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I love bacon.

On a beach trip with some college friends a few years ago, I was put in charge of breakfast. I made sure to buy eight pounds of bacon (one for every person on the trip) for our three days at the beach house. 

All the vegetables that other people brought for breakfast I then cooked up in the leftover bacon fat. 

Another year, my friends got me bacon-scented soap as a birthday present. I then received bacon-flavored bourbon the following year.

I’ve been known to “toast” bread by frying it in a pan of bacon fat.

Which is all to say: please take everything I’m about to tell you regarding low-carb diets for longevity with a grain of salt (ideally the kind of salt used to cure some organic, thick-cut bacon… mmmmm) because I have a deeply personal interest in bacon being labeled a health food.

But the salient question for this article is not “how much bacon can I eat before I die,” but instead: Can low-carb diets like Keto, Paleo, and Carnivore (not to mention Atkins and South Beach) help you live longer?

And, to be fair, most low-carb diets do not fall under the heading of “the bacon diet” (as much as I wish they did…).

In fact, for most low-carb diets—including Keto and Paleo (but not, obviously, Carnivore)—you replace carbohydrates, not with animal protein like bacon, but primarily with healthy fats. In fact, keto dieticians point out that too much protein can spike glucose and actually kick you out of ketosisWhen your body creates and burns molecules from fat called ketones for energy, instead of the sugar molecules produced by carbohydrates. Where the Keto Diet gets its name..

The reason people associate low-carb and keto diets with big juicy steaks instead of avocados and olive oil is likely because every single stupid media article on them is headed with an image of a massive slab of red meat.

You’ll notice the image we used for our header showcases eggs, veggies, and avocados.

But if low-carb diets aren’t just the replacing of bread and sugar with T-bones and NY strips (or, sadly, rashers of bacon), what are they?

What are low-carb diets?

One of the difficulties with low-carb diet research (and, indeed, diet research in general) is that there is no one, gold-standard definition of what a “low-carb” diet even is. 

Are you low carb if you get 40% of your calories from carbohydrates? Or only if you eat fewer than 20 grams a day of carbs? Or somewhere in-between?

All the above carbohydrate levels have been labeled as “low-carb” diets across various research papers and studies, which makes comparing results between them frustratingly difficult.

That said, among the scientists that are aware of this issue there seems to be an attempt to standardize definitions of low-carb diets to something like the following:

• Very Low-Carbohydrate Diet (VLCD): 20-50 grams of carbs per day (or even less than 20, in some cases) or about 10% and under of total daily calories coming from carbs (typically the level required to induce ketosis on the keto diet).
• Low-Carbohydrate Diet (LCD): Under 130 grams of carbs per day, or less than 26% of total daily calories from carbs.
• Moderate-Carbohydrate Diet: Under 225 grams of carbs per day, or less than 45% of total daily calories from carbs.

Of course, there remains significant room for variation within those definitions. For instance, you could replace the carbs with protein from plant or animal sources, or fat from plant or animal sources, or some ratio of both. That’s why everything from Paleo (eating only foods our paleolithic ancestors may have eaten), to Atkins (a low-carb diet popularized by Dr. Robert Atkins in the 70s), South Beach (a moderate carb diet allowing low-glycemic-index carbs), Keto (a very low carb diet aimed at inducing ketosis), and Carnivore (eating only animal products) all fall under the “low-carb diet” umbrella despite varying (sometimes widely) in their specific food recommendations.

But if you’re confused now, just wait until we get to the science around these kinds of diets!

Because boy, is it all over the place.

The science in favor of low-carb diets for longevity

Searching “low carbohydrate” on PubMed yields over 187,000 results, 7,197 of which were published just in 2020.

Trying to form a complete picture of how low-carb diets impact health and longevity is not a simple task. In fact, after my experience sifting through scores and scores of studies, I’d be wary of anyone who seems too sure about the effect of low-carb diets one way or the other. 

That said, there is some compelling evidence in the “low carb for life extension” camp.

Animal studies of low-carb diets

Most of the studies I could find for low-carb diets in animals specifically about lifespan were done on mice and rats.

For instance, two different 2017 studies in mice showed that low-carb, ketogenic diets extended longevity and healthspan in male mice, and lowered mid-life mortality and improved memory and healthspan in aging mice.

The first study showed a 13.6% increase in median lifespan for mice on the ketogenic diet (89% of their calories from fat) and found cancer rates significantly decreased in ketogenic mice as well.

The second study showed that, interestingly, cycling on-and-off from a zero-carb diet to a normal control diet every-other-week had better health effects than both a continuous zero-carb diet and the control (high carb) diet. For example, mice on the cyclic zero-carb diet were more likely to survive middle age, though they didn’t have a longer (or shorter) maximum lifespan. However they did also have improved memory and healthspan compared to mice on the control diet.

The other positive animal study specifically investigating lifespan with a low-carb diet was a 2015 paper on mice fed a ketogenic diet, which found, “Long-term KD [ketogenic diet] feeding caused profound and persistent metabolic changes, the majority of which are seen as health promoting, and had no adverse effects on survival in mice.” In other words, the mice fed a ketogenic diet generally ended up healthier without any negative repercussions. That said, the study didn’t find any life-extension effects compared to controls, only healthspan-promoting effects.

Other animal studies on low-carb diets didn’t target longevity specifically, but did look at how low-carb diets impact a variety of other health biomarkers.

For instance, lots of studies have found low-carb diets help prevent or suppress cancer in mice.

A 2015 study found that a low-carb diet that was also low-protein helped suppress tumors in mice, while a 2013 study found that a ketogenic diet enhanced chemotherapy responses in mice, and another found a keto diet plus hyperbaric oxygen therapy helped mice with metastatic cancer live 77.9% longer (while the keto diet alone helped them live 56.7% longer). A 2019 study seems to back up the findings that a low-carb keto diet improves the response to chemo and radiation therapy for mice with cancer.

Beyond cancer, low-carb diets appear to improve everything from viral immune response (crucial in the age of COVID), to glucose metabolism, to heart health and cardiovascular biomarkers in mice.

And to approach things from another angle, there’s also a fair amount of animal research implicating carbohydrates as exactly the kind of villains low-carb diet proponents paint them to be.  

For example, carbs may accelerate aging in yeast and in roundworms, and also seem to do so in rodents, fruit flies, and cell cultures.

Human studies of low-carb diets

As with most life extension interventions, it’s hard to measure the impact of diet on human lifespan in a rigorously scientific way because humans live so long, and because, especially with epidemiological observation studies (looking back on a population and trying to figure out why they had the mortality levels they do) there are so many other possible confounding variables.

That said, there are some proxies we can look at to make assumptions about the likelihood of low-carb diets extending human lifespan—or at least healthspan—like biomarkers for different disease risks and probabilities.

In that sense, low-carb diets would seem to have a fair amount going for them.

For example, a low-carb diet, even without weight loss or calorie restriction, seems to protect against diabetes risk. 

Low-carb diets appear to treat polycystic ovary syndrome (PCOS) in overweight women. 

They also may, similar to the results in animal models, help to prevent and treat cancer. 

One 2020 review concluded, “most preclinical and some clinical studies support the use of the KD [Ketogenic Diet] as an adjuvant cancer therapy” after finding that a low-carb diet in human case studies led to better responses to chemotherapy and radiation, and slower tumor progression. The study cautioned that these were small sample sizes and that bigger controlled studies are required.

This finding tracks with some recent 2021 research suggesting high levels of carbohydrates may lead to worse cancer outcomes.

Along similar lines, a 2017 study of 135,000 people across 18 different countries for over seven years called PURE (Prospective Urban Rural Epidemiology) concluded that:

“High carbohydrate intake was associated with higher risk of total mortality, whereas total fat and individual types of fat were related to lower total mortality. Total fat and types of fat were not associated with cardiovascular disease, myocardial infarction, or cardiovascular disease mortality, whereas saturated fat had an inverse association with stroke.”

Several studies and meta-analyses have found low-carb diets—contrary to what you might expect from increasing fat intake—may actually improve biomarkers of heart health cardiovascular risk. For example, a 2020 analysis cautiously suggested, “low-carbohydrate diets have a beneficial effect on cardiovascular risk factors but that the long-term effects on cardiovascular risk factors require further research.”

A 2019 randomized controlled trial found, “Low-carbohydrate, high-fat diets have a positive effect on markers of health…there were comparable improvements in weight loss and waist circumference and greater improvements in HDL-c and TG [triglycerides] with greater carbohydrate restriction.”

And it seems lean red meat may not worsen cardiovascular risk markers when replacing carbs with it in your diet, even in an already healthy diet.

Additionally, traditional Alaskan Inuit diets appeared to be much higher in fat and animal products than “modern” diets, but had much lower incidence of cardiovascular disease and, when more carbohydrates were introduced into their traditional diets, the amount of cardiovascular disease went up.

The paleo diet, which in the research would be considered a “moderate carbohydrate diet,” may lower all-cause mortality by as much as 23% (along with the Meditterranean diet, which we’ll be covering in a few weeks!).

Paleo also performed better in a 2021 meta-analysis of athletes than other healthy control diets in improving health biomarkers like weight, total cholesterol, low-density lipoprotein cholesterol, and triglycerides.

Speaking of weight, low-carb diets may help you lose more weight than low-fat diets over the course of six to twelve months, with a 2020 meta-analysis suggesting, “that low-carbohydrate diets are effective at improving weight loss, HDL and TG lipid profiles. However, this must be balanced with potential consequences of raised LDL and total cholesterol in the long-term.”

Low-carb diets may also help you keep the weight off better than low-fat diets as they don’t seem to lower your metabolism as much as low-fat diets do when you lose weight.

Another 2020 analysis suggested a low-carb ketogenic diet led to better “glycemic and lipid control among patients with Type 2 Diabetes,” and “also significantly contributes to their weight loss.”

Finally, as we’ll see in the next section on the science against low-carb, a lot of concern about low-carb diets stems from the possible long-term effects they may have on health. Critics often admit they do well for weight loss and biomarker improvement in the short-term compared to low-fat diets, but that long-term effects are not known or are possibly damaging and that most of the studies showing positive effects are short duration trials of 30-90 days, not multi-year studies showing safety.

Countering that, several studies seem to show no adverse long-term health impacts of low-carbohydrate diets, including if adhered to for 12 months, and for two years (at least in diabetics). And a 2020 review argued that low-carbohydrate diets seem to be safe, and possibly even protective against, a whole host of human diseases including liver failure, kidney disease, and stroke and heart attack.

The science against low-carb diets for longevity

For whatever reason the low-carb vs. low-fat diet debate is one of the most contentious in modern science. I didn’t encounter anything like the number of obvious partisans and studies outright contradicting each-other when researching caloric restriction. Every study “proving” the safety and health benefits of low-carb diets has another showing they’ll kill you, and vice-versa, and each one is thoroughly dissected and criticized by its opponents online.

Maybe, as the low-carb dieters proclaim, this is the result of an establishment that has been long-wedded to the low-fat dogma of the food pyramid and is simply lashing out at a new theory that disproves all their assumptions. Or maybe, as the anti-low-carbers insist, this is simply an example of responsible scientists pointing out the risks of high-fat, high-protein diets that don’t have enough fiber.

Regardless, there’s no shortage of research suggesting low-carb diets may not be the health panacea some think they are.

For instance, in fruit flies a high-carb, low-protein diet seems to increase lifespan, though the authors also suggested there may be “a threshold, over which an excessive proportion of carbohydrate intake shortens life.”

Hyperketonemia (super elevated levels of ketones like what you might get from keto diet) has been hypothesized to impair the health of dairy cows and lead to earlier onset of lactation diseases.

A much-reported on 2021 study found that long-term low-carb, ketogenic diets may lead to heart scarring and worse mitochondrial performance in rats, though low-carb proponents have argued the diets fed to the rats were unhealthy, with large amounts of soybean oil (a source of dangerous omega 6 fatty acids) and palm oil (in the form of cocoa butter). And again, there’s a study from 2019 directly contradicting the mitochondrial performance findings as well.

In humans, some retrospective epidemiological studies have implied low-carb diets are associated with a higher mortality risk in humans. For instance, a 2018 study suggested:

“Both high and low percentages of carbohydrate diets were associated with increased mortality, with minimal risk observed at 50-55% carbohydrate intake. Low carbohydrate dietary patterns favouring animal-derived protein and fat sources, from sources such as lamb, beef, pork, and chicken, were associated with higher mortality, whereas those that favoured plant-derived protein and fat intake, from sources such as vegetables, nuts, peanut butter, and whole-grain breads, were associated with lower mortality.”

A 2013 meta-analysis concluded: 

“Low-carbohydrate diets were associated with a significantly higher risk of all-cause mortality and they were not significantly associated with a risk of CVD mortality and incidence. However, this analysis is based on limited observational studies and large-scale trials on the complex interactions between low-carbohydrate diets and long-term outcomes are needed.”

A 2019 study showed low-carb diets may raise the risk of atrial fibrillation, but this seems to be the first finding of its kind and has not yet been replicated.

And, in the defense of (complex, low-glycemic index) carbs, a 2016 analysis found whole grain intake was associated with a lower risk of all-cause mortality and risk for diseases like cancer and cardiovascular disease.

A 2017 study also found that low-fat diets may lead to more energy expenditure than low-carb diets (contrary to a study referenced above) and so may be better for your metabolism.

There’s also a whole rash of studies about protein and animal fat/protein intake in particular, that take aim at high-protein, animal-based low-carb diets like Carnivore.

For instance, low-carb diets started in early adulthood (30s and 40s), and with animal fats and protein replacing carbohydrates, may lead to worse coronary artery calcium scores (a measure of heart health).

A 2010 analysis concluded, “A low-carbohydrate diet based on animal sources was associated with higher all-cause mortality in both men and women, whereas a vegetable-based low-carbohydrate diet was associated with lower all-cause and cardiovascular disease mortality rates.”

Additionally, it’s been suggested protein (and, specifically, methionine from protein) could be a major driver of aging itself and could have other negative effects like damaging kidney health.

Although, perhaps this is age-dependent, as a 2019 study in mice found, “Mortality in early and middle life was minimized at around one-part protein to two-parts carbohydrate, whereas in later life slightly greater than equal parts protein to carbohydrate reduced mortality.” Suggesting you should limit protein intake when young, and increase it as you age (possibly to prevent sarcopenia, or muscle wasting).

And to counterweight the Alaskan Inuit example provided above in the “for” section, a 2018 paper examining traditional hunter-gatherer societies that display markedly less cardiovascular and metabolic disease than people on modern Western diets found, “Diets in hunter‐gatherer and other small‐scale societies tend to be less energy dense and richer in fibre and micronutrients than modern diets but are not invariably low carbohydrate as sometimes argued.” In fact, in some cases 16–20% of total daily calories in hunter-gatherer societies came from honey—not exactly a low-carb food.

Of course, there’s also the research arguing that, if followed strictly, there’s no real health benefit to a low-carb vs. a low-fat diet. Both seem to converge at the same level of weight loss over time (though a low-carb diet starts out with more weight loss at first), and both show similar cardiometabolic protective effects as well.

How do low-carb diets increase lifespan?

Assuming low-carb diets do work to improve healthspan and possibly lifespan, by what mechanisms are they doing so?

• It’s possible low-carb diets mimic caloric restriction by lowering mTOR activation and IGF-1 (insulin-like growth factor 1) serum levels since ketones are less likely to activate either of these than carbs are. Both mTOR and IGF-1 are thought to be pro-aging, possibly because they promote the body’s growth pathway at the expense of cellular repair pathways.
• Low-carb diets may also lower inflammation, another potential major cause of aging, by producing a fat byproduct called beta-hydroxybutyrate which has been theorized to reduce oxidative damage in the body.

It’s also possible some combination of the above is the reason why low-carb diets extend lifespan.

How do I start a low-carb diet?

 First off, don’t start any diet intervention without first consulting your doctor, especially if you may have health conditions (like genetic nutrient deficiencies) or comorbidities that could make it dangerous, or if you’re pregnant.  

My take after losing myself in the research is that low-carb diets seem safe, but they may be best, like other longevity interventions such as intermittent fasting, when “pulsed” cyclically instead of adhered to constantly. A week on and a week off seemed to work pretty well in mice.

That said, women have different carbohydrate needs throughout their menstrual cycles, (and the above study on pulsing was only done on male mice), so women may want to start slower with restricting carbs and possibly avoid a one-week-on and one-week-off pulsing of a low-carb diet.

Additionally, it seems you may want the bulk of your fats from low-carb diets to come from plant sources like olive oil and avocados (which also seems to track with findings from the Mediterranean and other blue zone diets) vs. animal sources like (unfortunately) bacon. 

Diets that are both low in carbs and protein (at least in youth and middle age) seem like you may get the best of both worlds from a longevity and health standpoint, at least based on animal models. And this would seem to agree with certain aging theories about how things like constant mTOR activation and cellular growth, in addition to insulin resistance, are bad for lifespan.

Of course, when cycling off a low-carb diet it seems the types of carbs you’d want to add back in should be complex, low-glycemic-index carbohydrates that don’t spike blood sugar, like purple sweet potatoes and lentils, not Coca Cola and donuts.

It’s also worth noting the initial adaptation to a very low-carb diet, often called the “keto flu,” can be miserable as your body switches from burning carbs for energy to burning ketones. This adaptation period can last from 3-30 days according to one study, though anecdotal reports are that once the initial adaptation is done, it’s easier and faster for the body to “get back” into ketosis if you take a day or two off with higher carb intake.

There are as many low-carb diet options out there as there are different ideas on healthy nutrition (read: a ton) but a few helpful resources I’ve found if you want to get started with a low-carb diet yourself are:

• Reddit’s /r/keto FAQ: Great intro to the low-carb keto diet and links to lots of useful tools and resources.
• The Switch: Ignite Your Metabolism with Intermittent Fasting, Protein Cycling, and Keto by James Clement: One of our top longevity books and a good overview of low-carb diets as they relate to life extension.
• Ruled.me’s Keto calculator: Helps you plan out how many carbohydrates vs. fat vs. protein you may want to consume on a low-carb diet, relative to your age, size, activity levels, and goals.
• DietDoctor’s Low Carb Guide for Beginners: Great intro-level overview of the diet and how to do it.

What’s been your experience with low-carb diets?

I’ve personally tried Paleo, super strict Keto, Cyclical Keto, and Mediterranean-style diets and found benefits from all of them. 

I am likely to continue with a Cyclical Keto-type diet, with some healthy carbs like lentils added in on one or two days a week, and an emphasis on getting most of my fats from olive oil and avocados, while possibly lowering my protein intake on non-workout days. All that said I’d love to hear what you’ve noticed on any of these low-carb diets. 

Add your experiences in the comments!