4 Potential Side Effects of the Paleo Diet (2024)

The paleo diet is a way of eating meant to resemble the dietary pattern of human ancestors during the Paleolithic era, which took place about 2.5 million years ago (1).

Although their dietary patterns varied depending on geographic region, prehistoric people consumed diets rich in protein sources, like insects, game animals, and seafood, as well as plants like fruits, tubers, nuts, and seeds (1).

Obviously, the world — and our food supply — is a little different now, so the modern-day paleo diet focuses on consuming whole, minimally-processed foods.

People following a paleo diet avoid consuming grains, dairy, most types of added sugar, legumes, and certain oils, like canola oil (2).

A well-rounded paleo diet can absolutely be a healthy choice and may improve certain aspects of health. However, if you don’t plan a paleo diet carefully, it could lead to several side effects (3, 4, 5).

Here are 4 potential side effects of the paleo diet.

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1. Diarrhea

Most people don’t consume nearly enough fiber on a daily basis.

In fact, the majority of people in the United States only consume about 15 grams of fiber per day, which is well below the current recommendations. Experts say women under 50 should consume 25 grams per day and men under 50 should consume 38 grams per day (6).

This lack of fiber is because most modern-day diets are high in ultra-processed foods and low in fiber-rich foods, including fruits and vegetables.

A paleo diet is high in fiber-rich plant foods, like vegetables, fruits, and nuts. A person transitioning from a low fiber diet to a paleo diet could experience diarrhea as a result.

Higher fiber diets, like well-planned paleo diets, could cause diarrhea in someone who’s not used to higher fiber foods, like fruits, vegetables, nuts, and seeds.

This is because foods high in insoluble fiber can have a laxative effect, which can cause diarrhea (6).

For some people — such as those with irritable bowel syndrome (IBS) — eating a diet higher in fat, like a paleo diet, could also lead to digestive symptoms, including diarrhea (7).

​​In fact, one study found that eating a paleo diet increased participants’ risk of having diarrhea. In the study, 39 healthy women consumed either a paleo diet or a generally healthy diet that included grains and dairy for 4 weeks.

The women following the paleo diet had significantly higher rates of diarrhea compared with the general diet group (8).

Summary

Higher fiber diets, such as well-planned paleo diets, could cause diarrhea in someone who’s not used to higher fiber paleo-friendly foods like fruits, vegetables, nuts, and seeds.

2. Constipation

Some people think the paleo diet is a low carb diet that’s high in animal protein, but that isn’t necessarily the case.

Although the paleo diet cuts out common sources of carbs, such as grains, carb-rich snack foods, and added sugar, it’s rich in high fiber foods like vegetables, fruits, nuts, and seeds.

If a person is following a paleo diet that’s very low in carbs and isn’t eating enough fiber from produce, nuts, and other paleo-friendly fiber sources, they could experience constipation.

For example, if a person switches from a high carb, plant-based diet to a low carb paleo-like diet, they will significantly reduce their intake of fiber, which can negatively affect the digestive system (9).

Fiber adds bulk to stool and helps it move comfortably through the digestive tract.

Not eating enough fiber can lead to constipation, which can be painful and negatively affect your health (6, 10).

To ensure you’re getting enough fiber on the paleo diet, it’s important to eat plenty of fruits, vegetables, nuts, and seeds.

Summary

If you’re following a paleo diet that’s low in fiber-rich foods — like fruits, vegetables, nuts, and other paleo-friendly fiber sources — you could experience constipation.

3. Changes in gut bacteria

Findings from a recent study suggest that people following a paleo diet long-term may experience negative changes in their gut bacteria.

One study followed 44 people who were on a strict paleo or modified paleo diet and 47 people following regular diets for at least a year.

It found that the participants following the paleo diet had lower levels of certain beneficial bacteria and higher levels of harmful Hungatella bacteria (11).

Having higher levels of these bacteria may be harmful because of a compound they produce called trimethylamine (TMA).

Certain gut bacteria, including Hungatella, produce TMA after breaking down compounds found in red meat, eggs, and dairy. TMA is turned into trimethylamine N-oxide (TMAO).

Having higher levels of TMAO has been associated with an increased risk of certain conditions, like heart disease and kidney disease (12, 13).

The study above found that the participants who followed a strict paleo diet had higher levels of TMAO compared with the groups that followed a modified paleo diet or a regular diet.

What’s more, they found that the groups that followed a strict paleo or modified paleo diet also had a lower intake of resistant starch. When gut bacteria ferment this type of starch, they release beneficial compounds called short-chain fatty acids (SCFAs) (11, 14).

SCFAs act as a fuel source for the cells lining the colon and are involved in many other aspects of health (15).

Because of these considerations, researchers have suggested that following a diet that completely restricts grains and legumes for a long time period may not be beneficial for gut health (11).

However, more research is needed to fully understand how the paleo diet affects gut health.

Summary

Some research findings suggest that following a paleo diet long-term could negatively affect gut health by reducing the abundance of beneficial bacteria and increasing numbers of TMA-producing Hungatella bacteria.

4. Fatigue

Any poorly planned diet can cause you to feel “off” and have lower energy than usual.

If a paleo diet is too restrictive, it could lead to low energy levels and feelings of tiredness. Too restrictive means it doesn’t provide enough calories or certain nutrients, such as carbs.

Low carb intake may be one important cause of fatigue. Low carb diets, including the ketogenic diet, have been shown to cause fatigue. Research suggests this may be due to depleted glycogen stores that can happen with a low carb diet (16, 17).

Glycogen is a form of energy stored in your muscles and liver. Your body uses it as a quick fuel source (18).

Remember, the paleo diet is not necessarily considered a low carb diet. You can increase the carbs in a paleo diet by including more paleo-friendly carb sources, such as sweet potatoes, winter squash, and fruit.

Nonetheless, if a paleo diet doesn’t provide the energy and nutrients your body needs on a daily basis, you may experience fatigue.

Summary

A poorly planned paleo diet that doesn’t contain adequate calories or carbs may negatively affect energy levels.

How to follow a healthy paleo diet

Even though some studies have found that the paleo diet may lead to a few side effects, most studies have found that the diet is generally well-tolerated and isn’t associated with adverse health effects.

In fact, one study showed that the paleo diet may even be a better choice for psychological health compared with other popular diets.

The study analyzed the psychological effects of five dietary patterns, including vegetarian, vegan, gluten free, paleo, and weight loss diets.

People following paleo dietary patterns reported the lowest levels of disordered eating symptoms, food cravings, emotional eating, and negative emotions (19).

Interestingly, the paleo diet was associated with more positive psychological characteristics even when compared with a normal, unrestricted diet group (19).

Plus, the paleo diet may positively influence other aspects of health, including metabolic health (3, 4, 5).

So, how do you follow a balanced and nutritious paleo diet that’s unlikely to cause side effects?

Here are a few helpful tips:

• Consider a modified paleo diet. It’s less restrictive and allows a limited consumption of grains, legumes, and dairy. Findings from one study suggest that a modified paleo diet may be a better choice for gut health than the unmodified version (11).
• Make sure you’re consuming enough fiber. Some people don’t eat enough fiber when following a paleo dietary pattern. Adding in plenty of vegetables, fruits, and fiber rich products like chia seeds and flax seeds can help you increase your fiber intake.
• Eat regular, balanced meals. Consuming regular and balanced meals appropriate for your activity levels, body size, age, and weight goals is important for energy levels, blood sugar regulation, digestion, and more.
• Remember that the paleo diet is not a low carb diet. The paleo diet is not necessarily a low carb diet. It can be designed to be low carb, but you can also increase your intake of fruits and starchy vegetables to bump up your carb intake if needed.
• Work with a registered dietitian. If you’re interested in following a paleo diet but aren’t sure where to start, consider working with a registered dietitian. They can help you plan a paleo diet that covers your nutrient needs and helps you feel your best.

Additionally, any dietary pattern — including an unrestricted dietary pattern — has the potential to lead to low or deficient nutrient intakes.

Cutting out food groups like legumes, all grains, and dairy could increase your risk of underconsuming certain nutrients. However, studies show that paleo diets and modified paleo diets are of minimal nutritional risk compared with a standard eating pattern (20, 21, 22).

Still, it’s a good idea to keep an eye on your nutrients. For example, a person following a strict paleo diet should focus on getting enough calcium through nondairy sources, like leafy greens, seeds, sardines, and nuts.

Also, certain nutrient deficiencies are common no matter the type of diet you’re consuming. For example, vitamin D deficiency is one of the most common nutrient deficiencies worldwide (23).

Whatever diet you follow, consider having your vitamin D levels tested, and take vitamin D3 if your levels are low.

Summary

To minimize the risk of side effects and nutrient deficiencies, it’s important to focus on consuming regular, nutrient-dense, balanced meals. Adding in fiber and following a less restrictive modified paleo diet may also help.

The bottom line

The paleo diet can be a healthy dietary pattern. However, like any restrictive diet, it may lead to some side effects.

Studies show that some people may experience diarrhea, constipation, fatigue, and changes in gut bacteria when following a paleo dietary pattern.

To minimize the chances of experiencing side effects and to make sure you’re getting enough nutrition, eat regular and balanced meals that include a variety of nutritious, paleo-friendly foods.