Author Nina Evans, Research & Trainer in Public Health/25 April 2019
Independently both low carbohydrate and higher fibre diets have been shown to be beneficial for patients with Type 2 diabetes. Therefore it can be assumed that dietary intake with a combination of these two diets will have added benefits to health.
Some have suggested that it is the increasing intakes of refined carbohydrate alongside decreasing intakes of fibre over the 20th century which best explains the increase in the prevalence of Type 2 diabetes. Many have proposed that because of this the carbohydrate to fibre ratio may play an important role in human health. This blog will discuss what the carbohydrate to fibre ratio is, who is using it and whether we should be too.
Before we delve into the pros and cons of the ratio, let’s start off with some definitions. Carbohydrate is one of the three main macronutrients (alongside fat and protein) and is the only nutrient which directly influences blood glucose levels. There are two types of carbohydrate that provide dietary energy; sugars and starch.
Dietary fibre is also a type of carbohydrate, almost exclusively plant-based. Unlike starches and sugars, fibre is not digested in the small intestine and is often referred to as indigestible carbohydrate. We are recommended to have approximately 30g per day, with many of us eating below this (average is approximately 18g). High fibre foods are defined as those which have over 6g per 100g.
Carb to Fibre Ratio
The carbohydrate to fibre ratio is based on the idea that higher-fibre carbohydrate food has a lower glycaemic index and/or glycaemic load and tend to slow down the absorption of sugar and insulin spikes, compared to food with the same carbohydrate content with low fibre.
It is an easy equation to calculate, simply divide the total carbohydrate in grams by fibre (in grams). The smaller, the better (as it indicates a food with low carbohydrate but high fibre). There are, however, a few things to be aware of when calculating this ratio.
There can be some confusion when calculating this ratio between the United Kingdom and the United States. Using a nutrition label from the United States, it is the total carbohydrate which is stated on the label. Here in the UK however, nutrition labels have already subtracted fibre so total carbohydrate is presented as those which provide energy (sugars and starch; also known as net carbs). Therefore, if you are in the UK and trying to calculate the carbohydrate to fibre ratio, they may differ from results in the United States. To have consistency with research from the United States you would need to first add fibre to total carbohydrate (presented on the nutrition label) and then divide by fibre.
Recent research from the Harvard School of Public Health used a carb to fibre ratio to identify a whole grain food. The research was based on the American Heart Association 2020 Impact Goals and stated that a product should have at least one gram of fibre to every 10 grams of total carbohydrate. This is the ratio of fibre to carbohydrate in an actual whole grain (unprocessed wheat). To calculate the 10:1 ratio, just divide the total grams of carbs (if in the UK, plus fibre) by 10. If the grams of fibre is at least the answer, the product meets the 10:1 ratio.
Others suggest that the ideal ratio for cereal carbohydrate to fibre should be 5:1. However, there seems to be limited evidence for this.
Initial research by Gross et al. (2004) suggested that the increase in carbohydrate and decrease in fibre over the past forty years is associated with the Type 2 diabetes epidemic in the United States. After this association was highlighted, further research has looked into the carbohydrate to fibre ratio.
The ratio is used to help consumers identify products which are based on refined carbohydrates and often heavily processed. After evaluating 545 grain products in the US, the Harvard School of Public Health found that foods which met the 10:1 ratio also tended to have less sugar and trans fats than those which didn’t. Both of which have been shown to have negative implications on health.
Small (n=164) cross-sectional research suggests that carbohydrate to fibre ratio is closely associated with various clinical markers, including triglycerides and high-density lipoprotein cholesterol (however, the association was not strong) and with the metabolic syndrome. Further cross-sectional research suggested that a low carbohydrate to fibre ratio intake showed a significantly lower mean body weight and lean body mass, as well as lower mean fasting serum insulin levels, HOMA-IR and HbA1c.
A large prospective cohort study (n=70,025), showed the associated between carbohydrate and total fibre to be borderline significant. This was suggested it could have been due to nearly 85% of the study population having a carbohydrate to fibre ratio <10:1 at baseline.
So overall some research has indicated there are clinical improvements associated with the carbohydrate to fibre ratio, however, one of the main advantages of using the carbohydrate to fibre ratio rather than other classifications (such as glycaemic index) is its simplicity. It can be identified with an easy division which can be calculated by consumers simply looking at the nutritional information table.
A limitation of the carbohydrate to fibre ratio is that the benefits seen have been demonstrated through observational studies, which does not result in the determination of causality. As well as this all research presented assessing carbohydrate and fibre intake used food frequency questionnaires (FFQ) to assess diet, and as we know there are already established limitations with the usage of these.
Another limitation of the carbohydrate to fibre ratio is that the majority of the current research seems to be focused on whole grains, not all carbohydrates. This seems to be because the majority of research between fibre and metabolic health focuses largely on starchy carbohydrates. So, we have to question if this ratio is appropriate to use for all carbohydrates?
If we use fruit as an example, the ratio still seems to give a good indication of the type to help you make a better choice, especially if we are thinking of glycaemic control. For example, a banana has a carbohydrate to fibre ratio of 15.3 (total carbohydrate 20g per 100g + fibre 1.4g = 21.4/1.4 = 15.3). This does not meet the 10:1 or 5:1 ratio or classification of a food high in fibre. Raspberries, on the other hand, have a ratio of 1.7 (total carbohydrate 4.6g per 100g + fibre 6.7g = 11.3/6.7 = 1.7). This suggests that further research needs to look at the application of the carbohydrate to fibre ratio to other food than whole grains and cereals.
Further research also needs to clarify the discrepancies between what the ideal ratio is, is it 10:1 or 5:1? Until these questions have been answered, it may only be useful to use the carbohydrate to fibre ratio for its simplicity rather than technicality.
There currently seems to be some uncertainty and discrepancies with the carbohydrate to fibre ratio. Until further research from credible sources have been published; either for carbohydrate or just whole grains, it may be best using the carbohydrate to fibre ratio just as a foundation. For further information on ranking systems for carbohydrates, read our blog on glycaemic index and glycaemic load. You can access it here.
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