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A summary of evidence on the digestion, absorption and metabolism of white bread carbohydrates

Research Review No. 88

A summary of evidence on the digestion, absorption and metabolism of white bread carbohydrates

by

Ros Miller
Sara Stanner

British Nutrition Foundation, Imperial House 6th Floor, London WC2B 6UN

 

Abstract

Carbohydrates are an important source of dietary energy. In the UK, the proportion of energy derived from carbohydrates is close to the national dietary reference value (around 50% of total dietary intake). Carbohydrates are a relatively diverse group of compounds, classified according to molecular size and individual monomer units present, both of which can determine the site and rate of digestion and blood glucose response. Bread is rich in complex carbohydrates, particularly starch which is predominantly digested in the small intestine where it is broken down to its constituent glucose monosaccharide units. The rate of starch digestion mainly depends on the structure of the starch granules (ratio of amylose and amylopectin polysaccharides, protein and lipid content) and processing techniques (e.g. milling, refining and cooking). Bread made with refined, high amylopectin, low protein and/or low lipid wheat flour and baked to achieve an open crumb and thick crust is likely to result in most rapid starch digestion. Factors intrinsic to the consumer (e.g. degree of mastication, salivary α-amylase production and digestive transit time) and meal composition (e.g. protein, fat and fibre content of foods eaten at the same time or in previous meal) can also affect starch digestion and glucose absorption.

The glycaemic index (GI) is a measure of the rise in blood glucose after eating a specific food. Carbohydrate in a low-GI food is digested and absorbed at a slower rate than carbohydrate from a high-GI food, although there is large variation in glycaemic responses between and within individuals. White bread, as well as brown and wholemeal bread, is generally classified as a high-GI food due to the highly gelatinised starch it contains, low fibre content and porous physical structure, which is easily broken down during digestion. However, the GI can vary depending on the raw ingredients, processing method and what it is consumed with. Granary bread and some white breads (e.g. sourdough and pitta bread) have a ‘medium’ or ‘low’ GI rating. The GI may be reduced by the addition of fibre (e.g. intact grains or viscous soluble fibres – although the fibre type, dose and processing method appear to be important in terms of effect size), fat (e.g. olive oil) or with the presence of organic acids (e.g. from sourdough fermentation). Bread is commonly eaten with other foods (e.g. fat spreads, cheese and meats), which can reduce the glycaemic response to bread. A food with a low-GI is not always a healthier choice as low-GI foods can be high in fat and energy.

The original aim of classifying foods according to GI was to help improve glycaemic control in individuals living with diabetes. In healthy individuals, blood glucose concentrations are homeostatically controlled within a fairly narrow range. After a carbohydrate-containing meal, there is a very small increase in blood glucose in healthy individuals, with levels returning back to baseline after a couple of hours. Nevertheless, there is some evidence to suggest that glycaemic excursions within the normal physiological range may temporarily increase oxidative stress which could have an impact on the inflammatory response and blood vessel elasticity. In addition, upon review of the evidence, the Scientific Advisory Committee on Nutrition (SACN) found high-GI diets to be associated with an increased incidence of type 2 diabetes. However, this does not indicate causality and other factors (e.g. low fibre diet) may be responsible for this finding.

A number of acute studies of varying quality and design have investigated the effect of GI on satiety and appetite control. Over half have reported an inverse association between GI and satiety, with significant differences being reported for subjective satiety and hunger ratings and/or objective energy intake at a subsequent meal. However, a systematic review looking specifically at the effect of low- vs. high-GI breakfast meals failed to find a significant effect on subsequent energy intake, and upon reviewing the evidence on GI and appetite control, SACN also found no significant effect. Evidence to support a long-term impact on GI and appetite control (i.e. weight loss or maintenance) is also lacking, although one high-quality clinical study has shown a beneficial effect of a low-GI diet on weight maintenance. Low-GI foods are often higher in fibre and disentangling the potential effect of GI with that of increased fibre content is difficult. Furthermore, it has been hypothesised that the effect of low-GI foods/diets on appetite control observed in some studies may be underpinned by the fibre content of the food/diet rather than the glycaemic response. Fibre may help to increase satiety rating via metabolic signals sent between the gut and the brain, such as those transmitted by stretch receptors in the stomach (which sense physical fullness), gut hormones and short-chain fatty acids produced during fibre fermentation in the gut.

A number of studies have indicated that wholegrain bread (which is higher in fibre) is more satiating than white bread and adding fibre-containing flours or ingredients to white bread may increase satiety ratings (dependent on fibre type, dose and format). However, there is a lack of long-term studies investigating the effect of satiety-enhancing bread on long-term energy intake and body weight. Standard white wheat bread is commonly perceived amongst consumers to be associated with weight gain. However, the evidence to support this perception is somewhat limited. Most observational cohort studies indicate a possible positive association between white bread consumption and abdominal fat. However, it is difficult to determine from these studies whether it is the white bread per se causing the effect or other foods or behaviours associated with intake of white bread (e.g. low intake of fruit and veg, other high fibre foods and higher intake of energy-dense, high-fat foods).

Food-based dietary guidelines in the UK have recently been updated in light of the recent recommendations of SACN’s Carbohydrate and Health report (no more than 5% of dietary energy as free sugars for those aged over 2 years and an increase to 30 g fibre a day for adults). Updates include an increase in the starchy carbohydrate segment in the refreshed Eatwell Guide from 33% to 38% and greater focus on wholegrain and high-fibre foods. Globally, starchy carbohydrates are recognised as the cornerstone of the diet and most countries promote the consumption of wholegrains.

To conclude, research on the health impact of white bread is relatively limited. There may be a health benefit to consumers in selecting lower-GI options within a food category, such as wholegrain rather than white bread. However, the associations highlighted in the scientific literature between low-GI diets and health (e.g. reduction in risk of type 2 diabetes and weight maintenance) could be driven by other dietary and lifestyle factors, such as the fibre content of the diet. Both the GI and satiety rating of white bread appears to depend on the raw ingredients and processing method with improvements being particularly noted with the addition of specific fibres. Further research investigating the effect of incorporating different ingredients into bread on GI and satiety is currently underway and will help to increase our understanding of this topic. It is possible that satiety-enhancing breads, in combination with other approaches, could aid weight loss or weight maintenance but further long-term studies would be required to substantiate any health claims in this area.

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