Do Low-Carb Diets Hurt or Help Brain Function?

A closer look at the research.

Before I discuss what the research currently shows about low-carb diets, cognitive function and mental health, I need to get something out the way.

Whenever I write anything remotely critical about the effects of a popularized diet on brain function, two things invariably occur:

Steadfast advocates of said diet promptly declare they have been following it for X years, and self-appraise their mental and cognitive function as just fine. Therefore, according to these people, any data showing said diet to exert negative cognitive effects - even when derived from RCTs - is clearly bogus. These folks are basically asserting their personal anecdotal experiences override the findings of controlled research and that their experiences apply to everyone.

The English language has a number of terms to describe people like this, such as “clueless”, “dumb”, and “nincompoop”.

Steadfast advocates of said diet react angrily, their irrational and disproportionate outburst doing little to refute the contention that said diet can negatively impact cognitive function. It’s kind of funny when your staunchest critics actively help make your case. A memorable example was the last time I posted an article about low-carb diets and mental health, way back in 2010. In that post, I referenced untenable claims made for LC diets by folks like the infamous Robert Atkins and a guy called Michael Eades. The latter, along with his wife, rewrote the former’s New Diet Revolution, adding an anti-saturate angle and calling the resultant tome Protein Power. Long-time readers will know Eades and I don’t like each other. I think he’s a charlatan, and he thinks I’m unspeakably evil for publicly calling out his nonsensical “metabolic advantage” claims (the repeatedly disproved contention that LC diets offer some magical weight loss advantage over isocaloric higher-carb diets).

Anyways, my brief mentions of Eades in my 2010 post were evidently the straw that broke the donkey’s back, because he promptly posted an angry, long-winded masterclass in butt-hurt declaring I was a “man obsessed”. His evidence for this diagnosis was my mention of how many negative reviews had been posted for his book on Amazon, and what some of those reviews had to say. Eades, utter genius that he is, thought I had scoured through each and every one of the hundreds of Amazon reviews to ascertain the tally and read the negative ones. This, he triumphantly declared to his readers, was proof that I was batshit bonkers.

Never mind that at the very start of every Amazon review section is a clearly visible horizontal bar graph that tells you exactly how many 1, 2, 3, 4 and 5 star reviews have been posted for the item in question.

Lolz.

Furthermore, to access the reviews of a certain rating (e.g. all the 1 star reviews), you simply click on the easily discernible link in the bar graph for that rating.

Some of Eades’ less moronic readers pointed this out to him, instantly destroying his entire “obsessed” thesis.

Eades’ response was a dopey “oh…I didn’t know that.”

The Italian language has a number of terms to describe people like this, such as “ciuccio”, “piciolla”, and “scemo” (pronounced chu-cho, pee-shaw-la, and sheh-mo). Please don’t use that second term when meeting your Italian partner’s parents for the first time, it could result in strange looks. Half a dozen visits in and you should be fine, especially when referencing politicians, influencers and people who put pineapple on pizza.

Then there are my vegan critics.

Double lolz.

Back in February, I discussed the considerable evidence linking vegan and vegetarian diets to increased risk of mental disorders and antidepressant use. Right on cue, an angry vegan called “Maureen” showed up to assert this was simply untrue, and she proved it by embarking on a long-winded rant that had nothing to do with the actual article content. She admitted she barely even read any of it.

Then, to confirm just how mentally coherent she really was, she talked rudely to another reader in the comments section as if they wrote the article, even though that reader clearly wasn’t me.

Then a genius by the name of “Philip” sent me an email. I could tell right away Phil was an intellectual powerhouse, because his Instagram moniker is “wicked_vegan_chef” and his past endeavors included running nightclubs. They’re those places, thankfully falling out of favor nowadays, where people used to go to get shitfaced and make bad life choices, like hooking up with strangers or ending up in ED after antagonizing people they knew nothing about.

Like Maureen, Phil was incurably addicted to his own hot air. He ranted on and on, again subjecting me to an incoherent diatribe that discussed everything but the actual data in my article.

Needless to say, if I write an article highlighting the potential mental pitfalls of a said dietary approach, you’re not going to convince me I’m wrong by acting like a waffling loon or by displaying your laughable ignorance of how the world’s most popular retail website works. Rather, you’re merely strengthening my argument by providing a real life example of someone who follows said diet and acts like a complete dipshit.

Another thing to keep in mind when discussing the pitfalls of a certain diet is the difference between therapeutic use and general use. For example, if a certain diet is shown to help a condition like epilepsy, that doesn’t even begin to mean healthy non-epileptic folks should embrace said diet. The ketogenic diet has been shown to exert beneficial effects in epilepsy, but also comes with a lengthy list of documented side effects. In a situation where controlling epilepsy is the prime concern, and the risks of medication outweigh those of the diet, then use of a ketogenic diet is indeed warranted.

In a non-epileptic person who doesn’t fancy watching his athletic performance deteriorate like an OnlyFans girl’s self-respect, use of a keto diet is not just unwarranted but downright ill-advised.

Okay, so with that disclaimer out the way, let’s begin our examination of how low-carbohydrate diets might effect what goes on inside your cabeza. In this article, we’ll focus on the effects of ketogenic diets on measures of cognitive function, such as working memory, attention, speed of processing and accuracy. These are attributes you need to optimally perform daily functions, including work and study tasks.

This article will focus on the effects noted in volunteers free of known mental or cognitive issues, not people with neurological conditions like epilepsy. The effects of ketogenic diets on epilepsy are well known and I’m not going to belabor them here.

In a subsequent installment, we’ll look at the effects of carbohydrate restriction on conditions like anxiety and depression.

Low-Carb Diets and Cognitive Function

In a study published in International Journal of Obesity, overweight women consuming either a ketogenic or non-ketogenic 600-calorie/day low-carbohydrate diet were monitored for 28 days. This study was performed under metabolic ward conditions, meaning the subjects definitely followed the diets they were assigned.

Of course, contrary to what the tooth fairy metabolic advantage believers would predict, there was no difference in weight loss between the two groups.

Weight loss was not the only outcome measured; the study also examined the effect of the two diets on cognitive function. The researchers found that, in the first week, ketogenic dieters performed far worse on the trail making task, a widely used test that measures a range of neuropsychological skills including letter and number recognition, visual scanning, motor speed and sequencing ability. After the first week, the ketogenic group began to improve their test results, but by the time the study concluded at 28 days, their results were still lagging behind the non-ketogenic dieters (Wing et al, 1995).

Researchers from the CSIRO (Commonwealth Scientific and Industrial Research Organisation) in Adelaide, Australia compared the effects of a low-carbohydrate, high-fat diet with a conventional high-carbohydrate, low-fat diet on mood and cognitive function.

The low-carbohydrate diet provided 35% of total energy as protein, 61% as fat, and 4% as carbohydrate; the high-carbohydrate diet provided 24% of total energy as protein, 30% as fat, and 46% as carbohydrate. The diets were designed to be isocaloric and to have a moderate calorie restriction of around 30% for 8 weeks, however this was a free-living study.

After eight weeks, mean scores on the Beck Depression Inventory, Spielberger State Anxiety Inventory and Total Mood Disturbance of the Profile of Mood States psychological questionnaires had improved similarly on both diets. Working memory test scores increased similarly in both groups, while speed of processing improved to statistically significantly greater degree on the high-carbohydrate diet (Halyburton et al, 2007).

Noting that most studies on the topic were relatively short in duration, the CSIRO researchers later published a 12-month follow-up report (Brinkworth et al, 2009).

At one year, both diets had similar effects on working memory and speed of processing.

While this article is focused on cognitive processing, it’s worth noting scores on the Beck Depression Inventory, Total Mood Disturbance of the Profile of Mood States and anger-hostility, confusion-bewilderment, and depression-dejection subscales of the Profile of Mood States decreased (improved) initially in both diet groups and tended to remain low in the high-carbohydrate group. However, they drifted back toward baseline levels over time in the low-carbohydrate group.

At 12 months, scores on the Profile of Mood States subscales of anger-hostility, confusion-bewilderment, and depression-dejection and the Total Mood Disturbance of the Profile of Mood States score were significantly lower in the high-carbohydrate group than in the low-carbohydrate group.

On the Spielberger State Anxiety Inventory, differences were also evident between the diet groups at 12 months. A similar pattern of change was evident on the Beck Depression Inventory, although the difference between the diet groups did not reach statistical significance.

Even though the low-carb group fared worse in these mood state measures, the CSIRO researchers point out that the average scores for both groups at baseline and throughout the study remained within the normal range for healthy adults.

Danci et al (2009) studied nineteen women who self-selected a ketogenic diet or a diet similar to that recommended by the American Dietetic Association. Individuals who chose the keto diet were instructed to limit their daily carbohydrate intake to zero for one week, 5-8 grams during the second week, and 10–16 grams in the third week. Before, during, and after the diets, participants completed the Profile of Mood States questionnaire and a battery of cognitive tests.

During complete withdrawal of dietary carbohydrate, the keto group performed worse on memory-based tasks than ADA dieters. These impairments were eliminated after reintroduction of carbohydrates. Low-carbohydrate dieters reported less confusion and responded faster during an attention vigilance task than the ADA dieters.

So this non-randomized study found a mix of negative and positive effects.

Makris et al (2013) randomized forty-seven obese males and females to either an Atkins-style diet or one containing 55% carbohydrate. Over the course of the trial, there were no differences between groups in weight loss or performance on four computerized cognitive tasks: Stroop task (attention and information processing); Continuous Performance Task (CPT, attention and reaction time); word recall (verbal short-term memory), and; Wisconsin card-sorting task (problem solving, set-shifting and cognitive flexibility).

Iacovides et al (2019) was a small study featuring 11 males and females who followed high-carb and keto diets, each for 3 weeks in randomized crossover fashion. The main paper for the Iacovides study reported speed and accuracy responses for 4 cognitive tasks (Identification, Two Back Test, One Card Learning, and Groton Maze Learning Test).

Some scores were better after the high-carb diet, others were more favorable after the keto diet - however all such differences were marginal.

The researchers concluded “3 consecutive weeks in a sustained state of nutritional ketosis had no effect on vigilance, visual learning and memory, executive functioning, and working memory compared with 3 wk on an HCLF diet.”

Interestingly, the researchers also examined global cognitive function, which refers to a person's overall ability to process thoughts, including memory, speech, and learning new information. Clearly important, yet the researchers relegated it to secondary outcome status and only mentioned it in the separate supplementary material. When we open the supplementary file, we learn their analysis of global cognitive function indicated response time was significantly slower after the keto diet compared to baseline testing speeds.

Shaw et al (2022) examined the effect of a two-week keto diet in a small group of military personnel (n = 8) compared to a carbohydrate-based diet, in randomized crossover fashion. Cognitive performance (Psychomotor Vigilance task, 2-choice reaction time, and running memory continuous performance test) and mean weekly fatigue, vigor, and sleep quality measures were similar between the diets.

The researcher observed heart rate variability was reduced during the second week of the keto diet, suggesting induction to the diet increased physiological stress. Just what this means over the longer term is unclear. The researchers stated:

“HRV is a biomarker of interest within various populations to monitor stress-related impairments, such as the military, as lower HRV could have implications for cognitive performance. Nevertheless, we did not observe clear relationships between Δ weekly mean HRV and any of the cognitive performance variables within each diet. This is unsurprising given there was no difference between diets in cognitive performance variables, but this may change when under additional psychological and physiological stressors.”

More recently, Serio et al (2026) observed eight participants who completed two cognitive assessments immediately before and after 3 weeks of a eucaloric (weight-maintaining) ketogenic diet. This was not an RCT; no other diet was simultaneously compared to the keto regimen.

The results showed that participants performed a number of tasks and solved mathematical operations in the Automated Operation Span Task (Aospan) task faster after the keto intervention. However, they performed the Aospan task less accurately; participants had a lower absolute span score and solved fewer mathematical expressions correctly after the 3-week keto diet. The Aospan task is a cognitive experiment that measures working memory capacity. It requires participants to solve math problems while remembering a series of letters, testing memory ability under cognitive load.

Two Year Forecast: Brain Fog and Forgetfulness, Clearing With Cessation of Keto Diet

The longest of the abovementioned studies was 12 months. Recently, Neurology published a case report of 48 year old woman who presented with a two year history of gradually worsening memory loss and brain fog (Afzal and Salzman, 2024).

The patient described difficulty finding words and impaired ability to recall details of conversations or events. Formal cognitive testing revealed deficits in attention, anterograde memory, and executive functions such as task-switching and planning. Her labs, including vitamin B12, folate, TSH, comprehensive metabolic panel, CRP, and ESR, were normal.

The patient revealed she had adhered to a strict ketogenic diet for weight loss continuously for the past two years. She discontinued the keto diet, and within two months noticed significant improvement in her memory and cognitive function. Repeat cognitive testing was normal.

The researchers concluded:

“Providers should be aware of this potential neurocognitive side effect. Close monitoring of cognitive function in patients on long-term ketogenic diets may be warranted. Further research is needed to better characterize the impact of the ketogenic diet on cognition over time.”

Conclusion

Studies comparing the effects of ketogenic and non-ketogenic diets on cognitive attributes such as accuracy, processing speed and memory have found similar results. When differences are found, they tend to be in favor of the non-ketogenic diet, although the differences haven’t exactly been earth-shattering.

However, most of these studies were small, of relatively short duration, and involved carefully screened volunteers. Afzal and Salzman’s case study suggests ketogenic diets can indeed negatively impact cognitive function over the longer-term in at least some people.

The bizarre behavior of many low-carb advocates - including those who are so full of themselves they think someone would actually read each and every one the hundreds of Amazon reviews for their book, or who think it’s a great idea to become child molesters, or who form gushing man-crushes on convicted sex offenders (“Pussy is, indeed, low carb”!) - is also cause for concern. However, it’s hard to determine whether this behavior is a product of the diet, or because extreme personalities seem to be attracted to extreme diets. Perhaps it’s a mix of both.

It behooves me to note there are blow-hards and deviants out there following all manner of diets, and that I’ve met people following low-carb diets who didn’t behave like screwballs.

I must say, though, that in my 25 years or so of posting on the Internet, my most bizarre and demented trolls have come from the low-carb and vegan camps. There seems to be something about these two dietary philosophies that attracts and/or creates complete and utter loons.

Anecdotally, aside from the la la crowd, I’ve observed mixed responses to ketogenic diets. Some people report impaired cognition, others improved. Those in the latter category were often following diets high in refined or high-glycemic index carbs (wholemeal products, despite popular propaganda, typically have similar glycemic responses as their ‘white’ counterparts) prior to commencing low-carb regimens. Wildly fluctuating blood sugar levels can result in brain fog, lethargy and irritability, all of which make it hard to think and perform at your best.

At any rate, if you’re an organically healthy person with good glycemic control looking to maximize your cognitive function, there is little evidence to suggest cutting carbs will help, with some of the data hinting at possible negative effects.

The situation with depression and anxiety is a little more complex. Stay tuned.

References

Wing RR, et al. Cognitive effects of ketogenic weight-reducing diets. International Journal of Obesity, 1995; 19: 811-816.

Bogardus C, et al. Comparison of carbohydrate-containing and carbohydrate-restricted hypocaloric diets in the treatment of obesity. Endurance and metabolic fuel homeostasis during strenuous exercise. Journal of Clinical Investigation, 1981; 68: 399-404.

Danci et al. Low-carbohydrate weight-loss diets. Effects on cognition and mood. Appetite, 2009; 52 (1): 96-103.

Halyburton AK, et al. Low- and high-carbohydrate weight-loss diets have similar effects on mood but not cognitive performance. American Journal of Clinical Nutrition, 2007; 86: 580–587.

Brinkworth GD, et al. Long-term Effects of a Very Low-Carbohydrate Diet and a Low-Fat Diet on Mood and Cognitive Function. Archives of Internal Medicine, Nov 9, 2009; 169 (20): 1873-1880.

Makris A, et al. Similar effects on cognitive performance during high- and low-carbohydrate obesity treatment. Nutrition & Diabetes, 2013; 3, e89; doi:10.1038/nutd.2013.29

Iacovides S, et al. Three consecutive weeks of nutritional ketosis has no effect on cognitive function, sleep, and mood compared with a high-carbohydrate, low-fat diet in healthy individuals: a randomized, crossover, controlled trial. American Journal of Clinical Nutrition, 2019; 110 (2): 349–57.

Shaw et al. Cognitive, Sleep, and Autonomic Responses to Induction of a Ketogenic Diet in Military Personnel: A Pilot Study. AEROSPACE MEDICINE AND HUMAN PERFORMANCE, June 2022 ; 93 (6): 507-516.

Serio G, et al. Faster but less accurate: An explorative study on the effects of three weeks of ketogenic diet on cognitive functions in undergraduate students. PLOS One, Jan, 2026; 21(1): DOI:10.1371/journal.pone.0338877

Afzal S, Salzman D. Reversible Memory Loss and Brain Fog Associated with Prolonged Ketogenic Diet Use: A Case Report. Neurology, Apr 9, 2024; 102 (7, supp 1) 6118. https://doi.org/10.1212/WNL.0000000000206249