The Ketogenic Diet (Keto): What You Should Know About its Impact on Your Gut Health



Hey there, wellness seekers! If you've ever heard whispers about the ketogenic diet (or keto diet) but found yourself asking, "What's the fuss all about?" – you're not alone. Join us as we take a short dive into the fundamentals of the ketogenic lifestyle and its impact on your gut microbiome.

What is a ketogenic diet? 

A classic ketogenic diet plan restricts the amount of carbohydrates that you consume. So how many carbs can you have on a keto diet? Let’s break it down. In a keto diet [1]: 

  • 80-90% of your calories are from fat
  • 6-8% of your calories are from protein 
  • 2-4% of your calories are from carbohydrates

In contrast, a typical recommended diet specifies that on average 55% of calories should come from carbohydrates [2]. Carbohydrates are a major nutrient that your body uses for energy. This happens when your body transforms carbohydrates into glucose. 

Without carbohydrates coming in, your body must resort to another form of energy—ketone bodies [3]. These are formed from the extra fat that is being eaten, as well as from fat stores within your body. Your body will now use ketone bodies instead of glucose as its energy source—a state called nutritional ketosis. This is where the name, “ketogenic diet”, comes from. 

Using extra fat for the production of ketone bodies is what causes the weight loss that can occur while on the keto diet. 

The timing and duration of a keto diet can vary based on individual goals, preferences, and health considerations. Here's a breakdown of different approaches:

  • Short-Term Keto: Some individuals adopt a ketogenic diet for short periods, often as a targeted approach to achieve specific goals.
  • Long-Term Keto: Others choose to embrace the ketogenic lifestyle as a long-term commitment. For them, keto becomes not just a diet but a sustainable way of eating that aligns with their health and wellness values.
  • Cyclical or Pulsed Keto: Some individuals follow a cyclical keto approach, incorporating periods of higher carbohydrate intake. This may involve cycling in and out of ketosis on a weekly or monthly basis.
  • Targeted Keto: Targeted keto involves consuming a higher amount of carbohydrates around workouts to support physical activity while maintaining ketosis at other times.

Who should avoid a ketogenic diet?

Note that the keto diet is not for everyone, especially those with special nutrition requirements. In fact, this diet is mainly prescribed for very particular medical circumstances, such as managing epilepsy symptoms and diminishing neuroinflammation.

Pregnant or breastfeeding women should avoid the keto diet as they require special nutrition to nourish their little one. Interestingly, being pregnant actually pushes your body into ketosis even if you continue eating carbohydrates, because your body must shift the way it uses energy to help grow your baby [4]. 

The amazing process of a baby's development relies on glucose, the rich energy source that is formed from carbohydrates [5]. Did you know that 60% of a baby’s energy goes towards growing their brain [6]? This big job can’t rely on just glucose, and instead also pulls energy from ketones to get the job done. Making sure to eat a balanced diet of carbohydrates, proteins, and fats is the best way to provide your baby with the energy it needs. 

In addition, special nutrients that are essential during pregnancy to support prenatal health such as folic acid, vitamin C, and fiber are not present at necessary levels in the keto diet [7]. 

The origin of keto diets

Interestingly, the keto diet was first invented in 1921 for the treatment of epilepsy [1], [8]. Today, the keto diet is still thought to be a leading therapeutic for epilepsy, especially in children whose epilepsy cannot be treated with medication [9]. It isn’t fully understood how the keto diet helps epilepsy patients, but it is thought that the ketone bodies your body uses for fuel while on the keto diet may be able to somehow disrupt seizure activity [10]. The gut microbiome may be playing a role here as well, as the gut microbiome can be disrupted in epilepsy patients [11].

In recent years, the keto diet has gained popularity for its use in weight loss. Especially in obese individuals, the keto diet can reduce weight dramatically and quickly. One study testing the keto diet as a weight loss measure found that it significantly reduced weight in their participants [12]. People who use the keto diet may experience greater weight loss compared to those who focus only on lowering calorie consumption [13]. People on the keto diet also often report feeling less hungry [14], which can lead to further weight loss. This may be due to the ability of ketone bodies to stimulate the release of hormones that make you feel full [15].

A very low-calorie version of the keto diet can also be effective for weight loss. In one study, 100% of participants experienced weight loss during the 6-week very-low calorie keto diet intervention, with an average reduction in weight of 15.4 pounds [16], alongside other reductions in blood pressure and inflammatory biomarkers. However, this diet is extreme and is best used for weight loss only over short periods of time under medical supervision [17], as long-term use may have negative effects on your gut health [18]. 

How to use the keto diet 

This sounds like a good way to lose weight, however, it is important to note that unless followed very closely, a high-fat diet without the strict carbohydrate restriction can actually lead to weight gain. This is because you will miss the weight-loss benefits of ketosis.

To reach a state of ketosis, you must eat no more than 50 grams of carbohydrates per day. There are 50 grams of carbohydrates each in [19]: 

  • 1.5 cups of beans 
  • 2 small sweet potatoes
  • 2 slices of whole wheat bread
  • 2 bananas
  • 1 cup cooked brown rice 
  • 1/3 cup of white sugar 

People following a keto diet instead opt for lower carbohydrate options, such as [20]: 

  • Meats, fish, and poultry 
  • Eggs 
  • Cheeses 
  • Leafy greens (limited amounts to avoid carbohydrates present in natural sugars)
  • Berries (limited amounts to avoid carbohydrates present in natural sugars)

How does the keto diet impact the gut microbiome? 

The keto diet may negatively affect your microbiome—as carbohydrates are a very important food source for your microbes! Keep an eye on your microbiome health and watch out for negative gut symptoms if you choose to try out this diet. 

The gut microbiome is a dynamic ecosystem heavily influenced by dietary choices. Dietary changes impact the abundance and diversity of gut bacteria, altering their metabolic activities and fermentation processes. Consequently, the balance between beneficial and harmful microbes can be disrupted, potentially leading to both positive and negative effects on gut health and overall well-being.

Carbohydrates—food for the microbiome 

Carbohydrates can have a large impact on gut microbiome composition, often a positive one. Carbohydrates can be divided into two forms: digestible and indigestible [21]. 

Digestible carbohydrates, like starches and sugars, get degraded early in the gastrointestinal tract and are turned into an energy source for your body. These sugars, and even alternative sweeteners like saccharin and maltitol, which are also carbohydrates [22], can affect your microbiome composition.

Indigestible carbohydrates on the other hand, like dietary fibers, travel all the way to the end of your gastrointestinal tract to your colon—which is where over 90% of your microbes live. In your colon, these indigestible carbohydrates will be fermented by your microbes to support both microbe and host health.

The keto diet restricts consumption of all forms of carbohydrates, including fibers that come from vegetables—so it is not surprising that some processes in your microbiome could be disrupted. 

Microbiome changes on the keto diet 

Research has shown that levels of bacteria such as Bifidobacterium [23], [24], [25], Streptococcus, and Firmicutes [12] are reduced on a keto diet. 

The ability of bacteria to produce metabolites like short chain fatty acids (SCFAs) may also be reduced on a keto diet [26], as this process relies entirely on carbohydrates. This may be due to reductions in levels of beneficial bacteria such as Bifidobacterium, as they are producers of these metabolites [27], [28]. SCFAs are big players in a healthy gut, and help maintain a strong gut barrier and immune system [29], [30]. They can also exit the gut and support other processes, such as the communication between the gut and the brain [31], [32]. 

Alongside these decreases, researchers also see increases in bacteria such as Bacteroides [33], [34], [35] Escherichia coli [24], and Parabacteroides [12]. This makes sense, as bacteria such as Bacteroides and Parabacteroides are associated with eating high levels of animal fats and proteins (think meats, eggs, and cheeses!) [36] And in the keto diet, protein and fat make up the majority of your diet. Bacteroides and Parabacteroides can play both protective or disruptive roles in the gut, so as long as their levels don’t get too high they shouldn’t be a problem for your gut health. 

A very low-calorie diet may have similar effects on the microbiome, one that is characterized by an increase in Bacteroides and Akkermansia and a decrease in Actinobacteroidota and Firmicutes (recently renamed to Actinomycetota and Bacillota, respectively) [37], [38]. Akkermansia is important in a healthy adult gut—and can support a healthy immune system and metabolic health [39], [40]. Polyphenol-rich foods like berries, grapes, and cocoa have been shown to stimulate levels of Akkermansia [41].

It’s worth noting that most of these studies looked at kids with severe epilepsy or adults with obesity or diabetes. Extrapolating these findings to healthy people should be done with caution, as the outcomes may not necessarily be identical.

Do these microbiome changes matter?

These microbiome changes could have big outcomes for your health. Important bacteria, such as Bifidobacterium, are likely to be reduced if you cut out carbohydrates. One study showed that the gut microbiome shifts associated with keto diet, such as a reduction in Bifidobacterium, can reduce the presence of important inflammatory cells in mice [42]. While chronic inflammation is far fromideal, inflammatory cells are needed to help your body launch an effective immune response when you’re sick. In addition, another study showed that the keto diet has the potential to exacerbate symptoms of inflammatory diseases such as inflammatory bowel disease [43]. 

Keep your microbiome in mind when you’re weighing keto diet pros and cons 

The keto diet is a very-low carbohydrate diet designed to reduce epilepsy symptoms, especially in children, but has been used for weight loss in adults. Make sure to consult with your provider to make sure the keto diet offers the nutrients you need, especially in special times such as during pregnancy or breastfeeding.  

When you stop eating carbohydrates your gut microbiome does too! Carbohydrates have a lot of roles for your gut microbiome, such as the production of short chain fatty acids. Reducing your carbohydrate intake may be okay, but you should expect changes in the abundance of bacteria in your gut with this diet choice. 

An increase in uncomfortable gut symptoms may mean it’s time for a gut check to be sure your microbiome is tolerating the keto diet. And if you are thinking about starting a keto diet, it's a good idea to check in and optimize your gut health first.


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