The ‘keto’ diet is the latest trend in fashionable low carb diets promising fast weight loss and numerous health benefits. The original ketogenic diet has been used as medical nutrition therapy for over 80 years as a last resort for treating uncontrolled epilepsy in children - with close monitoring. A ketogenic diet is very low in carbohydrates – only allowing very low carbohydrate (non-starchy) vegetables, cream, butter, oils, meat, fish, nuts and seeds. You will need to say no to fruit, grains, legumes, milk, yoghurt, potato, sweet potato, pasta and of course birthday cake too.

What can you eat?

The true ketogenic diet involves a carbohydrate intake of five to ten per cent of total energy intake, usually 20-50 g of carbohydrate per day. To give you an idea – one slice of bread contains around 20 g of carbs. Unlike the typical low carb high protein diets, there is emphasis on a moderate protein intake (around 20 % or less of energy intake) but a very high intake of fats (80-90 % of energy).

How it works

Generally, the lower your carbohydrate intake, the more your body will rely on other sources of energy besides glucose derived from carbohydrates. Your body will commence to break down fat as an energy source. Fat can be metabolized for energy by many tissues in your body but in your liver it can also be converted to glucose and further broken down into another potent energy source, ketone bodies, in a process called ketosis. In a ketotic state your brain primarily depend on ketone bodies as its primary energy source. Your body also enters into the state of ketosis when you have severe energy restriction such as during fasting, starvation, or prolonged intense exercise without adequate carbohydrate intake.

Why keto?

For most people, the aim of a ketogenic diet is weight loss – by encouraging the body to rely on fat and ketones for fuel, instead of glucose from carbohydrates. There is a lot of individual variation in how long it takes to achieve ketosis, but by around three days you would be able to measure ketones in urine and notice them in your breath – they smell like nailpolish remover!

Does it work?

If the main reason you are trying a keto diet is for weight loss – this will work only if you are reducing your total energy intake as well. Studies do show that a ketogenic diet can allow faster weight loss short-term compared to traditional energy restriction – possibly because ketones are reported to reduce appetite, and higher protein or fat foods are typically more filling too. With any lower carbohydrate diet we see better weight loss for the first six months, but at 1-5 years, there is no difference in weight loss compared to the traditional restricted calorie approach.

The down side

Short term there are side effects (‘the keto flu’) including headaches, fatigue, dizziness, nausea, mood changes and constipation. Once your body adapts to lower carbohydrate intake and starts to use ketones however, these symptoms do reportedly subside.

Whenever you cut out many food groups from your diet, there is going to be a risk. Many high-fibre foods are limited, and intake of protective phytonutrients and B vitamins will be reduced too. Low intake of dietary fibre is strongly linked to bowel cancer, and fibre is crucial for supporting the healthy bacteria in the gut.

So, it may be a good way to kick start weight loss if you need to lose weight quickly for surgery. However, as yet there are no long term studies on the impact of a ketogenic diet on weight loss and health, or the effects of maintaining ketosis for a long period of time.

There is mixed research on the impact on heart health; however it is likely if a ketogenic diet is followed, then the intake of animal or saturated fats increases, which tends to increase health risk. Because carbohydrates impact blood sugar levels more than fats and proteins, by restricting carbohydrate you can see improvements in markers such as blood glucose and insulin, which means it is even more important that you are monitored if you have diabetes.

Should you do it?

Any diet that results in reducing your energy intake will help with weight loss and related metabolic changes. At this stage there is no research about the long term implications of a keto diet – but we know that if you are doing a keto diet for weight loss, the long-term eating pattern is more important. Unfortunately, the keto diet is very restrictive and unlikely to be superior to alternative diets, and probably a bit antisocial too. Most people calling their diet ‘keto’ are likely following a very low carbohydrate diet, which is helpful for initial weight loss short term. And, what works for one person, doesn’t work for everyone.

The bottom line

If you are only after the short term rapid weight loss then the keto diet is just one of many ways that you can achieve this – but the extreme restriction is not likely worth it in the long term, especially if you are unable to maintain it and return to your previous eating habits. This is one diet where it is crucial to get personalised advice – not just trust what you see on Google – and to monitor any side effects especially if you have any health conditions.