Ketogenic diets are everywhere – but are they any good for our health? Hatty Willmoth explores the evidence, and speaks to experts Hannah Sutter and Janie Perry, to weigh up the pros and cons.

Keto, ketones, ketogenic, ketosis – the ketogenic diet has a whole vocabulary of its own, and it can get quite complicated.

It’s a hot topic in nutrition and has been for some time, with some studies suggesting amazing benefits – and other warning against it.

So, what actually is a ketogenic diet?

Ketogenic diet: definition and explanation

The ketogenic diet is a very low-carbohydrate, usually high-fat, moderate-protein diet that is intended to lead to a biological process called ketosis.

Normally, the human body uses glucose as fuel: simple sugars derived primarily from carbohydrates.

By restricting carbs to between 20g and 50g per day – the equivalent of one or two slices of bread – there isn’t enough glucose to keep going, and the body starts looking elsewhere for its energy.

The liver begins turning fats into ketones, which the body can then use as an alternative fuel. When the body is using ketones in the place of glucose, this is called ketosis.

Something similar might happen during an extended period of fasting, when the body begins turning fat stored in the liver into ketones in order to survive.

But a ketogenic diet – when carbohydrate is restricted but other macronutrients are eaten – encourages the body to produce ketones without fasting.

Today, some people believe that the body prefers to use ketones over glucose and that ketosis may have wide ranging benefits for our health.

The first benefit of a ketogenic diet

The ketogenic diet may be more popular than ever now, but it’s far from new; it was developed over 100 years ago to treat epilepsy.

It was well-known that fasting was an effective treatment for epilepsy – but there’s only so long a person can go without food.

In the 1920s, a new therapeutic diet was developed to mimic the effects of fasting without the unfortunate side-effect of starvation, to be used to treat epileptic children.

That was the ketogenic diet – and it worked. In fact, it’s still used today in cases where children with epilepsy are not responding to other treatment.

But since the 1990s, interest has grown exponentially in the variety of ways a ketogenic diet might serve different groups of people.

Ketosis for weight loss, diabetes, PCOS, depression, Alzheimer's...

Ketogenic diets have often been studied as a weight-loss diet, and the evidence suggests it can be remarkably effective.

The ketogenic diet also seems to benefit those with metabolic conditions, such as insulin resistance or type 2 diabetes.

For those people, their bodies struggle to effectively process glucose with the hormone insulin – but less carbs mean less glucose, and less of a need for insulin.

Studies also seem to suggest that those with metabolic issues tend to develop better insulin sensitivity on a ketogenic diet, meaning they become better able to cope with glucose while mostly relying on ketones for energy.

A ketogenic diet has also been shown to benefit people with polycystic ovary syndrome (PCOS): a hormonal condition that effects the female reproductive system, and often coincides with excess weight and insulin resistance.

Ketosis can positively affect both weight and insulin sensitivity, which can have knock-on benefits for the presentation of other PCOS symptoms.

But beyond weight and metabolism, new evidence is emerging about the potential benefits a ketogenic diet may have for the brain, with some scientists hypothesising that ketones may be inherently beneficial to brain health.

Research is still in its early stages, but there have been some indications that a ketogenic diet may improve mental health conditions, such as depression and anxiety, and brain disorders such as Alzheimer’s.

Plus, ‘going keto’ usually involves cutting out ultra-processed foods, with all manner of benefits, from reducing inflammation to improving gut health.

Is the ketogenic diet misunderstood?

There seems to be a plethora of potential benefits to a ketogenic diet – and one person who can attest to this is Hannah Sutter, who has been keto on and off for 20 years.

Sutter is also the founder of three food businesses, making ketogenic food: for the NHS to give to drug-resistant epileptic children on a prescribed keto diet; as snacks sold to the public in shops like Whole Foods and Ocado; and as a home delivery service called Natural Ketosis.

She says that the keto diet is chronically misunderstood and wants to set the record straight. Firstly, she says: “There is not one ketogenic diet.”

Instead, as long as fat is being burned for energy rather than carbohydrate, that is ketogenic – but there are four different recognised medical ketogenic diets, she explains.

For instance, the classical ketogenic diet gets 60-80% of its calories from fat, with less than 10% from carbs; but on the modified ketogenic diet, only 50-55% of calories come from fat, and up to 20% from carbs. A modified ketogenic diet is also slightly higher in protein.

Sutter says: “The amount of fat and the type of fat depends on the type of ketogenic diet being followed.

“The biggest misunderstanding amongst the public and some health professionals is that all ketogenic diets need to be high in fat.

“For many using the ketogenic diet for weight loss or type 2 diabetes, you really do not want it to be high in fat.”

However, most ketogenic diets are much higher in fat that standard western diets, as a ketogenic diet that lacked fat would result in a serious calorie deficit, and we need a certain amount of energy to survive.

Sutter continues: “Another area for confusion is between a low-carb diet and a keto diet. All keto diets are very low in carbohydrate, but most low-carb diets are not ketogenic.”

Janie Perry is not a fan of ketogenic diets

Not everybody is pro-keto. Nutritional therapist Janie Perry says, in general, she is not in favour of ketogenic diets.

“I don’t think it’s a healthy long-term solution to weight loss for the majority of people,” she says. “Fruits and starchy vegetables tend to be high in sugars, and so these are limited.

“There are so many health benefits to a diet rich in fruit and vegetables, including helping with healthy bowel movements. As a consequence, constipation is often an issue.”

There is some evidence that ketogenic diets may negatively affect gut microbiomes, perhaps due to a lack of fibre.

This is a point that Sutter takes issue with. She says: “Another misunderstanding is that you cannot get any fibre or green leafy vegetables on a keto diet.

“A well-managed keto diet can deliver fibre and green leafy vegetables and some fruit.”

But Perry continues, adding: “It’s a diet that is very high in saturated fats, which isn’t for everyone. Coconut oil, butter and lard are encouraged.

“Poor sleep is a known side-effect of the ketogenic diet. Sleep – in my view – is one of the key pillars to optimum health.

“I’m also wary of a ketogenic diet in athletes with a heavy or intense training load. Carbohydrates play an important role in fuelling the body and the brain.”

But for Sutter, the main disadvantage of a keto diet is how out of step it is with the current food environment.

“You are surrounded by sugar and carbohydrate everywhere you go,” she says, “and the public view is that the diet is unhealthy and weird.”

Ketosis for the brain

As previously discussed, some people believe that ketones are good for the brain – and some research is beginning to indicate that this may be the case.

Sutter says that, for herself, she finds ketosis “energy giving” and “cleansing”. She says: “It is great for people that have brain fog or generally lack energy.

“The reliance on sugar and carbohydrate for energy gives rise to highs and lows, but with energy from fat, you get a steady supply, which doesn’t dip up and down during the day.”

However, Perry says: “Ketogenic diets can cause brain fog, fatigue, irritability and constant thoughts about food.”

This may be most relevant when the body is in the process of switching from glucose to ketones: ‘keto flu’ is a term used to describe that period, which has a plethora of well-known symptoms, including brain fog, fatigue and irritability.

Sutter says: “You need to stay in ketosis for a few weeks to get the real benefits and open your fat-burning pathways, which means planning your time in keto is key for success.”

She also admits that “working out the amount of fat you need as well as the amount of carbohydrate is time-consuming” and says this is why she created Natural Ketosis, her food delivery service.

Ketogenic diet for women

The suitability of a ketogenic diet may be influenced by sex and life stage. Perry explains that carbohydrates are “really important for a menstruating female”.

“Too low a carbohydrate intake can lead to hormonal imbalances,” she says. “The female body’s need for them fluctuates throughout the monthly cycle.”

In the second half of a female cycle, called the luteal phase, Perry says that the body’s reliance on carbohydrate is higher as it relies on glucose to produce enough of the hormone progesterone.

There is some evidence for this, but more research is needed.

Plus, this obstacle is only true for menstruating women. Perry says: “Once women reach the menopause, the need for carbohydrate becomes less as their bodies become less tolerant to glucose.”

In these cases, she advocates for a long-term, non-keto, low-carb approach of 50g to 100g of carbohydrates per day.

More research is needed on the ketogenic diet

A low-carb diet seems to be much easier to follow than a ketogenic diet. Some of the research has even been corrupted by the fact that participants struggled to adhere to a ketogenic diet and stay in ketosis.

And many studies call for further comparisons to be made between ketogenic and other diets, such as the Mediterranean diet.

Even Sutter says she often switches to low-carb, rather than a strictly ketogenic diet, mainly when she’s travelling or eating with friends.

But overall, not enough research has been done on the ketogenic diet, particularly about how it affects people long-term, so we can’t categorically say that it’s safe.

And there is some research that a ketogenic diet may have serious negative consequences long term, such as promoting premature organ ageing or raising LDL cholesterol, which in turn may increase the risk of heart disease and strokes.

The verdict: should you go on a ketogenic diet?

As always, nutrition is personal. There is some evidence that a ketogenic diet might be beneficial in some cases, for example: drug-resistant epilepsy, weight loss, metabolic disorders, and perhaps mental and brain disorders.

However, proceed with caution: we don’t yet fully understand all the positive and negative repercussions of sustained ketosis over a long period of time.

And not all ketogenic diets are equal; a vegan modified ketogenic diet looks very different to a carnivore ketogenic diet.

If you decide to make a dietary change, you may wish to first consult a doctor or registered nutritional therapist.

Perry says: “I would always suggest that a ketogenic diet is followed under the supervision of a trained professional, like a nutritional therapist, to ensure that it is done safely.

“If I am recommending a ketogenic diet, I like to have clients reintroducing carbohydrates from time to time to help limit the negative side-effects.”


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