Diabetes

Updated 09 November 2015

Low-carb vs. high-carb – which is the best diet for type 2 diabetics?

DietDoc discusses a recent study of nine meta-analyses on the effects of low-carbohydrate diets on type 2 diabetes.

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As I mentioned in a previous article, many people would summarise the current biggest issues in the field of nutrition as: “Obesity, Banting and Carbs!” 

A study entitled “A critical review of meta-analyses of low-carbohydrate diets in people with type 2 diabetes” was recently published in a leading diabetes journal, Diabetic Medicine.

Because November is “Diabetes Awareness Month”, the results of this study may shed light on the type of diets that would be of the greatest benefit to diabetics.

Study design

The abovementioned study assessed the results from all the available meta-analyses on low carbohydrate diets in people with diabetes. A meta-analysis is simply the process of combining various studies into one to assess the overall outcome.

Hamish Van Wyk, who is a Registered Dietitian and Diabetes Educator at the Department of Health in Port Elizabeth, and his colleagues went one step further and combined nine meta-analyses to study the effects of low-carbohydrate diets on type 2 diabetes.

Read: Low-carb diet debate continues

Strict criteria

The authors set very strict criteria for the studies included in their analysis:

- Only randomised controlled studies were used. This means that a control group is used to monitor if the effect of the treatment is due to the treatment or if it is caused by some other factor. The allocation of test subjects to either the treatment group or the control group is done randomly to increase the probability that any effects that occur can be attributed to the treatment and not to chance alone. 

- The studies had to continue for at least 4 weeks or longer to determine if the subjects were able to stick to their diets or not.

- The subjects had to be older than 18 years of age and diagnosed with type 2 diabetes

- The carbohydrate intake of the test and control subjects had to provide 45% or less of their total energy requirement, which also included diets of less than 50g of carbs a day.

- A dietary intake assessment at the end of the study period had to be included to ensure that what was eaten was in fact a high or low carbohydrate diet as well as to determine if the subjects were eating as few carbs as specified at the start of the study.    

Nine meta-analyses were identified, containing 153 studies, of which only 12 met the strict criteria.

Unattainable carbohydrate intakes

At 1 year the average carbohydrate intake in the very-low carbohydrate diets (i.e. less than 50g of carbohydrate per day), was between 132g to 162g of carbs a day. This is 2½ to 3 times more carbohydrate per day than had been specified.  The researchers questioned if such a strict carbohydrate restriction can actually be achieved.

Read: Do you follow a low-carb diet ?

On the other hand, high carbohydrate diets were also not sustainable and most subjects were eventually eating moderate amount of carbohydrates.

Even in studies where a high carbohydrate intake was specified, the amounts of carbohydrate eaten were smaller than the subjects ate before they started on the high-carb diet. In other words, even on the high-carb diets, the subjects restricted their carbohydrate intake to some degree.

It also seems apparent that most people tend to eat a moderate amount of carbohydrate, no matter if they start out on a very-low or a high-carbohydrate diet.

Protein or fat?

In some studies the protein intake was increased, while in other studies, the fat intake was increased, with no clear superiority between the two. Studies that increased fat intake primarily increased plant based oils (e.g. the Mediterranean Diet).

Even in the studies where saturated fat (generally animal based fat) intake was encouraged, the subjects ate fewer saturated fats than before they started their diets. The researchers concluded that encouraging the public to reduce their carbohydrate intake and increase their saturated fat intake, is not based on data obtained so far with low-carb diets in type 2 diabetics.

In an interview, van Wyk mentioned that one of the largest studies on fat intake in relation to carbohydrate intake recently showed that saturated fat increases the risk of cardiovascular disease when this type of fat is used in place of vegetable based oils or whole grain carbohydrates. 

Read: Bring on the butter: saturated fats may not cause heart disease after all

However, van Wyk pointed out that he is concerned because the results also showed that if refined carbohydrates (e.g. sugar, white bread and sifted maize meal) were used in place of saturated fats there was no change in cardiovascular disease.

Thus, one could say that refined carbohydrates are just as potentially harmful as saturated fats. As most people in South Africa live on maize meal and other refined carbohydrates, these results of the study should be of great concern to health professionals.

Clinical markers

No significant differences in metabolic markers such as blood pressure, cholesterol, BMI, waist circumference, inflammation and insulin production and sensitivity were found between the two diets.

Blood glucose control

The researchers found no difference between blood glucose control between the low and high carbohydrate diets (despite the fact that some of these subjects reduced their carb intake by as much as 115 g/day compared to the high-carb diet subjects).

No real difference?

Van Wyk stated, “Patients overall are unable to stick to very-low carbohydrate diets. Even in the high-carbohydrate diet studies, the amounts of carbohydrate eaten were less than those eaten by the patients before the start of the studies. “

His personal experience at a primary healthcare clinic is that patients eat as much as 400g of refined carbohydrates a day. Consequently he encourages his diabetic patients to reduce their refined carbohydrate intake and tries to individualise each patient’s diet. 

Van Wyk believes that it is impossible to prescribe the same amount of carbs to all diabetic patients and that expecting each patient to reduce his or her carb intake to less than 50g is not sustainable for most people long term.

Read more:

Low-carb diet helps obese diabetics

To carb or not to carb

Pondering the low-carb lifestyle

Reference:

Van Wyk H, Davis RE, Davies JS (2015). A critical review of meta-analyses of low-carbohydrate diets in people with Type 2 diabetes. Diabetic Medicine, 2015 Sept 28. doi:10.1111/dme/12964.

Dr Ingrid van Heerden is a registered dietician and holds a doctoral degree in Nutrition and Biochemistry. She believes that "we are what we eat" and offers free nutrition and weight loss advice via her DietDoc service on Health24.com. Read more of her articles.