Studies over many years suggest that up to 25 per cent of children on the diet will have their seizures well controlled and may be able to decrease their medications or discontinue them, according to the Johns Hopkins Ketogenic Diet Fact Sheet from the US university in Maryland. A further 30 to 40 per cent will see their seizure frequency halved, while 25 to 30 per cent who try the diet will find, after a month or two, that it is not effective.

The keto diet’s perceived benefits

Halle Berry, a diabetic, says the diet helped her manage her condition. “If you’re like me, you can possibly reverse diabetes,” she posted on Instagram, listing the diet’s benefits – from better physical endurance, better skin, appetite control, migraine control, and more energy, to better mental performance.

It “actually forces your body to burn fat like crazy. I also believe it’s been largely responsible for slowing down my ageing process,” the 53-year-old Academy Award winner posted in 2018 for her 6.2 million followers, urging them to give the keto lifestyle a try.

Berry’s claims are exciting but have yet to be fully proven. The Johns Hopkins Patient Guide to Diabetes says the keto diet may work in the short term for some people with type 2 diabetes, but it’s not the only way to lose weight and manage glucose.

Like many keto dieters, Berry also practices intermittent fasting (IF), in which you eat only in a short window during the day. Studies suggest that IF, too, may play a role in some of the benefits she attributes to the keto diet.

Kourtney Kardashian, 41, recently told US magazine Health that she loves a keto diet, “though I’m not doing it now. I noticed my body change for the better. I [also] love intermittent fasting. I try to do that all the time”.

Jason Ewoldt, a wellness dietitian for the Mayo Clinic Healthy Living Programme, has seen a growing interest in the keto diet in the past two years.

“People are coming to us saying, ‘hey, I’ve heard of this keto diet, what’s it all about and should I be doing it?’”

Many, upon learning what it takes to follow the diet, decide it’s not for them, based on their lifestyle and goals, Ewoldt says. “Others dip their toe in and give it a try, knowing full well it might not be a long-term answer.”

Weight loss and fad diets: why keto diet, Paleo diet and Atkins diet don’t work

Most do not stick with it, he says, adding research shows the hardest part is sustaining the diet long term.

How the keto diet works

A look at the details suggests why. The classic keto diet is known as a 4:1, as it provides four times as much fat as protein and carbohydrates combined. Fat is energy-rich, with 9 calories per gram compared to 4 for carbohydrate or protein, so portions on this diet are smaller than normal.

Although different versions of the diet have sprung up, usually 75 per cent of daily calories are from fat, 20 per cent from protein, and 5 per cent from carbohydrates – just 20 to 50 grams a day, less than you find in four slices of bread or a cup of pasta.

An elevated level of ketones in the blood is called ketosis. Although the reasons why are still unclear, this state can help lower the frequency of seizures in epileptic patients. Some early research suggests it may help control blood sugar in diabetics. And, not surprisingly, it helps people shed fat.

Two types of keto test strips are widely available, to help check ketone levels in the blood or urine to know whether you have achieved ketosis.

What keto dieters can and cannot eat and drink

By now it’s well known that bacon gets a stamp of approval on this diet. But Ewoldt and those who follow it warn that it requires a high degree of discipline. For starters, high-carb foods – including staples like rice, noodles, pasta, bread, cereal, potatoes, and most fruits – are out. If you’re a sugar addict like many of us, you’ll have to leave that carbohydrate off your plate and out of your beverages, too.

Keto diet only works for a week before problem that can cause obesity and diabetes, study shows

The Dietary Guidelines for Americans recommends carbs make up 45 to 65 per cent of total daily calories; the typical American consumes 200 (7oz) to 300 grams of carbs a day. So a keto newbie faces a drastic change in eating habits.

The few carbs you have should come from non-starchy vegetables such as leafy greens, broccoli, cauliflower, peppers, asparagus, mushrooms, celery, zucchini and green beans.

If you’re prepared to eat by these rules and ratios, you may well experience a drop in belly fat, though study results are mixed.

A rocky transition for keto dieters

The downside? It takes two to three weeks for ketosis to kick in, Ewoldt says. Expect some unpleasant side effects, including nausea, constipation, headaches, bad breath (a chemically acetone smell, from the excess ketones) and fatigue, while your body transitions from using carbs to using more fatty acids and ketones, he says.

“It’s almost like you have the flu, hence the term ‘keto flu’. It’s not like a universal thing for everybody and it’s not the same symptoms for everybody.”

Keto and Paleo diets: what they leave out might just be what you need

The keto diet has been associated with an increase in LDL, or “bad” cholesterol, Ewoldt says, but the research is conflicting. It has likewise been associated with an increase in HDL, the “good” cholesterol.

Meeting the diet’s requirements means cutting out many healthy foods, making it difficult to meet your micronutrient needs. There are now many versions of the diet, as lines blur between low-carb and keto diets. So some people may follow an unhealthy high-fat diet and not be in ketosis, and people can be in ketosis with different levels of protein and carb intake.

Seek expert advice about the keto diet

Before starting this diet, or any diet, it’s best to consult a trusted expert to see whether it meets your needs and who can help devise a workable plan so you can safely stick with it.

“We do know from research, if you’re looking a year out, and this group is doing a calorie-controlled diet, and this one’s doing a keto diet, those on the keto diet tend to lose a little bit more weight,” Ewoldt says. “But if you’re looking at two years out, the weight discrepancies kind of dissipate. The reason being, researchers theorise, is those people on the keto diet aren’t doing the keto diet; they basically fell off because it is such a strict diet to adhere to.”

There’s little evidence to show that this type of eating is effective – or safe – over the long term for anything other than epilepsy, he adds.

“There are no long-term studies on the keto diet. What does it look like 15 years from now? That’s a very good question.”

A taste of the keto diet

For Harrison Lebowitz, the keto diet proved an effective way to drop a few stubborn pounds.

The 62-year-old lawyer, writer and entrepreneur leads an active lifestyle in the sunny US state of California, but despite working out religiously his whole life, had noticed his waistline was expanding.

“Three days a week I would do an intense workout in which I would burn 1,000 calories in an hour. My metabolism has slowed over the years, so I ended up putting on some weight. When I was 20 years old and burning off 1,000 calories and I could eat whatever I wanted to. Now, I burn off 1,000 calories and I only eat one meal a day,” he said.

When his fiancée, Ana Alfaro, dropped by to watch his beach volleyball session, she asked a fellow player in his mid-50s how he had achieved his “ripped” physique. His response was the keto diet.

“We’d both heard about the diet but hadn’t explored it,” Lebowitz said. The couple did some research and took up the diet themselves.

“It was like the most unhealthy thing I’ve ever heard of … Massive butter, meats are good, fats are good. It was counter-intuitive.”

The couple swapped out wheat flour, using almond flour, substituted crumbled pork rinds for breadcrumbs, and followed the diet quite strictly for seven months.

Surprisingly, they ate more vegetables than they had before – especially cauliflower. Put through the processor, you can create a dish that “tastes exactly like mashed potatoes” but with far fewer carbs, Lebowitz says. And to meet the fat requirements, “you end up putting a lot of butter on it”.

He missed having “a really good pizza … You can have one with a cauliflower crust, but it just wasn’t the same. You can have as much bacon as you want, but I was missing certain other things, including some fruits. You could lick a stick of butter and that’s fine. I was missing wheat-based products – breads, a good baguette.”

He dropped five to 10 pounds (2.3 to 4.6kg) despite not having cut back on his food intake.

“Having all that bacon, all that meat, all that butter, from everything that I’ve read, I thought there’s got to be some bad side effects here,” Lebowitz said. When a blood test did show an increase in his bad cholesterol, he began to question the diet.

They put the diet on pause when they went to Mexico for a couple of weeks last summer. “It was going to be fairly impossible to try to do the keto diet there”, where the cuisine features wheat and cornflour, beans, beer and other high-carb ingredients, Lebowitz says.

When the coronavirus pandemic spread to California and they were asked to shelter at home, the couple returned to having their usual carbohydrate levels.

Lebowitz would be supportive if Alfaro wants to return to the keto lifestyle. “I’m just nervous about the cholesterol side effect.”

Besides, he says, “At this age, why should we cut back on some things we enjoy?”

This feature is part of a short series on the keto diet. Next we profile people for whom the keto diet has had a powerful impact – including a father and his epileptic son, and a physical education teacher who shed 20kg in two years on the keto diet.