Written by Aisling Moran BSc (Hons)
25th Feb 2022 • 8 minute read

A ketogenic is a type of low-carb, high-fat diet. It was initially designed to help treat childhood epilepsy but has gained popularity among the general population. This is largely due to claims that it can help with everything from weight loss and heart disease to brain cancer. Learn how the keto diet works and if there's evidence to support these claims.

What is a ketogenic diet?

A ketogenic (or keto) diet is a very-low-carbohydrate, high-fat diet

This diet was originally developed in the 1920s as a way to manage childhood epilepsy. But it has gained a lot of popularity among the general population due to various health claims.

Types of keto diets 

While all types of keto diets require you to limit your carbohydrate intake, there are several variations. These include:

  • a standard keto diet — usually about 70% fat, 20% protein, and 10% carbohydrates
  • a high-protein keto diet —usually about 60% fat, 35% protein, and 5% carbohydrates
  • a cyclical keto diet — more carbohydrates are allowed on some days, like 2 days a week
  • a targeted keto diet — more carbohydrates are allowed around workouts
  • a medium-chain triglyceride (MCT) keto diet — promotes getting most of your fats from MCT oil 

National dietary guidelines generally recommend about 50% of your calories to come from carbohydrates. This equates to about 260g a day, while keto diets require you to limit this to 20-50 g a day.

What can you eat on a keto diet?

If you’re following a keto diet, foods you’re allowed to eat include:

  • cheese
  • eggs
  • butter and oils
  • meat, chicken, and fish
  • avocado
  • low-carb vegetables — like broccoli, spinach, and mushrooms
  • low-carb fruits in small amounts — like strawberries and blackberries
Ketogenic diet foods

What can’t you eat on a keto diet?

If you’re following a keto diet, foods you’re meant to limit include:

  • high-carb vegetables — like potatoes, butternut squash, and carrots
  • grains (including whole grains) — like rice, pasta, bread, quinoa, and couscous
  • sugary foods and drinks
  • beans, peas, and lentils
  • alcohol
  • foods with bulk sweeteners — like sorbitol or xylitol 

What happens when you follow a keto diet?

Usually, your body relies on carbohydrates as its main source of fuel. By limiting your carbohydrate intake, your body eventually starts to burn fat. This results in the production of ketones — which your body uses as fuel. This process is called ketosis. 

For ketosis to begin, it usually takes 3-4 days of limiting your carbohydrate intake to less than 50 g a day. Ketosis can also occur if you’re fasting or exercising for a long period of time.

Signs and symptoms of ketosis

Bad breath

Sometimes called ketosis breath, bad or “fruity” breath can happen due to raised ketone levels. In particular, it’s due to a ketone called acetone — which you expel through your breath and your urine. 

Keto flu

A drastic drop in your carbohydrate intake might lead to what’s called the keto flu — usually happens 2-7 days after starting a keto diet. This isn’t a recognised medical condition but refers to experiencing a collection of symptoms like:

  • headaches
  • poor concentration
  • irritability
  • diarrhoea
  • weakness
  • muscle cramps
  • constipation
  • difficulty sleeping

Gut issues

The high-fat content of a keto diet sometimes causes people to experience nausea and diarrhoea. The low fibre amounts of fibre you’re eating can also lead to constipation.

Epilepsy and the keto diet

A keto diet is still sometimes used to treat childhood epilepsy — usually if a child isn’t responding to their medication. 

It’s not fully clear the mechanisms behind how it works. One possible mechanism might be that for a seizure to happen, your brain requires energy that’s available quickly — like glucose. This process is slower with ketones, which might help reduce the number of seizures you have.

Because of its restrictive nature, teenagers and adults can find it difficult to follow a keto diet. A modified Atkins or a modified ketogenic diet might be recommended as it’s considered more flexible. 

It’s really important to work with your doctor and a dietitian if following a keto diet for medical purposes.

A keto diet might also be used for certain metabolic disorders, like:

  • glucose transporter type 1 deficiency syndrome (GLUT1DS) — glucose has trouble moving into the brain so it doesn’t have enough fuel 
  • pyruvate dehydrogenase deficiency (PDH) — a neurodegenerative disorder

A keto diet has also been suggested as a treatment for certain types of brain cancer. But evidence is quite mixed — some studies even found that ketones might encourage the growth of cancer cells. Until there’s more evidence to support this as a treatment, it’s best to avoid doing this.

Weight loss and the keto diet

People often lose weight on a low-carb diet like a keto diet — this is one of the reasons it has become so popular. But it seems that this weight loss is only short-term, as it’s common to regain the weight you’ve lost. Research also shows that weight loss is often due to eating less calories, rather than it being due to a low-carb intake. 

It’s also important to consider the quality of carbohydrates you ate before starting a keto diet. If your diet was previously high in less nutritious carbohydrates, like cakes and pastries, excluding these from your diet is likely to have a positive effect. But it’s possible that you’d have similar effects if you were to replace these less nutritious carbohydrates with something like whole grains.

A keto diet can also cause you to quickly lose weight when you start this diet. This early weight loss is mostly due to losing water weight. This happens as the glycogen (your glucose stores) in your muscles and liver is being depleted. And because this glycogen is bound to water, this water is lost too. The larger your body size, the more water weight you’re likely to lose.

Heart disease and the keto diet

Some research shows an improvement in cholesterol levels on a keto diet. Similar to weight loss, the reduction in calories sometimes seen on a keto diet is possible for this. 

The amount of carbohydrates you eat doesn’t seem to have an impact on your risk of death from heart disease. But the types of carbohydrates you eat does — sugary and refined carbohydrates like white bread and cake are linked to an increased risk. So when eating carbohydrates, opting for things like whole grains is important.

Diabetes and the keto diet

Following a low-carbohydrate diet to manage diabetes has been used as an approach by dieticians to managing diabetes for a number of years. There are some reports that it‘s helped reverse diabetes.

Evidence has sometimes shown that a keto diet might be useful for people with type 2 diabetes. It might help to improve average blood sugar levels (HbA1c), kidney function, and aid weight loss. It’s not clear whether it’s a low-carb intake or a loss in weight that’s responsible for this. 

It’s important to note that a keto diet can increase the risk of hypoglycaemia (very low blood sugar levels) if you’re taking medication for type 2 diabetes

You should always work with your doctor and dietitian if following a low-carb diet to manage your diabetes.

Long-term health and the keto diet

A large study looking at carbohydrate intake and your risk of all-cause mortality (any cause of death), showed that low-carbohydrate or high-carbohydrate diets increased the risk of all-cause mortality. While people with a moderate carbohydrate intake (50-55% of their diet) had the lowest risk.

When carbohydrates are reduced, nutrient-dense foods like fruits, vegetables, and whole grains are also reduced. This means you’re getting less fibre, vitamins, minerals, and antioxidants — which are all essential to your long-term health. This is why it might be linked to an increased risk of all-cause mortality.

If you choose to follow a low-carb diet, opt for more plant-based protein and fat sources. Substituting carbohydrates with more animal-based proteins and fats is linked to an increased risk of all-cause mortality.

Mental health effects of restrictive diets

A keto diet is very restrictive, so people find it very difficult to stick to it long-term. Sticking to a restrictive diet can have an impact on your social life too — for example, it can make eating out or going to a friend’s for dinner more awkward.

If you have a “slip-up” and eat more than 50 g of carbohydrates, it can lead to feelings of guilt and shame. It’s also very difficult to keep your body in ketosis, and just one higher carbohydrate meal can take you out of this metabolic state. So these feelings of guilt and shame can be heightened on a keto diet. 

It can also make you view foods as “good” or “bad” — creating an unhealthy mindset about food. And if you have a history of disordered eating, following a restrictive diet can be triggering.

Environmental impact of the keto diet

In 20129, a recent EAT-Lancet report summarised what needs to happen on a global scale to be able to safely feed our expanding population — coined the planetary health diet. The main message is to move towards a more plant-based diet. Some high-level advice from the report is to:

  • have at least 5 portions of fruit and vegetables a day
  • include plenty of whole grains
  • limit red meat and choose more plant-based proteins over animal-based proteins
  • include 2-3 tablespoons of legumes (peas, beans, and lentils) a day
  • limit dairy intake to 1-2 portions a day
  • choose mainly unsaturated fats

Eating sustainably on a keto diet can be challenging. The high amounts of animal proteins and fats that often come with a keto diet does have a negative effect on the environment. So try to opt for more plant sources like avocado and nuts if possible.