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Carbohydrate quality

Carbohydrate quality is important and this positive approach to improving diet has great potential to improve public health.

What is carbohydrate quality?

Carbohydrate quality can significantly influence health. Rather than being one thing, carbohydrate quality can be characterised by several components. The most widely studied include:

  • Dietary fibre
  • Wholegrains
  • Pulses/legumes
  • Glycemic Index (GI)/Load (GL)

The benefits of quality carbohydrates are due to the nutrients they contain such as fibre and phytonutrients such as antioxidants, and/or their reduced glycemic impact that contributes to better metabolic health.

A worldwide not-for-profit organisation called the International Carbohydrate Quality Consortium (ICQC) exists to support, summarise and disseminate science in this field.

Carbohydrate quality and nutrient adequacy

Two trials have explored the relationship between carbohydrate quality and nutrient intake. An analysis of data from the Spanish PREDIMED study used a Carbohydrate Quality Index (CQI) that assessed four aspects:

1. glycemic index (GI);

2. ratio of wholegrains to total grains;

3. ratio of solid vs total (solid + liquid) carbohydrate (liquid carbohydrates are sweet drinks); and

4. dietary fibre.

The researchers found highest consumers of quality carbohydrates (with the highest CQI scores) had the lowest prevalence of inadequate micronutrient intake. In a repeat of similar methodology using the CQI, analysis of data from the SUN (Seguimento Universidad de Navarra) cohort also found those with the highest CQI scores had the lowest prevalence of nutrient inadequacy.

Carbohydrate quality and health

In research commissioned by the WHO (World Health Organisation) to inform the development of updated carbohydrate recommendations, a series of systematic reviews and meta-analyses examined the relationship between carbohydrate quality and health. It included 185 prospective studies and 58 randomised controlled trials with a total of 4635 adult participants.  

The researchers found a 15-30% decrease in all cause and cardiovascular disease mortality, incidence of coronary heart disease and stroke, type 2 diabetes and colorectal cancers in the highest consumers of fibre compared with the lowest. They also found lower body weight, lower systolic blood pressure and lower total cholesterol in the highest fibre group.

The risk reduction was greatest at fibre intakes between 25 and 29g a day, with higher amounts probably conferring greater benefits against cardiovascular disease, type 2 diabetes, colorectal and breast cancer. The evidence between carbohydrate quality and better critical outcomes were graded as moderate for dietary fibre, low to moderate for wholegrains, and low to very low for glycemic index (GI) and glycemic load (GL). The authors conclude GI or GL might be less useful as an overall measure of carbohydrate quality.

Glycemic Index and health

While the authors of the above paper found the evidence around GI and GL less than convincing, others have found the evidence more compelling, especially around the risk of type 2 diabetes. The journal Nutrients ran a special issue in February 2020 of 12 research papers addressing the relationship between GI and health outcome.  Here are some key findings.

A dose-response meta-analysis of prospective cohort studies on the relationship between GI, GL and the risk of type 2 diabetes found GI was strongly associated with incident type 2 diabetes. The risk increased by 90% from the lowest to the highest GI exposure worldwide, and the relative risk was found to be much larger than consuming low levels of fibre or wholegrains. An investigation of causal effect using the Bradford Hill criteria (used to test causation) concluded there is substantial evidence that GI and GL are causally linked to the risk of type 2 diabetes. Neither dietary fibre, cereal fibre or wholegrains are reliable predictors of GI or GL.

In their introductory remarks to the special issue, Professor Jennie Brand Miller and Annette Buyken comment that many health professionals still don’t understand the implications of GI on food choices: “four decades after the publication of the first comprehensive list of GI values, many health professionals still believe that sugar and sugar-sweetened beverages have greater impact on PPG (post prandial glycemia) than starchy foods like bread and potatoes. This belief has no scientific basis. Most starchy foods have a GI greater than 70, while most sugary foods are less than 70, and on the whole, we eat twice as much energy in the form of starch as added sugar…”

A point worth noting is that refined starches are readily digested and absorbed and 100% by weight glucose.  Sugar is approximately 50% by weight glucose, the remainder being fructose which has much lower impact on PPG.

How does reduced glycemic impact help?

Post prandial glycemia (PPG) -elevated blood glucose after eating - is the likely mechanism through which higher GI foods exert their metabolic harm.  High PPG is implicated in the development of obesity and chronic diseases including type 2 diabetes and cardiovascular disease.

High PPG causes glycemia and insulinemia that contribute to oxidative stress, inflammation, fatty liver, reduced fat oxidation, lipogenesis and endothelial dysfunction. Pharmacological intervention to reduce PPG in people with impaired glucose tolerance using acarbose (an alpha-glucosidase inhibitor) has been shown to decrease progression to diabetes by one quarter.

Wholegrains and fibre

The WHO Global Burden of Disease Study 2017 identified low intakes of wholegrains was one of the top four dietary risk factors in Australia. Read more about study in Issue 46 of Sweet bites.

In their 2017 consensus statement on wholegrains, the ICQC stated there is strong epidemiological evidence associating wholegrain consumption with reduced all-cause mortality, reduced risk of obesity, type 2 diabetes and cardiovascular disease. Evidence from prospective cohort studies show increasing wholegrain consumption from zero to two servings a day is associated with health benefits.

In a recently published paper included in the top ten research summaries in this issue, a systematic review and meta-analyses of dietary fibre and whole grains in diabetes management found all types of fibre help management of all types of diabetes and improve glycaemic control, blood lipids, body weight, inflammation and reduce premature mortality. Greatest improvements were found in moving from low to moderate or high intakes. They suggest increasing fibre by 15-35g might be a reasonable target to reduce premature mortality.

Both the Australian Dietary Guidelines and the New Zealand Eating and Activity Guidelines recommend “mostly wholegrain” grain foods.

Australia:

“Enjoy a wide variety of nutritious foods from these five foods groups every day: (including) Grain (cereal) foods, mostly wholegrain and/or high cereal fibre varieties”

New Zealand:

“Enjoy a variety of nutritious foods every day including grain foods, mostly whole grain and those naturally high in fibre.”

The Heart Foundation (Australia) Position Statement on Heart Healthy Eating Patterns encourage people to eat “plenty of vegetables, fruits and wholegrains”.

Wholegrains or cereal fibre?

It is difficult to disentangle the benefits of cereal fibre (fibre from cereal grains) versus the benefits of wholegrains, and this represents an area of difficulty in interpreting the literature and in setting food and nutrition policy. The Australian Dietary Guidelines have an “and/or” statement for wholegrains and fibre.

FSANZ permits health claim for fibre on food labels, whereas wholegrain content claims are currently managed under an industry Code of Practice by the Grains and Legumes Nutrition Council (GLNC). Fibre is part of the Health Star Rating (HSR) calculation whereas wholegrain content is not, although many support its inclusion.

Recommending wholegrains is consistent with food-based dietary guidelines rather than nutrient based guidelines. The Heart Foundation (Australia) say evidence suggests that improving the entire eating patterns, not just simply altering one nutrient or food, is required to promote cardiovascular health.

Effect of processing

A New Zealand research group investigated the effect of processing of whole grains on glycemic control in people with diabetes. In a crossover design intervention, they found less processed whole grain wheat, oats, and brown rice (e.g. dense grainy bread, wholegrain oats) improved measures of glycemia compared with finely milled versions (e.g. wholemeal bread, instant oats) over two weeks. Their results suggest finely milling wholegrains may reduce their health benefits. This result raises the question as to whether adding fibre as an ingredient (such as synthetic carbohydrate polymers like polydextrose) to highly processed foods has the same health benefits as fibre naturally present in whole foods. Different types of fibre have different benefits so consuming a variety of types is important.

Pulses and legumes

Pulses and legumes are healthy foods that attract few arguments about their dietary status and can well and truly be considered quality carbohydrates. They have higher protein content than grains, they are high in fibre, rich in phytonutrients and have a low GI - even when processed. They’re also affordable and sustainable. Now, more than ever, they are in the spotlight because of the trend toward eating less meat and more plant foods. The evidence for their health benefits is also solid, showing strong associations with metabolic health markers and a reduced risk of cardiovascular disease.

Are there other carbohydrate components that influence quality?

The type of starch in food is an important yet frequently overlooked component (read more about starches in issues 16 and 24). Starch type and how it is processed (by manufacturers or by cooking at home) has a big influence on the GI of foods.

The type of starch in most rice varieties is high GI, especially when cooked in the traditional way until fluffy, and explains why most rices, white or brown (wholegrain), have a high GI. It’s the same for potatoes– most are high GI, especially when boiled or mashed (there are some lower GI varieties including Carisma developed in Australia). The type of starch - amylose vs amylopectin - in a food is an important predictor of GI: amylopectin is extensively branched leaving more area available for digestion into glucose which conveys a high GI, whereas amylose is linear with a smaller area exposed for digestion conveying a low GI.

Resistant starch found in legumes, green bananas, oats, barley and cooked and cooled pasta and potato, is classified as dietary fibre with well-known gut health benefits. However refined starches have a high glycemic impact and dental caries-causing potential. Refined starches are a popular food ingredient in potato, rice, and corn (extruded) snacks and puffed breakfast cereals, and as an additive (thickeners and stabilizers).  Some are obvious on the label such as ‘corn starch’ while others have complicated names, such as ‘hydroxypropyl distarch phosphate’. Starches are also not listed in the Nutrition Information Panel; to determine starch content you need to deduct total sugars from the total carbohydrate content. While research on refined starches in the food supply and their influence on health are lacking, they are likely to have adverse effects.

Conclusion

It is clear carbohydrate quality is an important aspect of healthy eating. Quality carbohydrates reduce chronic disease risk and the evidence is particularly compelling for dietary fibre. There may be some inconsistencies in the literature on the impact of GI and wholegrains, however there is high likelihood of benefit and low risk of harm from recommending all aspects of carbohydrate quality.

 

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