A diet developed in the 1920s to treat children with epilepsy is suddenly all the rage.
The ketogenic diet, or "keto diet", has reportedly been endorsed by celebrities and even athletes are giving it a go.
There are hundreds of people selling ketogenic diet plans online and on social media, with big promises of the results to be expected.
The keto diet got its name because ketones are the source of energy that the body uses when it's burning fat. Ketones are produced in weight loss regardless of the type of diet you are following.
So, actually, anyone who is losing weight is actually on a keto diet.
It doesn't matter to your body whether the fat it's burning is from your existing reserves or from the high fat meal that you just ate. And the production of ketones doesn't necessarily mean you are burning body fat.
So when keto dieters add fat to their diet through bulletproof coffee or coconut oil, it is burnt as fuel instead of body fat – which defeats the object of the diet achieving weight loss.
By adding additional fat to the diet, your energy balance will remain positive regardless of the fuel (carbohydrate, fat or protein) and this will promote weight gain, as is demonstrated by children on ketogenic diets when they gain weight despite the fact that their urine shows that they are producing ketones.
Inducing ketosis – a natural state for the body, when it is almost completely fuelled by fat – in therapeutic diets is a skill that needs the close supervision of a dietitian in a specialist clinic.
This is because the diet is not balanced and can easily lead to nutrient deficiencies, nausea, vomiting, headache, tiredness, dizziness, insomnia, poor exercise tolerance and constipation – sometimes referred to as keto flu.
The effects of maintaining ketosis for long periods of time are unknown. But concerns include the impact on important gut microbes that are likely to be starved of essential fibre required for healthy balance.
The potential effect of this on long-term health is still not clear.
Most people calling their diet a keto diet are simply following a low or very low carbohydrate diet. Low carbohydrate diets can be helpful, at least in the short term, for some people to lose weight.
However, as with the true ketogenic diet, most people can't stick with a very low carbohydrate diet for long.
The latest research shows that it's the ability to stick to the diet that matters.
So if a low carbohydrate diet or keto diet is a practice that works for you and you are able to maintain it for as long as it takes to lose excess body fat – and you are meeting your nutritional requirements – then the science says that this should be encouraged.
But it's worth noting that, as yet, there hasn't been enough research into the ketogenic diet to support its use in some medical conditions – so people using the diet to treat diabetes or polycystic ovarian syndrome, should consult their doctor before trying it, as it can affect blood sugar levels.
People with pancreatic or liver problems, or problems with fat metabolism should also avoid the ketogenic diet. This is because the diet is so high in fat that it puts added pressure on both organs, which are essential for fat metabolism.
It is also worth considering that eating a well balanced, keto diet is actually very expensive. For most people, following a low carbohydrate diet, rather than a no carbohydrate diet, is much more practical – as it will also allow for the inclusion of fruit and all vegetables.
This represents much better dietary balance and usually leads to people sticking with it for longer.
As always with weight loss, in the end it all comes down to taking less energy in than you burn. In the UK, the National Diet and Nutrition Survey says that on average, people get about half of their energy from carbohydrates.
So by cutting out the source of half of your energy from your diet – even if some of that energy is replaced by fat – you are likely to reduce your energy intake, which leads to weight loss.
But if you can't sustain the keto diet, don't worry, you are in the majority. Try considering why you eat, rather than what you eat. Tackling convenience buying and emotional eating is the key to successful weight loss for most people.