Why the keto diet may not be safe, especially for some people

The keto diet is a very high-fat, very low-carbohydrate diet that excludes healthy sources of carbs like whole grains, beans, fruit, and starchy vegetables.

Because it cuts out so many foods containing fiber, vitamins, and minerals, and can increase how much saturated fat you eat, the keto diet is probably not healthy for most people in the long term.

In particular, it’s not recommended for pregnant and breastfeeding women or those with certain health conditions, like IBS, osteoporosis, or kidney problems.

The keto diet can improve markers of health — such as cholesterol and other blood fat levels — in people with obesity without major side effects. But the studies that show this have been short-term and often required medical supervision to ensure participants stayed healthy.

Before going on any diet plan, you should first think about whether you really need to lose weight. If your body mass index (BMI) lies within the underweight range or at the lower end of the moderate range, you don’t need to lose weight.

Instead, we believe in helping you to understand your personal responses to food to empower you to achieve your health and weight goals.

To find out more about how eating the right foods for your metabolism can benefit your overall health and your weight, take our free quiz. 

What is the keto diet?

The keto diet is a very low-carbohydrate, high-fat diet. Most keto diet plans suggest that 10% or less of the total calories you eat each day should come from carbs, with around 70% from fat and 20% from protein.

More specifically, this means less than 50 grams of carbs per day.

Your body’s main source of energy is glucose, a sugar that you get from carbs. But when you restrict carbs and your glucose stores are used up, your body switches to using substances called ketones, which it gets from fat.

This process is called ketosis, and the idea is that it can lead to you burning body fat. 

However, when you go from a carb-burning to fat-burning state, you release stored water. The fast weight loss that some people see when they first start keto can be due to this rather than fat loss.

Foods allowed on the keto diet include sources of protein like meat, seafood, and cheese. Fat can come from oils, nuts, butter, dairy, seeds, and red meat. 

Keto-friendly vegetables include leafy greens, cabbage, broccoli, cauliflower, tomatoes, and celery, although even these contain some carbs, so there’s a limit to how much of them you can eat. 

Foods to avoid on the keto diet include any that contain a significant amount of carbs.

That includes healthy foods like whole grains, beans, most fruit, and vegetables like potatoes, squash, carrots, onions, and corn, as well as lentils, rice, pasta, and bread. 

Sugar is a carbohydrate, so many condiments, dressings, and sauces are out, as are candy and desserts. 

Is the keto diet safe?

Some scientists have studied the short-term safety of the keto diet, but there hasn’t been a huge amount of research into its longer-term effects.

Given this lack of data, there is the potential for the keto diet to cause harm in the long term, but more research is needed. 

Given that it’s not a balanced diet and cuts out so many nutritious foods, there’s good reason to believe that continuing with it for a significant amount of time would not be healthy. 

It’s also possible that extreme diets like keto can have long-term effects on your metabolism and ability to lose weight, even if you only follow them for a relatively short time. 

Short-term safety

Some people report unpleasant side effects when they start the keto diet. These include the so-called “keto flu,” which can make you feel quite unwell, as well as others:

• Keto flu: Symptoms include headaches, achy muscles, and fatigue, which are associated with the change from burning carbs to fat. 

• Bad breath: Ketones are acidic and can cause bad breath.

• Constipation: Removing high-fiber foods from your diet can lead to constipation.

• Low blood sugar: Low carb consumption can lead to low blood sugar and, in turn, symptoms like dizziness, shakiness, and heart palpitations.

Despite this, some researchers looking into the effects of keto have concluded that it’s relatively safe in the short term. 

One study assessed the effect of a keto diet in 83 people with obesity for 24 weeks. The participants received 20–30 grams of carbohydrates daily in the form of green vegetables and salad and 80–100 grams of protein from meat, fish, poultry, eggs, shellfish, and cheese. 

The rest of their diet was made up of fats (20% saturated fat and 80% healthier polyunsaturated and monounsaturated fats). At the halfway stage, an extra 20 grams of carbs were added to their diets.

By the end of the study, the participants had lost significant amounts of weight and saw healthy changes to the levels of sugar, cholesterol, and other fats in their blood.

The researchers found no signs of the unhealthy side effects seen with certain weight-loss drugs and concluded that it was safe to use the keto diet over this period of time.

However, this was a relatively small study, focusing specifically on people with obesity with a high BMI of between 36 and 39, and it involved continued medical and nutritional supervision. Most people who go on restrictive diets don’t have these luxuries. 

Long-term safety

There’s not much good long-term research around the safety of the keto diet. However, there is plenty of evidence that people who go on restrictive diets tend to end up regaining the weight they’ve lost. 

Sticking with a very restrictive diet can be challenging for many people. Even short-term restrictive dieting can decrease your metabolic rate long term, which is the amount of energy used by your body to perform its basic everyday functions.

These changes can last for years, making you more likely to gain weight in the future. 

There are also lots of potential nutritional issues associated with a diet that increases some types of foods and restricts others in such an extreme way.

Increasing your intake of fat in the form of dairy products and meat will introduce more saturated fat in your diet.

While there are some links between saturated fat and raised levels of LDL or “bad” cholesterol — and, in turn, an increased risk of heart disease — more recent studies don’t support this. 

How to eat healthy fats

If you are going to increase your fat intake, it’s a good idea to focus on foods with high quality healthy fats, like avocado and olive oil. 

ZOE’s research has found that some people’s bodies are better at responding to high levels of fat than others. If your body struggles to cope with increased levels of fat, then the keto diet may put you at higher risk of dietary inflammation. 

Cutting out healthy plants, meanwhile, can lead to a lack of important nutrients including certain vitamins, minerals, and fiber.

Plants that contain fiber are a great source of substances known as prebiotics, which feed the “good” microbes that live in your gut, helping them to out-compete your “bad” gut microbes.

ZOE scientists have identified 15 “good” gut microbes and 15 “bad” gut microbes. The good microbes are associated with a healthier heart and metabolism, while the bad microbes are linked to a higher risk of heart disease, type 2 diabetes, and excess weight.

The ZOE program helps you identify the best foods and food combinations for your body. It will also tell you the best foods for the unique combination of bugs that live in your gut, known as your gut microbiome.

Unpublished research from ZOE shows that when people closely followed our personalized gut-healthy nutrition program, around 80% of participants didn’t feel hungry and had more energy.

The program led to an average weight loss of 9.4 pounds after 3 months.

Find out more about the ZOE program and how it can help you.

Who really shouldn’t do keto?

Some people face particular health risks from the keto diet. They include pregnant or breastfeeding women and people with certain health conditions, including:

• People with osteoporosis: Ketosis can lead to a condition called acidosis, where there is too much acid in your bodily fluids. Acidosis can remove some of the minerals from your bones, increasing the risk of fracture. People who already have weakened bones, like those with osteoporosis, should avoid keto. One study found that athletes who followed the keto diet for a short time showed signs linked to less efficient bone repair after exercise. 

• People with IBS: Because keto restricts many high-fiber foods that help keep you regular, it can affect how often you poop. People with irritable bowel syndrome (IBS) who are prone to constipation may have worse symptoms on keto.

• People with kidney problems: Our kidneys filter fat and protein, which can both be increased on the keto diet. For people with existing kidney problems, pushing their kidneys to work harder by processing the extra protein can make things worse.

• People who are underweight: If your BMI is in the underweight range, following a restrictive diet is not recommended. Work with your doctor if you have concerns about your diet. 

• Pregnant or breastfeeding women: Women who are pregnant or breastfeeding need a healthy range of nutrients, so they should not go on restrictive diets.

• Children: A balanced diet is particularly important at a time when your body is growing. Restrictive diets also increase children’s risk of developing eating disorders later in life. 

• People with a history of eating disorders: If you've had an eating disorder in the past, following a restrictive diet is not recommended, as this may increase your risk of disordered eating. Work with your doctors to find the best dietary choices for you.

Summary

The keto diet is a very low-carbohydrate, high-fat diet. It aims to take your body from burning the glucose produced by carbs to burning ketones from the fat you eat and your body fat.

It often leads to weight loss to begin with, but some of this is probably due to the release of excess water rather than fat. 

As with all restrictive diets, you’re likely to regain the weight once you stop keto. This may be partly due to a drop in your metabolic rate — how fast your body uses energy — which can last for a long time afterward.

People on the keto diet sometimes experience so-called “keto flu” when they first start out on the diet, as well as bad breath, constipation, and low blood sugar.

Some research suggests keto may be relatively safe in the short term, but these studies have involved medical supervision and giving participants food supplements to keep them healthy. 

Keto is particularly unsuitable for pregnant and breastfeeding women, people with a history of eating disorders, and those with certain health conditions.

The keto diet may contain high levels of unhealthy saturated fats that are linked to an increased risk of heart disease.

It also cuts out healthy foods like whole grains, beans, fruit, and starchy vegetables. These contain nutrients including fiber that feed your “good” gut bugs and are good for your general gut health.

The 15 “good” gut bugs identified by ZOE are associated with better heart and metabolic health as well as less belly fat.

ZOE’s at-home test can tell you which “good” and “bad” bugs live in your gut, and we can give you personalized advice about foods to improve your gut health. Following the ZOE program can boost your energy and lead to gradual weight loss.

Find out more about the ZOE program and how it can help you.