low carb and keto

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Low carbohydrate and ketogenic diets have demonstrated short-term benefits for weight loss and type 2 diabetes, however their long-term impact on cardiovascular health remains unclear.

Take home messages

Low carbohydrate diets vary widely in carbohydrate content, ranging from very low (<50g carbohydrates a day) to moderate (150g per day).
A ketogenic diet is a very low-carb-high-fat diet containing <50g of carbohydrates per day (or with carbohydrates providing no more than 10% of total energy).
The evidence for benefits of low and very low carbohydrate diets is inconsistent. There is no demonstrated advantages for cardiovascular health or lipid profiles.
Short-term benefits (three to six months) may be seen:
- in people with type 2 diabetes who are overweight or obese and aiming to reduce their weight or blood triglycerides and/or improve blood glucose control.
- in people wishing to lose weight, especially when the diet is also high in protein – but the benefits are no different to those seen with any other calorie-controlled diet. Adherence to the diet is key for weight loss.
Long-term benefits of 12 months and over are not known as the evidence is lacking.
Optimal cardiovascular health is associated with plant-based diets that are low in saturated fats and refined carbohydrates and high in whole grain cereals, fruit and vegetables, plant proteins such as nuts, beans and pulses, and unsaturated fats. These are difficult to achieve when carbohydrate is restricted.
- Moderately low carbohydrate diets, with 150g of carbohydrates per day, can allow for more of these cardioprotective foods to be included.
Contraindication: As well as being very low in carbohydrates and therefore healthy plant foods, Ketogenic diets also tend to be very high in saturated fats without close dietetic supervision. This means they are not recommended for people with hypertriglyceridaemia and hyperlipidaemia, especially genetic hyperlipidaemias such as FH.
If someone wishes to trial a low carb diet, what the carbohydrates are replaced is key to cardiovascular health outcomes:
- Allow for a moderate carbohydrate intake rather than a very low carbohydrate intake.
- Carbohydrate allowances should be met with whole grains, fruits, and vegetables, beans, and pulses.
- Unsaturated fats should be prioritised over saturated fats.
- Plant sources of protein and fish should be used in place of meat and other animal proteins.
- Discourage this way of eating in the long-term as long-term evidence is not available.

What is a low-carb diet? National dietary recommendations The scientific evidence Cardiovascular risk

Vegan low-carb Contraindications Practical implications

Low carbohydrate diets, including ketogenic diets, have become popular over the past decade. In more recent years, the focus has turned to their potential benefits for managing type 2 diabetes and reducing body weight^1–3. Given that obesity and type 2 diabetes are significant risk factors for cardiovascular disease (CVD), and CVD is the primary cause of death in people with type 2 diabetes, it’s helpful to examine the impact of such diets on metabolic risk factors and cardiovascular outcomes¹.

What is a low carbohydrate diet?

Low carbohydrate diets vary significantly in carbohydrate content, with some containing no more than 50g of carbohydrates a day whilst others contain up to 150g a day. In the UK, usual carbohydrate intakes are around 220g daily for adults⁴.

Ketogenic diets, such as the Atkins Diet, contain very little carbohydrate (<50g, or provide no more than 10% of total energy) and are very high in fat^1,5.

50g of carbohydrates is equal to just two to three slices of bread or one serving of rice or pasta daily.

Most studies of low carbohydrate diets have examined their effects on type 2 diabetes, while the strongest evidence supporting ketogenic diets is for children with drug-resistant epilepsy⁶.

Categories of carbohydrate intakes¹

Carbohydrate category	Amount of carbohydrate
Carbohydrate category	g per day	% total energy
Very low (Ketogenic)	20 to 50	≤10%
Low	>50 to <130	>10% to <26%
Moderate	130 to 230	26% to 45%
High (usual intakes)	>230	>45%

A recent review highlighted the significant variation in the amount of carbohydrates in 'low carbohydrate diets,' which ranged from 50-130g of carbohydrates daily2 or 14% to 50% of total energy1.

National dietary guidelines and carbohydrates

UK and other national guidelines recommend that healthy carbohydrates provide around 50% of total daily energy intake (~250g). Refined carbohydrates and sugars in all forms, especially sugar sweetened beverages, are actively discouraged.

The main healthy food sources of carbohydrates are tubers such as potatoes and yams, whole grain cereals such as wholemeal bread/roti, brown rice, and whole grain pasta and breakfast cereals. However, other foods such as fruit and vegetables, beans and pulses and nuts are also excellent sources of carbohydrates.

Diets rich in healthy carbohydrates have been associated with an array of health benefits including improved blood lipid profiles^7,8, reduced risk of some cancers^7,9 and reduced risk of cardiovascular disease^10,11.

What does the evidence say?

In the short-term, low carbohydrate diets may have benefits for people who are overweight or obese with type 2 diabetes in terms of weight loss, blood glucose control, reduced blood triglycerides and lower blood pressure.

A recent review of the evidence concluded that 'lower' and 'low' carbohydrate diets are effective in the short-term (three to six months) for managing body weight and improving glycaemic control and cardiovascular risk factors in overweight or obese patients with type 2 diabetes^1,2.

However, it cautioned against continuing low carb diets beyond 12 months, as the longer-term implications are unknown. The review highlighted that lower and low carb diets are not superior to other diets; the key determinant of how effective they are is how long the person can sustain them.

A 2022 Cochrane review pooled data from 61 randomised controlled trials (RCTs) of 6,925 overweight and obese individuals, with and without type 2 diabetes, compared low and very low carbohydrate diets with normal carbohydrate diets¹². It found no discernible differences in weight loss, HbA1c, or LDL cholesterol in the short-term (<12 months) or the long-term (one to two years).
Another review of 19 meta-analyses investigating various diets found no difference between low and high carbohydrate diets for weight loss¹³. Even though low carbohydrate diets did lead to remission of type 2 diabetes in 20% of patients, very low calorie (400-500kcals per day) diets were far more effective, resulting in a 54% remission rate.

Contrary to the interest in low and very low (ketogenic) carbohydrate diets, the most recent dietary guidelines from the American Heart Association emphasise the lack of evidence to support their endorsement for heart health¹⁴.

The cardiovascular consequences of low carb diets

It is essential to consider what replaces the carbohydrates in low carbohydrate diets.

National and international recommendations for lipid management and cardiovascular disease prevention advocate healthy plant-based diets, such as the Mediterranean, DASH and Portfolio diets^14–16. These evidence-based diets demonstrate long-term cardiovascular benefits. They revolve around healthy carbohydrate-rich plant foods such as whole grains, fruits, vegetables, beans and pulses, with carbohydrates making up about 50% of total energy.

Adopting low-carbohydrate diets automatically reduces the intake of healthy carbohydrate foods which have proven cardiovascular benefits. Additonally, carbohydrates are often replaced with foods that have the opposite effect on cardiovascular health, especially over the long-term. Many studies in the SACN 2021 review did not address this issue¹. Without professional supervision, it’s likely that people will swap healthy carbohydrates with high saturated fat and protein-rich foods like red meat and butter, which are well established high risk factors for raised LDL cholesterol and CVD risk^17–21.

In 2019, the US National Lipid Association Nutrition and Lifestyle Task Force assessed the impact of low carb and very low carb diets on body weight management and cardiometabolic risk factors³. They found mixed effects on LDL cholesterol levels, with some studies showing an increase, possibly related to the high saturated fat content. They expressed concerns about the use of ketogenic diets, especially for individuals with hypercholesterolemia, and particularly familial hypercholesterolemia (FH), as it was deemed contraindicated for these conditions.

Could a low carb vegan diet be healthy?

A metabolic trial assessed the impact of a structured, calorie-restricted, vegan low-carb diet on blood lipids²². This diet, known as 'Eco-Atkins', emphasised limiting saturated fats and sugars while optimising whole grains, legumes and low-carb vegetables, with a focus on unsaturated fats and plant proteins.

47 overweight participants were randomised to 'Eco-Atkins' or a high-carb, lacto-ovo-vegetarian (LOV) diet and all received pre-prepared meals for four weeks. Although weight loss was similar in both groups, 'Eco-Atkins' significantly lowered total cholesterol by 8.1% (p=0.002 ) and total to LDL cholesterol ratio dropped by 8.7% (p=0.004). After a further six-months where participants ate a diet of their choice, 23 participants completed the trial and experienced further benefits with Eco-Atkins. This study highlights the short-term efficacy of healthy, moderately low carbohydrate vegan diets on lipid profiles.

However, long-term adherence was challenging, with less than half the participants completing the trial when meals were not provided for them. Given that only a small proportion of the UK population are vegan²³, and even fewer follow national healthy eating guidelines, it is unlikely that a healthy, low carb vegan diet could be achieved by most people in the long-term²⁴.

Contraindications for low carb diets

Some individuals may experience extreme effects of ketogenic diets due to genetic factors. These diets are not suitable for:

people with known hypercholesterolemia and FH, as they have, or may have, a genetic predisposition to raised LDL cholesterol levels
people with a history of hypertriglyceridemia linked to acute pancreatitis or severe hypertriglyceridemia.

In these cases, it’ is imperative to strongly discourage low carb diets, especially very low carb, high fat diets.

What does this mean in practice?

Talk to patients for whom low carb diets are contraindicated about their predisposition to CVD and why adopting these diets comes with risks.
Find out why your patient may want to try a low carb diet and let them know that these diets lack proven benefits for cardiovascular health and are no more effective than other diets for weight loss or blood sugar improvement. Over the long-term, depending on what is replacing carbohydrates, these diets are likely to result in dyslipidaemia and pose a risk to cardiovascular health.
For patients without diabetes and within a healthy weight range, advise them on the limited benefits of a low carb diet.
Encourage patients to embrace evidence-based, plant-based diets for improving blood lipids and cardiovascular health^14,15,25,26.
If a patient still wants to try a low carbohydrate diet, a tailored approach is necessary to ensure the overall quality of the diet is as cardioprotective as possible:
- Advocate for a 'lower' rather than a 'low' or 'very low' carbohydrate intake.
- Help them ensure the carbohydrates they are consuming are unrefined: whole grains, beans, pulses, low carb vegetables, nuts and seeds.
- Discourage refined grains, sugary foods and drinks.
- Ensure the main fats consumed are unsaturated, such as olive, sunflower or rapeseed oil, and the fat naturally present in nuts and seeds.
- Encourage healthier protein sources such as fish, nuts, beans and mycoprotein in place of red and proceed meat.

In summary

Low and very low carbohydrate diets, characterised by restricted carbohydrate intake and increased consumption of protein and fats, have gained attention for their short-term benefits, including weight loss, improved satiety, and better glycaemic control, particularly among overweight or obese individuals with type 2 diabetes. However, the long-term effects of these diets remain uncertain. Daily carbohydrate restrictions can range from <50g to 150g daily. Ketogenic diets are very low in carbohydrate (<50g per day) and high in fat.

While short-term studies suggest that low-carb diets may not adversely affect blood lipids, concerns arise regarding their potential impact on cardiovascular health over time.

Low carbohydrate diets, especially ketogenic diets (<50g carbohydrate per day), limit the consumption of foods known to be cardioprotective, such as fruits, vegetables, whole grains, nuts, and legumes. Furthermore, the common practice of replacing carbohydrates with foods high in saturated fats, such as meat and butter, have been proven to unfavourably impact on blood lipids and increase the risk of CVD.

The lack of long-term studies, particularly those extending beyond 12 months, prevents definitive conclusions regarding the effects of low-carb diets on lipid profiles and CVD outcomes.

The short-term benefits of low-carb diets are comparable to other calorie-controlled diets like the Mediterranean diet and Portfolio diet, both of which have demonstrated long-term benefits. Thus, while low-carb diets may offer short-term advantages, considerations for their potential long-term impact on cardiovascular health warrant further investigation and comparison with other dietary approaches.

References

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Diabetes UK. Low-carb diets position statement for professionals (May 2021). Diabetes UK. Accessed February 7, 2024. https://www.diabetes.org.uk/for-professionals/supporting-your-patients/clinical-recommendations-for-professionals/low-carb-diets-for-people-with-diabetes
Kirkpatrick CF, Bolick JP, Kris-Etherton PM, et al. Review of current evidence and clinical recommendations on the effects of low-carbohydrate and very-low-carbohydrate (including ketogenic) diets for the management of body weight and other cardiometabolic risk factors: A scientific statement from the National Lipid Association Nutrition and Lifestyle Task Force. Journal of Clinical Lipidology. 2019;13(5):689-711.e1. doi:10.1016/j.jacl.2019.08.003
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Published: June 2024