstream mean more ketone bodies are available to perform signaling functions. This may explain why metabolic switching is a special driver of healthier aging, particularly brain aging as Mark Mattson suggests.

“If the signaling activities of ketone bodies wind up being important for something like memory in humans, then an intermittent exposure to ketosis or a ketogenic diet might actually be better, because you are essentially getting higher drug levels,” Newman said.

Takeaways

In summary, we still have a lot to learn about the functions of ketone bodies. For example, we still need research studies on what effects ketone bodies have irrespective of changes in insulin levels, Newman says. What are the biochemical and physiological effects of taking exogenous ketones? That remains to be studied in depth. We also don’t know how ketone bodies impact cells in different tissue types, for example endothelial vs central nervous system cells. And finally, we need to study the impacts of ketone bodies in female vs. male organisms. Newman used only male mice in his cycling ketogenic diet experiment.

However, it’s clear that ketone bodies are a rising player in the healthspan-promoting impacts of interventions like fasting and exercise. Ketones may even change gene expression patterns that could benefit us into old age. It’s certainly one more reason to get into ketosis and kickstart ketone production with intermittent fasting and moderate to high intensity exercise, at least intermittently!