Can some diets harm your heart?

Keto, paleo and plant-based diets have increased in popularity over the years with potential health benefits reported. However, recent research suggests that some diets may harm your heart in the longer term. Dr Mohssen Chabok, our consultant cardiologist, explores popular diets, such as keto, paleo and vegan, and their health benefits and harms in this article.

What is the keto diet?

The keto diet – short for ‘ketogenic’ – is a low-carbohydrate, high-fat diet. People following this diet typically consume 70-80% of their daily calories from fat, 5-10% from carbohydrates, and 10-20% from protein.

This is in stark contrast to standard dietary guidelines which suggest that just over a third of your daily calories should come from carbohydrates, with fruits and vegetables also contributing towards at least a third of your daily calories. The remaining recommended amounts are made up of protein (around two handfuls per day), dairy (2-3 portions per day) and a small amount of healthy fats.

“Depriving the body of carbohydrates causes it to enter a state known as ketosis, during which the body breaks down fat molecules to produce energy,” explains Dr Mohssen Chabok. “This process can lead to weight loss which has popularised the keto diet, particularly among people who are aiming to lose weight within a short period.”

How does the keto diet affect your heart health?

While losing excess weight can have a positive impact on your cardiovascular health, the increased amounts of fat consumed in the keto diet can have the opposite effect.

In fact, the keto diet has been associated with an increase in LDL cholesterol levels (also known as “bad” cholesterol). High levels of LDL cholesterol can increase your risk of experiencing heart disease and strokes. With cardiovascular disease responsible for 25% of deaths in the UK, protecting your heart health is crucial.

Studies have shown that severe high cholesterol (defined as over 5mmol/L or 190mg/dL) was approximately doubled in people on a keto diet, with 10% of them experiencing severe high cholesterol. This is high compared to a figure of 5% for people following a more balanced diet with the recommended level of carbohydrates.

Research also suggested that people following the keto diet had a sixfold increased risk of developing heart disease, leading to some specialists recommending that those on the keto diet should have their cholesterol checked regularly.

Further research by cardiologists in China, which was presented at the American College of Cardiology’s Annual Scientific Session, looked at the long-term effects of a low carbohydrate diet over the course of 20 years. This revealed an association between a diet low in carbohydrates and an increased risk of atrial fibrillation (an irregular and often fast heartbeat).

Atrial fibrillation can increase your risk of stroke by up to five times, further highlighting the potential negative effects of a low-carb diet.

What is the palaeolithic diet?

The palaeolithic diet (often shortened to ‘paleo’) is centred around eating foods which were thought to be commonly eaten in the Stone Ages, around 2.6 million years ago, which could have been hunted or gathered. Our cave-dwelling ancestors did not have access to sugar, salt, frozen foods or dairy, and instead are thought to have mainly consumed:

• lean meat
• fish
• eggs
• fruits
• vegetables
• roots
• nuts

Sticking to foods which could have been hunted or gathered by our ancestors eliminates processed foods and preservatives, and promotes a diet lower in sodium as no salt is added to any food.

What effects does the paleo diet have on your heart health?

The paleo diet is praised for its elimination of processed foods and is also often promoted for gut health. However, the paleo diet is now thought to have negative effects on cardiometabolic markers and gut microbiota, which provide indications of your heart and gut health, due to a lack of fibre from whole grain sources.

A study published in the European Journal of Nutrition revealed that people strictly following the paleo diet long-term experienced higher levels of trimethylamine-n-oxide (TMAO) than people following the paleo diet loosely, as well as people who ate a standard balanced diet. TMAO is a metabolite from the gut which is associated with cardiovascular disease.

The study also showed that the paleo diet “may not be beneficial for gut health,” due to the changes in gut microbiota caused by a lack of whole grains.

The paleo diet allows an unlimited amount of red meat. However, diets heavy in red meat have been linked to an increased risk of cardiovascular disease and cancer. The paleo diet also doesn’t include whole grains or legumes which are linked to better cholesterol levels, as well as a lower risk of stroke.

“As with any diet, ensuring that you eat a balanced range of foods is the best approach,” says Dr Chabok. “This avoids cutting out entire food groups, such as carbohydrates in the keto diet, or whole grains in the paleo diet, and ensures your body is getting all the nutrients it needs.”

How do vegan and vegetarian diets affect your heart?

With more than three million people in the UK eliminating meat from their diets, and 600,000 of those following a plant-based programme, vegetarian and vegan diets are much more widely known than keto and paleo diets.

Vegetarian diets simply involve leaving out any meat and fish, while vegan diets exclude meat, fish, dairy, eggs, and any other food which is created from animal products. A pescetarian diet is also meat-free but does allow fish and other seafood to be eaten.

While vegan diets are widely seen as very nutritious, unhealthy plant-based options are becoming more popular with vegan alternatives to cakes, burgers and pizzas readily available in supermarkets and restaurants. As with the paleo diet, it’s important to note that just because a commonly-perceived ‘healthy’ diet includes particular types of food, it doesn’t necessarily mean it’s all nutritious.

With this said, well-balanced plant-based diets have been shown to lower the risk of heart attacks, strokes and heart failure. This is particularly true if a vegan diet is followed in young adulthood to help lower the risk of heart problems in middle age. Other similar research has demonstrated a link between a plant-based diet and lower cholesterol levels in postmenopausal women, ultimately leading to a lower risk of heart disease.

Contrastingly, a study published in September 2019 in the BMJ has suggested that vegetarians with low vitamin B12 levels (found in egg, salmon, beef, milk, and cheese) may experience a higher risk of strokes. It is possible that this is due to their diets lacking some animal-based nutrients and healthy fats, however the evidence from the study is not conclusive. The authors themselves acknowledge that further research in the area is required to provide definitive answers around stroke risk for vegetarians.

Looking at vegan diets in particular, a 2021 study was published in the Journal of the American Heart Association. The research followed almost 5,000 participants aged 18-30 for 30 years, and those who ate a healthy plant-based diet were 52% less likely to develop cardiovascular disease over the next three decades. Findings showed that nutritionally rich plant‐centred diets are linked to a decreased risk of cardiovascular disease, further demonstrating the benefits of vegan and vegetarian diets on heart health.

What is the Mediterranean diet?

The Mediterranean diet is named after the area from which it derives: the Mediterranean Basin. The diet is common in countries such as Portugal, southern Spain and Italy, and Greece. It focuses on plant-based foods and healthy fats, generally including foods such as:

• fruits and vegetables
• beans, nuts and lentils
• wholegrains (e.g. brown bread and brown rice)
• fish and other seafood
• cheese and yoghurt
• unsaturated fats (e.g. olive oil)
• red wine with meals

The Mediterranean diet features little to no sweets or sugary drinks, no butter and no processed foods. It also generally includes little to no meat and, if meat is eaten, then poultry is recommended over red meat.

How does the Mediterranean diet affect your heart?

The Mediterranean diet has long been described as one of the best diets for your heart health, thought to lower your risk of a heart attack, stroke or early death from cardiac disease. 

“We touched on one of the main negatives of the keto and paleo diets in that they include an unlimited amount of red meat – which has been linked to an increased risk of cardiovascular disease – so one of the advantages of the Mediterranean diet is that it doesn’t recommend red meat at all,” explains Dr Chabok. “People are advised that if they want to eat meat and follow this diet, then a small amount of poultry is better than red meat.”

The Mediterranean diet also includes whole grains and legumes which have both been linked to better cholesterol levels and a lower risk of strokes.

The Mediterranean diet and six other low-fat diets were researched for a 2023 study in the BMJ journal and, amongst 35,000 participants across three years, the Mediterranean diet resulted in the most reduced levels of heart attacks and strokes.

The Mediterranean diet also reduced risk factors commonly associated with cardiovascular disease, including type 2 diabetes, high blood pressure, high cholesterol and obesity. “Managing these factors helps to prevent cardiovascular disease from occurring further down the line, so the earlier a Mediterranean diet is adopted, the better the effects are for your heart health,” says Dr Chabok.

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