PERSPECTIVES WITH DR ALAN BARCLAY

DO HIGH CARB DIETS INCREASE THE RISK OF DIABETES? 
High carbohydrate diets, and as a consequence foods and drinks high in carbohydrate, are the current dietary villains. Book store shelves are lined with the latest low-carb fad diet books. Similarly, our supermarket shelves are becoming increasingly stocked with food and drinks that proudly declare that they are low, or at least lower, in carbohydrate, on their food labels and associated marketing materials. Fad diets are great for business.

The theory behind the fad is that consumption of carbohydrate (starches and sugars) increases blood glucose and insulin levels, ultimately increasing insulin resistance, and therefore making it harder for our bodies to utilise fat (from our diets as well as what’s stored in our fat cells). Consequently, they make us get fatter. Because carbohydrate has the most profound effect on blood glucose and insulin levels, many people also think that reducing carbohydrate consumption may correspondingly reduce the risk of developing type 2 diabetes. However, as explained previously in GI News, proteins in food can also increase insulin secretion and fats can increase insulin requirements by increasing insulin resistance. Unfortunately, both foods and human physiology are much more complicated than what is portrayed in the latest fad diets.

Excess weight – particularly carried around the middle (central obesity) – is a well-established risk factor for type 2 diabetes, and randomised controlled trials conducted around the globe have proven conclusively that losing at least 7% of initial body weight will prevent its development in people with pre-diabetes (elevated fasting blood glucose or impaired glucose tolerance).

One of the best examples of this can be found in the Diabetes Prevention Program, which was a large (more than 3000 people), multi-centre randomised controlled clinical trial, which commenced in the USA in the mid-1990s. Participants were over 25, had a BMI over 24 kg/m2, and had pre-diabetes. They were randomly assigned to one of three groups: (1) placebo; (2) metformin; or (3) intense lifestyle.

People assigned to the intense lifestyle group were encouraged to achieve and maintain over 7% weight loss (based on their initial body weight) and were counselled to follow a reduced-energy (calorie/kilojoule), low-fat (also low in saturated fat) diet and to engage in more than 150 minutes of moderate-intensity physical activity each week. After an average of 2 years and 9 months, people in the lifestyle group had a 58% (3 out of 5) decrease in risk of developing type 2 diabetes, and due to these outstanding results, the trial was discontinued. Careful analysis of the data demonstrated that weight loss through reduced energy intake and increased physical activity was the main driver of their reduced diabetes risk.

Participants’ dietary intake was estimated at the beginning of the study (baseline) and 1 year later using a validated food frequency questionnaire. At baseline, body weight was inversely associated with total carbohydrate consumption and positively associated with total and saturated fat consumption. In other words, diets higher in total and saturated fat and lower in carbohydrate were associated with increased body weight. Similarly, weight loss at 1 year was strongly associated with increased carbohydrate consumption – in particular from foods that were naturally high in dietary fibre like fruits, vegetables and legumes. Weight loss was also associated with decreases in total and saturated fat consumption.

There are several potential reasons why increasing carbohydrate from fruits, vegetables and legumes are associated with weight loss:

• High carbohydrate and high fibre foods generally have a lower energy density (Calories or kilojoules per gram of food), but are very filling. 
• The dietary fibres stimulate the production of short-chain fatty acids in the colon, which may alter energy metabolism in our liver, muscles and fat tissues. 
• Soluble dietary fibres from these foods slow down the passage of the carbohydrate from the stomach and intestine and slow their absorption into the blood. 
• The carbohydrate and fibre provide important fuel for the microbiome, which in turn effects energy balance. 

The authors overall conclusion was: “Given the widespread public perception that carbohydrates are detrimental in increasing diabetes risk and the increasing prominence of low-carbohydrate diets for weight loss, the current findings are critical to the development of evidence-based recommendations for optimal dietary approaches to prevent diabetes.”

Based on the best available evidence, dietary patterns that encourage the regular consumption of quality carbohydrates do not increase the risk of developing type 2 diabetes. Quite the contrary.

Read more: 

• A High-Carbohydrate, High-Fiber, Low-Fat Diet Results in Weight Loss among Adults at High Risk of Type 2 Diabetes 
• Prevention or Delay of Type 2 Diabetes – Standards of Medical Care in Diabetes—2018

 
Alan Barclay, PhD is a consultant dietitian. He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of  The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).

Contact: You can follow him on Twitter.