Dietary Lectin exclusion: The next big food trend?

Abstract

Until recently, with the exception of coeliac disease, gastroenterologists have not been particularly interested in the role of diet in the management of gastrointestinal disorders. However, patients have always felt that diet must play a part in their symptoms and, in the absence of any medical interest, have turned to alternative dietary practitioners for help, which can often have no evidence base. Fortunately, with the advent of the FODMAP diet (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) and the realisation that diet can have a profound effect on the microbiome, medical opinion is now changing. Nevertheless, research on the various diets that are now available is often completely lacking. Lectins are carbohydrate binding proteins which are widely distributed in nature and are found in a whole variety of commonly consumed foods. It seems likely that the exclusion of lectins from the diet could become the next “food fashion” for alternative practitioners to promote, especially as there is some evidence to suggest that certain lectins may be harmful to health. It is, therefore, the purpose of this viewpoint to try and stimulate research on the dietary effects of lectins, which is currently minimal, so that we can pre-empt a situation where we are unable to give patients or the public evidence based advice on this topic.

Key Words: Dietary lectins, Exclusion diets, Gastrointestinal system, Harm, Carbohydrate

Core tip: Patients with gastrointestinal problems, as well as the general public, are being offered an increasing number of different diets which claim to improve their health, often without any evidence to support a beneficial effect. Lectins are carbohydrate binding proteins which are found in many foods and some of them, such as those found in red kidney beans, can cause gastrointestinal symptoms if not cooked properly. Consequently, it is possible that a lectin exclusion diet could become fashionable in the future and research is needed to find out under what circumstances, if any, such a diet may be advisable.

INTRODUCTION

The majority of patients with gastrointestinal problems, especially those of a functional nature, consider that diet is important either in the cause of their symptoms or at least in their exacerbation. Unfortunately, until relatively recently the medical profession has largely ignored the role of diet in gastroenterology other than to advise patients to eat more fibre. However, in 1994 we showed that cereal fibre can actually exacerbate the symptoms of irritable bowel syndrome (IBS)[1]. In addition, we also found that fruit and vegetables could cause problems and assumed that this was likely to be as a result of their fibre content[1]. Despite these findings being published in the Lancet, this lack of interest in the contribution of diet to gastrointestinal health persisted with the void gradually being filled by alternative dietary practitioners as well as the marketing of a whole variety of tests for the detection of food allergies and intolerances. The proliferation of these alternative sources of advice coupled with the advent of the Internet may also partly explain why even healthy members of the general public have now become interested in the possible harmful effects of some dietary components. As a result of this, a bewildering array of diets are now fashionable.

GLUTEN

The exclusion of gluten is attracting particular attention. Obviously, the role of gluten in coeliac disease is beyond doubt but there is now interest in the possible effects of gluten in those individuals with the genetic predisposition to coeliac disease and even concepts such as non-coeliac gluten sensitivity[2]. Furthermore, large numbers of apparently healthy individuals are now adopting a gluten free diet[3]. Consequently, it is absolutely essential that the medical profession start undertaking good quality research on the role of gluten and other dietary components in health and disease so that the public can be given evidence based advice about their diet rather than having to trawl through questionable information on the Internet.

FODMAPs

Fortunately, the advent of the low FODMAP diet (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) for the treatment of IBS has at last galvanised interest in dietary interventions amongst the gastroenterology community and, hopefully, this will stimulate further research on other dietary interventions in both gastroenterology as well as other specialties. FODMAPs are carbohydrates that are poorly absorbed by the gut and are, therefore, fermented by bacteria. This leads to symptoms, especially in patients with IBS, possibly by exacerbating the gut hypersensitivity that exists in these patients[4]. It seems likely that the detrimental effect of fruit and vegetables on symptoms of IBS that we previously reported is more likely to be due to FODMAPs rather than fibre as we surmised, although a dual effect may be a possibility. FODMAPs are contained in cereals, fruits, vegetables, and legumes as well as milk products and there is now reasonably good evidence that FODMAP restriction in IBS can improve symptoms[5]. However, there are some concerns about whether this diet can lead to changes in the gut microbiota that may not be entirely desirable[5]. Given the sometimes dramatic reduction in symptoms that can be seen in patients on a low FODMAP diet it is possible that, in the future, it might be tempting for even healthy individuals to experiment with this approach.

LECTINS

Lectins are carbohydrate binding proteins that are widely distributed in nature and occur in a variety of foods such as cereals, fruit, vegetables, animal products and fish[6]. There is no universally accepted classification of lectins but they do have characteristics that differ from other proteins. Some lectins such as ricin, which is derived from the castor bean, are highly toxic with the ingestion of even miniscule quantities of ricin proving fatal whereas other lectins are relatively harmless[7]. Lectins, such as phytohaemagglutinin and concanavlin A, agglutinate red cells and act as lymphocyte mitogens with these properties having been used in the laboratory for many years. Much of the research on the results of consuming lectins is relatively old and largely confined to the effect of plant lectins on animals where, for instance, they can survive transit through the gut and have a variety of local and systemic effects[8-10]. Furthermore, in contrast to animal proteins, lectins are resistant to heat and even cooking can fail to inactivate them unless it is above 100 °C for as long as thirty minutes or more[11]. Animal studies have shown that lectins, which have an affinity for gut epithelium, can interfere with absorption of nutrients with these effects sometimes being called “anti-nutritional”[12,13]. For instance, phytohaemagglutinin, which is found in high concentrations in the red kidney bean, has a range of effects on the gut including decreased acid secretion, crypt hyperplasia, changes in the brush border and even an indirect effect on the pancreas and these effects on the gut are not confined to this particular lectin[10,14-16]. There is also evidence that some lectins may affect the gut microbiota as well as having systemic effects such as the modulation of inflammation and immune function[17,18]. It should be noted that these latter properties may not necessarily always be negative indicating that the therapeutic potential of some of these proteins might also be worth exploring.

Despite this evidence of the detrimental effects of lectins in animals, their potential to cause harm in humans has received surprisingly little scientific attention although “food poisoning” due to red kidney bean consumption has been reasonably well documented[19]. However, much more needs to be known about which lectins are harmful and the effects of dose and duration of consumption. It is also interesting to note that many of the foods that are excluded in the low FODMAP diet are those that also contain lectins. This raises the possibility that it may not just be the FODMAPs that are causing problems in those who benefit from their exclusion.

CONCLUSION

As a result of their potential for toxicity and their “anti-nutritional effects” it is almost inevitable that lectin exclusion could well become a big food fad[13]. Consequently, now is the time to resume research on this ubiquitous family of proteins so that we fully understand their role in health and disease. This would then enable us to advise our patients and the general public accordingly, rather than having to play “catch up” after everybody starts wondering whether they should be excluding some or adding others to their diet.