What is a keto diet? A beginner's guide

What is the keto diet? A beginner’s guide

The ketogenic diet, better known as the ‘keto’ diet, has become a popular way of eating that involves consuming a very low amount of carbohydrates. When following the plan, carbs are replaced by fat for energy, with the goal being to promote the body’s fat burning process. There are several versions of this diet, but the one most researched is the ‘standard ketogenic diet’.

Having originally been used to treat drug-resistant epilepsy in children, given a keto diet appears to reduce the frequency and intensity of seizures, the diet went mainstream when people realised it may also be helpful for weight loss.

Read more about popular diets such as the 5:2 diet and the Atkins diet. Check out the top keto diet foods to try, our delicious ketogenic diet recipes, from keto breakfasts to keto dinners.

How does the keto diet work?

With a very strict carb component, the aim is to push the body into a state of ‘ketosis’. Diets that use this approach include the Atkins diet, the Dukan diet as well as LCHF (low carb, high fat) diets such as the Banting diet. The ratios of fat, protein and carbs, and other specific features of each of these diets vary. The keto diet stands apart because of its high fat content with only moderate amounts of protein.

What is ketosis?

Under normal circumstances our body uses glucose from carbs to fuel our activity, keep us warm and support essential organs. When you deprive the body of its main source of energy, it will find an alternative. The brain demands glucose, and when insufficient amounts are available it pulls stored glucose from the liver and muscles. After three to four days when these reserves are exhausted, the body turns to stored fat and the liver converts the fat into ketones – this process is called ‘ketosis’.

How quickly you reach ketosis depends on a number of factors including your body mass index (BMI), your body fat percentage and your resting metabolic rate.

In order to trigger ketosis, the carbs you eat need to be heavily restricted – down to no more than 20-50g per day. To put this in perspective an average banana contains 20g and a medium baked potato 41g, so clearly this is a diet that demands careful planning and strict compliance.

Most organs of the body are able to use ketones as an alternative energy source, even the brain, which unlike other organs has a minimum glucose requirement.

A set amount of protein is included in the diet. This is because the building blocks of protein, known as amino acids, can be converted to glucose. For this reason, the amount of protein in the diet (typically 10-20%) is set to preserve lean body mass, including muscle, and not to disrupt ketosis.

What are the benefits of ketones?

Ketones help the body maintain an efficient fuel source even when there’s a calorie deficit, and they also appear to have a protective antioxidant effect.

How to follow the keto diet?

Before you start the diet, it may be worth focusing on liver-supportive foods like garlic and onions. This is because the liver has to work hard to produce ketones. It may also be helpful to reduce your intake of sugar, caffeine and alcohol prior to kicking off your keto journey.

When following a keto diet be sure to include plenty of non-starchy vegetables, such as kale and spinach, which are very low in carbs but contribute valuable vitamins and minerals.

If you are interested in adopting this diet you should consult your GP to confirm it is appropriate and safe for you to do so.

What foods to eat on a keto diet?

Foods that are generally allowed include:

• meats including high fat, red and processed varieties
• fish and seafood as well as oily varieties like salmon
• eggs
• oils, cream and butter
• nuts and seeds
• high-fat dairy foods including cheese
• certain fruit such as avocado, peppers, tomatoes and some berries,
• condiments like salt, pepper, herbs and spices
• low-carb, non-starchy vegetables including leafy greens cauliflower and onions

What foods to avoid on a keto diet?

A typical keto diet reduces carbs to less than 50g per day so, unsurprisingly, when you’re reducing your carb level to this degree you will need to eliminate the obvious carb-heavy options such as:

• grains and cereals including bread, pasta, rice and most conventional baked goods
• sugary foods like ice creams, cakes and confectionery
• root and tuber vegetables including potatoes, sweet potatoes and carrots
• most fruits including pineapple, grapes and mango

Some of the less obvious items to skip include:

• beans and pulses including peas, lentils, chickpeas
• sweetcorn
• low-fat and diet products and ready meals
• condiments like barbecue sauce, ketchup, honey mustard, hot honey and teriyaki sauce

You will also need to watch what you drink – fruit juices, fizzy drinks, cordials and even milk-based drinks will need to be avoided, as well as alcoholic beverages like beer and wine.

What are ‘net carbs’?

In the keto community, ‘net carbs’ is a familiar term that refers to the amount of carbs that are absorbed by the body and contribute to calories. The figure is calculated by subtracting the amount of indigestible carbs (insoluble fibre and some sugar alcohols) from total carbs. However, this calculation is rarely reliable or an exact science.

For those in the UK, you should be aware that our food labels list ‘fibre’ separately to 'total carbs'.

What’s the evidence of the keto diet?

The Standard Ketogenic Diet* (SKD) which appears to be the most researched does appear to help people lose weight and control blood sugar in the short term. In addition to weight loss, there may be metabolic improvements in insulin resistance and blood pressure as well as cholesterol and blood triglyceride levels. As a consequence, there is a growing interest in the use of low-carb diets for type 2 diabetes.

That said, although there are several theories as to why keto diets promote weight loss, they have not been consistently demonstrated in the research. In fact, there have been few long-term studies on the keto diet. Although, a 2013 study did suggest that those on a very low-carb diet achieved a greater weight loss over a longer period than those following a low-fat diet.

*The SKD is typically made up of 70-75% fat, 20% protein and 5-10% carbs.

Does the keto diet work for weight-loss?

Keto diets appear to lead to weight loss and may improve insulin sensitivity in those with diabetes. When compared to a low-fat diet, a keto diet appears to achieve greater reduction in body weight.

Weight loss at the start of the diet may be rapid, in some cases as much as 10 pounds (4.5 kg) in two weeks. This initial loss is typically due to the diuretic effect of the diet (water is lost as stored muscle glucose is used up) and is subsequently followed by fat loss. As ketosis continues, you may experience less hunger and because ketosis is a calorie-consuming process (there is an increased calorie demand to convert fat and protein to ketones) you may experience further fat loss.

How long weight loss is maintained, however, is dependent on your ability to adapt your dietary habits once you start to introduce a more balanced and healthy approach to eating.

What are the other benefits of the keto diet?

Research suggests the keto diet may have additional health benefits for epilepsy and, due to its use for weight loss, is likely to impact:

• Heart disease – following a keto diet may help reduce risk factors for heart disease such as blood pressure, cholesterol levels and body fat
• Cancer – one area currently under investigation is whether a keto diet may help as an adjunct to cancer therapy, evidence is emerging, but more quality research is needed
• Alzheimer’s disease – a keto diet may help slow the progression of this condition
• Polycystic ovarian syndrome (PCOS) – this complex endocrine and metabolic condition may benefit from improving insulin levels and for this reason a keto diet may be helpful

Is the keto diet healthy? Our nutritionist’s view…

Ketosis is a normal adaptive response which enabled humans to withstand periods of famine. Today, this natural mechanism is being exploited by its weight loss benefits. Following such a diet means you replace carbs with foods rich in fat and protein, and if followed over an extended period of time, this may have unfavourable consequences for some people. These may include dehydration, electrolyte disturbances and hypoglycaemia as well as symptoms which combined have been coined ‘keto flu’.

The keto diet also contradicts most people’s understanding of a healthy, balanced diet which typically promotes the consumption of carbs, protein and fat. The carb restrictions mean you may find it hard to reach your five-a-day. Eating high-fat foods is likely to increase your saturated fat intake, which UK government guidelines recommend we limit to 30g for men and 20g for women. High levels of dietary protein are thought to be an issue if you have an underlying kidney condition – although most keto diets supply moderate rather than high levels of protein.

It’s also worth noting that because the diet restricts carbs it’s typically low in dietary fibre which may have a negative impact on gut health including reducing the presence of gut-friendly bacteria. In this case, it’s worth making sure you are consuming plenty of gut-friendly foods such as leafy greens, fermented vegetables and certain fats like butter which provides butyric acid – a gut-supportive short-chain fatty acid.

The symptoms associated with ketosis are often temporary and relate predominantly to dehydration, because of the water loss experienced in the early stages of the diet. These may include headache, dry mouth, bad breath, fatigue and nausea.

Finally, sticking with the keto diet long-term can prove challenging – food choices are limited which can severely restrict eating out with family and friends.

Sample keto meal plan for a week

To help get you started we’ve created a sample meal plan, use this as a guide only, aim to vary your protein and vegetable sources as much as possible while following a keto diet:

Day 1

Breakfast: Bacon & avocado frittata

Lunch: Keto pizza

Dinner: Peanut butter chicken curry

Day 2

Breakfast: Keto smoothie

Lunch: Keto pizza

Dinner: Summer courgetti & meatballs

Day 3

Breakfast: Keto smoothie

Lunch: Thai broccoli rice

Dinner: Crustless quiche

Day 4

Breakfast: Keto smoothie

Lunch: Thai broccoli rice

Dinner: Tarragon, mushroom & sausage frittata

Day 5

Breakfast: Keto smoothie

Lunch: Courgette frittatas

Dinner: Quick prawn, coconut & tomato curry

Day 6

Breakfast: Courgette frittatas

Lunch: Roasted vegetable soup with halloumi ‘croutons’

Dinner: Greek island salad with chicken & avocado

Day 7

Breakfast: Scrambled eggs with basil, spinach & tomatoes

Lunch: Roasted vegetable soup with halloumi ‘croutons’

Dinner: Baked salmon & leek parcel

Who should not follow a keto diet?

Diabetics and anyone with a blood sugar management issue should discuss the potential implications with their GP and healthcare team before embarking on such a regime. Similarly, anyone who meets one or more of these criteria:

• have kidney and/or liver disease or a family history of such
• is under 18 years old or elderly
• has a pre-existing medical condition
• are pregnant or breastfeeding
• or are on prescribed medication

Please note: if you're considering attempting any form of diet, please consult your GP first to ensure you can do so without risk to health.

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This article was reviewed on 7 January 2025 by Kerry Torrens.

Kerry Torrens BSc. (Hons) PgCert MBANT is a BANT Registered Nutritionist® with a post graduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Nutrition and Lifestyle Medicine (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including Good Food.

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