The keto diet is a low-carbohydrate, high-fat diet. The lack of carbohydrates forces your body into a metabolic state called ketosis, where it burns stored fats for energy. Normally your body’s preferred energy source is sugar (glucose) in the bloodstream, which comes from the breakdown of carbohydrates.
The keto diet is not new. It has been used by doctors to treat untreatable epilepsy in children for 100 years. The keto in keto diet stands for ‘ketogenic’.
How does a keto diet work?
During normal digestion, carbohydrates are broken down to sugars, including glucose, primarily in the small intestine and then absorbed into the bloodstream. The glucose is then taken up by the body’s cells and muscles. Some glucose is burned up for energy in the body cells and the brain and some is stored as glycogen for use later.
If carbohydrates are restricted, after a few days the body uses up all it’s sugar stores (glycogen) and is faced with low circulating blood glucose. So it turns to its stored fat reserves and starts breaking them down.
So essentially, a keto diet (technically ‘ketogenic’ diet) forces the body to start utilising its fat stores for energy and this can lead to weight loss.
In breaking down its fat reserves, ketone bodies are produced by the liver from fatty acids. There are 3 ketones produced in the body: acetoacetate, beta-hydroxybutyrate (BHB) and acetone.
This metabolic process is known as ‘ketosis’. The body is forced to use the ketone bodies as its primary energy source until more carbohydrates become available to provide glucose.
Ketosis can also be triggered by fasting or starvation. More on that later.
An added bonus of a ketogenic diet is that it seems to suppress appetite. Normally, any diet that restricts energy and causes weight loss will create an increase in appetite.
How do I get into ketosis?
In a regular Western diet about 55% of daily energy intake is from carbohydrates. There’s nothing wrong with this. The Australian Dietary Guidelines suggest adults get 45-65% of their daily energy intake from carbohydrates. Perhaps not the type of carbs we’re actually eating, but that’s another story.
But what percentage of carbs do we need to go into a ketogenic state (ketosis)? Typically, a keto diet limits carbohydrates to 10% of energy intake. Most people will need to stay under 50 g of carbohydrates per day. The keto diet requires that a person gets the majority of their daily kilojoules from fat.
It normally takes a few days to get into a state of ketosis, however, it only takes a small breach of your carbohydrate allowance to push you out of ketosis. So, to remain in ketosis you need to substantially deprive your body of carbohydrates.
Ketosis is different from ketoacidosis, which is a life-threatening condition where ketone bodies are produced in high amounts, changing the pH of the blood to become acidic.
Ketosis is brought about by reducing carbohydrate intake as mentioned, and also by fasting.
How do I know when I am in ketosis?
Aside from experiencing the known side effects of the keto diet (see below), which provide a clue that you are in ketosis, you can also verify it by testing your ketones.
Ketone testing can be done by testing your blood, urine or breath. The easiest way initially is to use ketone testing urine strips, such as Ketostix or Keto Diastix. These can be obtained from the pharmacy. You simply urinate on the strip and wait for a colour change. The deeper the colour, the higher the concentration of the ketone called acetoacetate. This works best when you first embark on the keto diet. As your body adapts, you will produce less acetoacetate.
Blood ketone meters are the most accurate of these 3 methods, but even they do not measure total blood ketones.
Keto breath meters are the least accurate way to measure ketones. They measure acetone on the breath, then use that to estimate the amount of ketones being produced.
Can fasting help with the keto diet?
Fasting, which is where the body is deprived of food, can help to trigger the body to move into ketosis, where it flicks the switch from using glucose for energy to using ketones. Fasting may help people who are having problems getting into ketosis, but it is not mandatory on a keto diet.
There are a couple of methods of fasting:
- Intermittent fasting – such as in the 5:2 diet, where you fast intermittently, such as on 2 days of the week, and eat ‘normally’ on the other 5 days
- Time-restricted eating – where you eat only during a specific time window during the day, e.g. 14:10, where you fast for 14 hours and eat within a 10 hour window
- Periodic fasting – where you don’t eat at all on some days. This is not usually suitable for people new to fasting.
What are the macros on a keto diet?
A keto diet typically has the following composition of dietary macronutrients:
- Fat: 55-60%
- Protein: 30-35%
- Carbohydrate: 5-10%.
What carbohydrate foods are avoided on keto diet?
To reach a state of ketosis you will need to restrict carbohydrates and generally people on the keto diet avoid the following carbohydrate-rich foods:
- Grains, such as oats, wheat, rye, corn, and quinoa
- Foods made from grains, such as pasta, pizza, bread, crackers
- Fruits that are high in sugar, e.g. pears, grapes, bananas, peaches, mangos
- Starchy vegetables – usually those growing underground, e.g. potatoes, sweet potatoes, parsnips, and also sweetcorn
- Legumes (beans and peas) – chickpeas, baked beans, green beans, kidney beans, lentils, black beans etc
- Sugary drinks and fruit juice
- Added sugar and sweeteners in foods.
What can I eat on a keto diet?
The keto diet differs from other low-carbohydrate diets such as Atkins and South Beach, which focus on protein, rather than fat.
Fat is a requirement of the keto diet – a high percentage of total kilojoule intake comes from fat. Protein is a small part of the diet and carbohydrates usually form less than 10 per cent of daily kilojoules.
Oils and fats, such as peanut butter, avocados, butter, cream, coconut oil, olive oil and avocado oil can be eaten liberally. Vegetable oils are not as useful. Eggs and cheese are good keto diet foods. Nuts and seeds, e.g. walnuts, almonds and chia seeds, are good. Some protein in the form of beef, salmon and chicken thighs (dark meat) is good. Fish and seafood are allowed on the keto diet.
Non-starchy vegetables, such as broccoli, brussels sprouts, cauliflower, green beans, celery and asparagus are OK to eat on a keto diet.
You should ensure you drink adequate water on a keto diet – usually more than you would normally drink. The reduction in fruit and vegetables will mean you are getting less water. In addition, the keto diet is known to have a diuretic effect, and dehydration can be a problem.
Is the keto diet better than other diets for weight loss?
The keto diet can certainly result in short-term rapid weight loss, however, like all restrictive diets it can make it difficult to meet long term nutrient needs for key vitamins and minerals, according to Dietitians Australia. They also note that it is difficult to maintain due to the drastic restrictions on food groups such as fruit and vegetables.
Systematic reviews of randomised controlled trials of ketogenic diets versus low-fat diets show greater weight lost on ketogenic diets, however, often this was not statistically significant.
Typically on a diet, a person’s body will adapt to the reduction in energy intake by lowering the basal metabolic rate (how many kilojoules the person burns at rest). But, it has been shown that a low-carbohydrate diet can result in less lowering of a person’s BMR (basal metabolic rate) compared with a low-fat diet. Meaning the body will burn more kilojoules at rest on a low-carbohydrate diet than on a low-fat diet.
Side effects of the keto diet
Common short-term side effects of a keto diet include bad breath, constipation (due to the lack of fibre), fatigue, dizziness, headache and nausea. The lack of sugar can cause confusion, low mood and irritability. Difficulty sleeping may also be a problem – known as ‘keto insomnia’.
‘Keto breath’ smells fruity, metallic, or of acetone (used in some nail polish removers).
In general terms, people sometimes refer to the symptoms as ‘keto flu’. The symptoms normally resolve after a few days or weeks. Staying hydrated can help with symptoms.
In the long term, potential side effects include fatty liver and kidney stones. Low-carbohydrate diets have also been linked to an increased risk of atrial fibrillation, a heart rhythm disorder.
Health effects of the keto diet
The keto diet was developed many years ago to reduce seizures in children with untreatable epilepsy, and there is longstanding evidence that it works in this regard.
The keto diet can also improve blood sugar levels in people with type 2 diabetes, and improve insulin resistance, in the short term. But it can also have some detrimental effects. Dietitians Australia counsel that there is no single optimal diet for all people with diabetes.
The effect of the keto diet on cholesterol levels is unclear. Some studies report that it increases LDL-cholesterol (‘bad’ cholesterol), some that it decreases it, while others report initial increases in cholesterol levels which then fall away. Triglyceride levels do not seem to be affected.
The keto diet is under investigation as a potential therapy for Alzheimer’s disease and other brain degenerative disorders. It is known that impaired glucose uptake is a part of Alzheimer’s disease – brain scans show areas of reduced glucose metabolism in patients with Alzheimer’s disease. And it’s thought that this problem processing glucose may play a key role in the development of Alzheimer’s. Because the keto diet forces the body to use ketone bodies not glucose for fuel it is thought that it may help.
Following a keto diet may put you at increased risk of bowel cancer due to the lack of wholegrains in a keto diet.
There are some meta-analyses that suggest consuming a low-carbohydrate diet, like the keto diet, is associated with an increased risk of death.
Who should not do a keto diet?
Due to the high saturated fat content of the keto diet, it may not be suitable for people with heart disease or diabetes. In any case, you should discuss it with your doctor first if you are considering trying a keto diet. And people who have diabetes and who take insulin or oral hypoglycaemic agents may need their medication adjusted before starting a keto diet. There is an increased risk of having a ‘hypo’ on low-carbohydrate diets like the keto diet.
People on blood pressure medication may need it adjusted if they go on a keto diet.
Anyone with liver problems, pancreatitis, conditions which affect fat metabolism and some enzyme disorders should not go on a keto diet.
Anyone with gallstones or who has had their gallbladder removed should seek medical advice as to whether a keto diet is suitable for them.
Pregnant women should not do a ketogenic diet, as prolonged ketosis in pregnancy has been linked to developmental problems for the baby. It’s not known if a keto diet is safe when breastfeeding, so it’s best avoided at that time also.
Last Reviewed: 16/07/2020
1. Dietitians Association of Australia. Hot Topics. Ketogenic diet (August 2017). https://daa.asn.au/voice-of-daa/hot-topics/
2. Rettersjoj K, Svendsen M, et al. Effect of low carbohydrate high fat diet on LDL cholesterol and gene expression in normal-weight, young adults: A randomized controlled study. Atherosclerosis 2018 (Dec): 279: 52-61 https://www.atherosclerosis-journal.com/article/S0021-9150(18)31432-1/fulltext
3. Seidelmann SB, et al. Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis. The Lancet Public Health 2018; VOLUME 3, ISSUE 9, PE419-E428, SEPTEMBER 01, 2018. https://www.thelancet.com/journals/lanpub/article/PIIS2468-2667(18)30135-X/fulltext
4. Harvard Health Publishing. Should you try the keto diet? Updated Dec 2019.
5. O’Neill B, Raggi P. The ketogenic diet: Pros and cons. Atherosclerosis 2020 (Jan): 119-126.
6. Ting R, Dugré N, Allan GM, Lindblad AJ. Ketogenic diet for weight loss. Can Fam Physician. 2018;64(12):906. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6371871/
7. Ketogenic diet in Alzheimer’s disease. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6720297/
8. Masood W, Annamaraju P, Uppaluri KR. Ketogenic Diet. [Updated 2020 Mar 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499830/
9. Diabetes UK. Keto diet safety. Updated Jan 2019. https://www.diabetes.co.uk/keto/keto-diet-safety.html
10. Ebbeling Cara B, Feldman Henry A, Klein Gloria L, Wong Julia M W, Bielak Lisa, Steltz Sarah K et al. Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial BMJ 2018; 363 :k4583. https://www.bmj.com/content/363/bmj.k4583
11. Abbasi J. Interest in the Ketogenic Diet Grows for Weight Loss and Type 2 Diabetes. JAMA. 2018;319(3):215–217. doi:10.1001/jama.2017.20639
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