How does the keto diet work?
With only a very strict carb component to the diet, the aim is to send 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 source. The brain demands glucose, and when insufficient amounts are available it pulls stored glucose from the liver and muscles. After 3-4 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 very 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?
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 starting.
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 sort of diet you should consult your GP to confirm it is appropriate and safe for you to do so.
What foods are allowed on a ketogenic diet?
Foods that are generally allowed include high-fat meats, processed meats, fish, oils, lard, butter, nuts, high-fat dairy such as cheese, and low-carb vegetables such as leafy greens, as well as specific fruits such as avocado.
What foods are avoided on a ketogenic diet?
A typical keto diet reduces carbs to less than 50g per day so, unsurprisingly, when you are drastically reducing your carb levels you will need to eliminate all bread, pasta, rice, cereals and most conventional baked goods. Less obvious perhaps is the need to skip legumes, beans, root vegetables, sweetcorn, most fruits as well as starchy veggies, such as potatoes.
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You will also need to watch what you drink – fruit juices, fizzy drinks, cordials and even milk-based drinks will need to be avoided.
What are ‘net carbs’?
In the keto dieting community ‘net carbs’ is a familiar, but unregulated 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 a keto diet work?
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.
Is the keto diet healthy? Our nutritionist’s view…
From an evolutionary perspective, ketosis is a normal adaptive response which enabled humans to withstand periods of famine. Today, this natural mechanism is being exploited by low-carb weight loss regimes. 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 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’ll find it hard to reach your 5-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, long-term compliance can prove challenging – food choices are limited which can severely restrict eating out with family and friends.
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. They’re likely to experience complications because of the effect on blood sugar. 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 16 October 2023 by Kerry Torrens.
A registered Nutritional Therapist, Kerry Torrens is a contributing author to a number of nutritional and cookery publications including BBC Good Food. Kerry is a member of the The Royal Society of Medicine, Complementary and Natural Healthcare Council (CNHC), British Association for Applied Nutrition and Nutritional Therapy (BANT).
All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.
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