You may have read that the ketogenic or ‘keto’ diet – where up to 80 per cent of what you eat is fat – doubles your risk of heart attack, blocked arteries or stroke.

A new study has found a suggestive link between the diet and heart disease.

The keto diet involves a very small amount of carbohydrates (no more than 10 per cent), a moderate amount of protein and otherwise mostly fats.

There are known side-effects and shortcomings with the diet – and an increased risk of heart disease is said to be one of them.

But it’s a contentious area. Or at least a complex one.

The ketogenic diet has been used as a treatment for intractable epilepsy since the 1920s.

How does the keto diet work?

Most people get a significant part of their diet from carbs: Potatoes, rice, bread, sugars from fruit and milk and, regrettably, highly processed foods.

These foods convert to glucose. Some of this is burnt off as fuel and some is stored as fat.

In the keto diet, in the absence of carbs, the body enters a metabolic state called ketosis, where fat is burnt for energy. This can start happening within a couple of days, but may take a week or two.

Rapid weight loss tends to follow.

Movie stars and other celebrities are apparently keen on the keto diet as a way of preparing (dropping a ton of weight) before a big project.

Others might go the keto route when the gossip rags start commenting on their chubbiness.

The ‘keto flu’

In the short term, some people have complained of suffering with the ‘keto flu’ – where the body reacts to the radical change in diet with symptoms including diarrohea, constipation and vomiting.

Less common, according to usually reliable Healthline, are poor energy and mental function, increased hunger, sleep issues, nausea, digestive discomfort, and decreased exercise performance.

Longer-term side-effects can include low protein in the blood, extra fat in the liver, kidney stones and micronutrient deficiencies.

An explainer from the University of Chicago cites low blood pressure and kidney stones as other side-effects. It notes that the keto regime “is not safe for those with any conditions involving their pancreas, liver, thyroid or gallbladder”.

What about the heart?

A new study suggests that a “keto-like” diet may be associated with higher blood levels of LDL cholesterol (the unhealthy sort).

According to a statement from the American College of Cardiology: “The study also found “a two-fold heightened risk of cardiovascular events such as chest pain (angina), blocked arteries requiring stenting, heart attacks and strokes.”

Lead author of the study was Dr Iulia Iatan, attending physician-scientist at the Healthy Heart Program Prevention Clinic, St Paul’s Hospital and University of British Columbia’s Centre for Heart Lung Innovation.

Dr Iatan and her colleagues weren’t, strictly speaking, looking at the keto diet. Instead, they more broadly looked at low-carb and high-fat diets (LCHF).

They defined an LCHF diet as consisting of no more than 25 per cent of total daily calories from carbohydrates and more than 45 per cent of total daily calories from fat.

What happened in the study

The researchers analysed data from the UK Biobank, a large-scale biomedical database, containing in-depth genetic and health information from half a million UK participants.

Upon enrolment in the biobank, 70,684 participants completed a one-time, self-reported 24-hour diet questionnaire and, at the same time, had blood drawn to check their levels of cholesterol.

The researchers identified 305 participants who, during the 24-hour reporting period, met the study’s definition of an LCHF.

These participants were matched by age and sex with 1220 individuals who reported eating a standard or balanced diet.

This resulted in 73 per cent of the participants in each group being women and the group’s average age was 54 years.

Those on an LCHF diet had an average body mass index (BMI) of 27.7; those on a standard diet, 26.7. A BMI of 25 to 30 falls within the overweight range.

The results

Compared with participants on a standard diet, those on an LCHF diet had significantly higher levels of LDL cholesterol. There were also higher levels of apolipoprotein B (apoB), a protein that helps carry cholesterol and fat throughout the body.

Dr Iatan said that “previous studies have shown that elevated apoB may be a better predictor than elevated LDL cholesterol for risk of cardiovascular disease”.

After an average of 11.8 years of follow-up – and after adjustment for other risk factors for heart disease, such as diabetes, high blood pressure, obesity and smoking – people on an LCHF diet had more than two times higher risk of having several major cardiovascular events.

These included blockages in the arteries that needed to be opened with stenting procedures, heart attack, stroke and peripheral arterial disease.

In all, 9.8 per cent of participants on an LCHF diet experienced a new cardiac event, compared with 4.3 per cent of those on a standard diet.

A limitation of the study is that participants provided dietary information at only one point in time, which should be considered when interpreting the study findings, Dr Iatan said.

She says her findings warrant further investigation of LCHF diets and heart health.

Does the study stack up?

We have to keep in mind that we don’t know exactly how many of the participants were on a true keto diet. And how many experienced the benefits of weight loss.

Does this matter?

Dr Daniel Soffer, is a Penn Medicine lipidologist – a doctor who studies fatty substances called lipids in your blood and looks for ways to manage or treat conditions linked to them.

In a 2022 interview, he said population studies and clinical trials had overall found “the impact the keto diet has on lipids is modest”.

Most commonly, he said, research shows keto produces modest reductions in triglycerides, modest changes in heart-protective high-density lipoprotein (HDL, or “good”), and minimal changes in low-density lipoprotein (LDL, or “bad”) levels.

But, and here’s the kicker, some keto diets involve healthier fats than others.

“Some people are careful about the types of fats they eat, while others are not,” he said.

The results of the new study probably reflect this reality.