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The facts on low-carb high-fat diets and diabetes

Low-carbohydrate diets, including low carbohydrate high fat (LCHF) and ketogenic (keto) diets have been gaining popularity among people with diabetes, as a way of managing blood glucose levels and weight. But are they really the answer to optimal diabetes management? 

What is a low-carb diet? 

There’s no formal definition of low carb. This term can be used for diets as low as 20g of carbs per day or for those with a more modest reduction in carbs (less than 45% of total energy intake).

Low carb and very low-carb diets typically restrict or exclude a wide range of foods including wholegrains, starchy vegetables, legumes, most fruits and some dairy foods and replacing them with non-starchy vegetables, meat, poultry, seafood, eggs, olive oil, coconut oil, nuts, and dairy foods such as cheese, butter, cream and Greek yoghurt. They often encourage choosing higher fat cuts of meat and full-fat dairy foods.

Is a low-carb diet the best way to manage my diabetes?

When we eat carbohydrate, it is broken down during digestion to glucose, which then appears in the bloodstream. It might therefore seem logical that reducing carbohydrate intake will help to lower blood glucose levels. And to some extent, this is the case.

But if you have type 2 diabetes, the reason you don’t deal with carbs in the same way as someone without diabetes is because of the underlying insulin resistance. This means that any dietary or lifestyle changes you make should be aimed at improving how insulin works in your body – treating the underlying problem of insulin resistance rather than just the symptoms of high blood glucose.

Some diets that help you to lose weight may improve insulin resistance and lower blood glucose levels. Low-carb diets are no exception and evidence shows that low-carb diets (including very low-carb diets) can help improve blood glucose levels and blood fats and help reduce medication needs in people with type 2 diabetes. But other diets, including Mediterranean and low-fat vegetarian diets, have shown similar benefits. This suggests that low-carb diets are not the only option and that cutting carbs isn’t necessary for managing diabetes.

How does a low-carb diet affect type 2 diabetes? 

There is evidence that low-carb diets, particularly those high in animal foods, can worsen insulin sensitivity, increase the risk of type 2 diabetes and increase overall mortality. Diets high in saturated fat are associated with increased insulin resistance and risk of type 2 diabetes. And restricting carbohydrate intake and consuming more fat and protein, particularly from animal sources, can cause negative changes in gut bacteria, potentially increasing inflammation and risk of digestive diseases including bowel cancer (32–35).

There is a lack of research to tell us whether low-carbohydrate diets are safe and effective for people with type 1 diabetes.

Current recommendations on low-carb diets for diabetes 

Diabetes Australia, the American Diabetes Association and Diabetes UK have all published recommendations on the use of low-carbohydrate diets for people with diabetes. They state that while there is evidence for short-term benefits of low-carb diets for type 2 diabetes, in the long run (12 months or more) they appear no more effective than higher carbohydrate eating patterns. They also point out research to suggest that low-carb diets are hard to sustain –an important consideration when making dietary changes to optimise your health.

Other recommendations:
  • The American Diabetes Association says that low-carb diets are not appropriate for certain groups including women who are pregnant or lactating, children and those with renal disease or disordered eating behaviour.
  • Diabetes UK also caution against children eating a low-carb diet because of serious concerns about the impact on growth and Australian researchers found that this diet can affect the growth of children with type 1 diabetes.
  • Diabetes Australia recommend that people with diabetes who wish to follow a low-carb diet should do so in consultation with their diabetes healthcare team, including seeking advice from an Accredited Practising Dietitian experienced in diabetes management.

The pros and cons of going low carb 

Different research show that there are many carbs that shouldn’t feature regularly in a healthy diet. These include refined starches (such as biscuits, cakes, pastries, highly processed breakfast cereals and white bread) and added sugars (such as confectionary and soft drinks). It’s likely that most of the short-term benefits of going low carb come from eliminating these foods from your diet.

However, low-carb diets also restrict most of the foods we know are beneficial for good health including wholegrains, legumes, fruits and some vegetables. Research has shown that eating more of these foods is associated with a reduced risk of many chronic diseases including cardiovascular disease, type 2 diabetes and some types of cancer. The long-term effects of following a low-carb diet are unknown, but this type of eating plan is very different to the diets of long-lived healthy populations, who all consume a mostly plant-based diet.

Low-carb diets can also have some unpleasant side effects including constipation, bad breath, headaches and dehydration because of ketosis (breaking down fat for energy). Low-carb diets can also lead to poor energy levels and fatigue, making exercise difficult, particularly endurance exercise.

A word of caution about diabetes medications and low-carb diets 

If you choose to follow a low-carb diet and you are taking insulin or some types of oral medications, it is important that you see your doctor before changing your diet. A low-carb diet can increase the risk of hypos if you are taking insulin or sulphonylureas and your medication doses will need adjusting.

Another type of diabetes medication, called sodium-glucose co-transporter-2 inhibitors (abbreviated to SGLT2i) has been found to cause a serious condition called diabetic ketoacidosis (DKA) in some people, particularly when they restrict their carbohydrate or overall food intake. So make sure to check in with your doctor beforehand as you will likely need to stop taking this medication if you adopt a low carb eating plan.

In summary 

Low-carb diets are one way to lose weight and improve blood glucose levels, at least in the short term, but they are not the only way and may come with longer-term health risks.

There are certainly benefits of cutting down on refined starches and added sugars – few people would argue with this. But there’s also good evidence for the health benefits of many quality carbohydrate foods including legumes, wholegrains, fruit and vegetables, and research points towards a plant-based diet being one of the best types of eating plans for long-term health and chronic disease prevention.

If you choose to follow a low-carb diet, try to choose more plant-based sources of protein and fat (including avocado, nuts, seeds, olive oil, tofu and tempeh), and fill up on plenty of non-starchy vegetables and salads.

For more information on managing diabetes, check out our fact sheet. Consult with your doctor or health care professional for further advice.
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