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.
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.
