Tuesday, March 18, 2014

Lectins and health - a review


Have you heard of lectins? They are a type of protein found in small amounts in over 30% of plant foods including grains and legumes. While they are not well known, popular fad diets including the Paleolithic diet (Paleo diet) cite lectins as a main reason to exclude grains and legumes from the diet. As the topic of lectins is making headlines in the Paleo world, GLNC has reviewed the science on the role of lectins and lectin containing foods within a balanced diet. 

Background
The Paleo diet is an eating plan which is intended to closely reflect that of our hunter gather ancestors – GLNC’s Position Statement on the Paleo Diet can be found here. A quick Google search on lectins, directs you to the websites, and blogs of prominent Paleo’s (avid Paleo dieters)  from around the world. Paleo’s describe lectins as toxic and destructive anti-nutrients which damage the intestine, cause a leaky gut leading to obesity  and diseases (1, 2). The presence of lectins in grains and legumes is one factor which underpins the Paleo recommendation to exclude these foods. However, examination of the scientific evidence as a whole indicates there are flaws in this argument.

The science doesn’t stack up
The beliefs which underpin the avoidance of lectin containing grains and legumes are based on studies which have limited generalisability to humans. The studies used to support the argument against lectins were mostly conducted in animals, using doses of purified lectins which could not be achieved through foods alone and some studies even injected the lectins directly into the bloodstream.(3-13) In the studies which observed negative effects in humans, they involved people with pre-existing intestinal damage and/or the participants of the study ate foods which were not adequately prepared for human consumption i.e. unsoaked legumes.(14, 15)

It is inaccurate and misleading to apply the results of such studies to humans, let alone use them to support the dietary recommendations for large populations. Upon reviewing the broader science on lectins, including a comprehensive review of the literature published last month, there is no convincing evidence to indicate that lectins cause harm. The claims made by proponents of paleo diets to avoid grains and legumes due to these foods continaing lectins are not supported by the scientific eveidence. (3, 16)

Interestingly, there is some evidence that low doses of dietary lectins, consumed within adequately prepared foods may in fact have health benefits including anti-cancer effects and assist weight management by helping to promote a feeling of fullness. (17, 18)

Grains and legumes promote health
In contrast to claims to avoid grains and legumes, there is a significant body of evidence, including large population studies (in humans) which indicate that higher intakes of grains, mostly whole grain and high fibre are associated with improved nutrition and a reduced risk of a disease (19, 20) – including cardiovascular disease, obesity, type 2 diabetes and certain cancers. Comprehensive reviews of scientific trials also demonstrate legumes have beneficial effects on the markers of chronic disease such as blood pressure, cholesterol levels and blood glucose control.(21-23)

Grains and legumes provide a range of essential nutrients including carbohydrates, protein, vitamins, minerals as well as dietary fibre and phytonutrients. In support of the Australia Dietary Guidelines and in line with the evidence on the nutrition and health effects of grains and legumes, the Grains and Legumes Nutrition Council (GLNC) recommends that Australians should enjoy grain foods 3 – 4 times a day, choosing at least half as whole grain or high fibre grain foods, and enjoy legumes at least 2 – 3 times each week. 

To view the complete scientific topic summary prepared by GLNC contact GLNC via email: contactus@glnc.org.au or phone: 1300 GRAINS

 References: 

  1. Sisson M. The Lowdown On Lectins 2013 [January 2014]. Available from: http://www.marksdailyapple.com/lectins/#axzz2qKZy6KHQ
  2. Cordain L, Toohey L, Smith MJ, Hickey MS. Modulation of immune function by dietary lectins in rheumatoid arthritis. The British journal of nutrition. 2000;83(3):207-17.
  3. Pusztai A, Bardocz S. Biological effects of plant lectins on the gastrointestinal tract: metabolic consequences and applications. Trends in glycoscience and glycotechnology. 1996;8:149-66.
  4. Liener IE. Nutritional significance of lectins in the diet. In: Liener IE, Sharon, N., Goldstein, I.J., editor. The lectins: properties, functions, and applications in biology and medicine. Orlando, Florida: Academic Press; 1986.
  5. Vasconcelos IM, Oliveira JTA. Antinutritional properties of plant lectins. Toxicon. 2004;44(4):385-403.
  6. Pusztai A, Ewen S, Grant G, Brown D, Stewart J, Peumans W, et al. Antinutritive effects of wheat-germ agglutinin and other N-acetylglucosamine-specific lectins. British Journal of Nutrition. 1993;70(01):313-21.
  7. Lam SK, Ng TB. Lectins: production and practical applications. Applied microbiology and biotechnology. 2011;89(1):45-55.
  8. Mishkind M, Keegstra K, Palevitz BA. Distribution of wheat germ agglutinin in young wheat plants. Plant physiology. 1980;66(5):950-5.
  9. de Moya CC, Grant G, Fruhbeck G, Urdaneta E, Garcia M, Marzo F, et al. Local (gut) and systemic metabolism of rats is altered by consumption of raw bean (Phaseolus vulgaris L. var. athropurpurea). British Journal of Nutrition. 2003;89(3):311-8.
  10. Hamid R, Masood A. Dietary lectins as disease causing toxicants. Pakistan Journal of Nutrition. 2009;8(3):293-303.
  11. Pusztai A. Dietary lectins are metabolic signals for the gut and modulate immune and hormone functions. European journal of clinical nutrition. 1993;47(10):691-9.
  12. Kilpatrick D. Immunological aspects of the potential role of dietary carbohydrates and lectins in human health. Eur J Nutr. 1999;38(3):107-17.
  13. Banwell J, Howard R, Kabir I, Costerton J. Bacterial overgrowth by indigenous microflora in the phytohemagglutinin-fed rat. Canadian journal of microbiology. 1988;34(8):1009-13.
  14. Miyake K, Tanaka T, McNeil PL. Lectin-Based Food Poisoning: A New Mechanism of Protein Toxicity. PloS one. 2007;2(8):e687.
  15. Rodhouse J, Haugh C, Roberts D, Gilbert R. Red kidney bean poisoning in the UK: an analysis of 50 suspected incidents between 1976 and 1989. Epidemiology and infection London, New York NY. 1990;105(3):485-91.
  16. van Buul VB, F.J.P.H. Health effects of wheat lectins: a review. Journal of Cereal Science. 2014.
  17. De Mejía EG, Prisecaru VI. Lectins as bioactive plant proteins: a potential in cancer treatment. Critical reviews in food science and nutrition. 2005;45(6):425-45.
  18. Marinangeli CP, Jones PJ. Pulse grain consumption and obesity: effects on energy expenditure, substrate oxidation, body composition, fat deposition and satiety. British Journal of Nutrition. 2012;108(1):46-51.
  19. Council GLN. What’s to Gain from Grains? An update of the scientific evidence 2013.
  20. Council GLN. Lifting the Lid on Legumes. A guide to the benefits of legumes. 2013.
  21. Jayalath VH, de Souza RJ, Sievenpiper JL, Ha V, Chiavaroli L, Mirrahimi A, et al. Effect of Dietary Pulses on Blood Pressure: A Systematic Review and Meta-analysis of Controlled Feeding Trials. American Journal of Hypertension. 2014;27(1):56-64.
  22. Bazzano LA, Thompson AM, Tees MT, Nguyen CH, Winham DM. Non-soy legume consumption lowers cholesterol levels: A meta-analysis of randomized controlled trials. Nutrition, metabolism, and cardiovascular diseases : NMCD. 2011;21(2):94-103.
  23. Sievenpiper JL, Kendall CW, Esfahani A, Wong JM, Carleton AJ, Jiang HY, et al. Effect of non-oil-seed pulses on glycaemic control: a systematic review and meta-analysis of randomised controlled experimental trials in people with and without diabetes. Diabetologia. 2009;52(8):1479-95.
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Lentils reverse damaging effects of high blood pressure

Large population studies indicate that people who eat beans and soy foods at least four times a week are less likely to develop heart disease.1 It was thought that legumes had this effect due to their ability to reduce LDL cholesterol.2 However, a recent study in animals has brought a new idea to light.

A meta analysis of intervention studies on humans published this month indicates eating legumes significantly lowers blood pressure in people with high blood pressure as well as in people with normal blood pressure.3 The analysis of eight different trials found that when an average of one cup of legumes, including faba beans, lentils, chickpeas and beans, was substituted for foods of the same energy value there was a 2.25 mmHg reduction in blood pressure. A reduction of 2 mmHg is clinically relevant for lifestyle modification given that similar reductions are achieved with a low salt diet (4-5mmHg).4

A Canadian study in animals has found a new clue that may explain just how legumes are having this effect on blood pressure. The study, conducted in rats, showed that adding lentils to the diet can effectively block the increase in blood pressure that occurs with age. The findings also indicate that eating lentils can reverse the changes that occur in blood vessels as a result of high blood pressure.5

According to lead investigator Dr. Peter Zahradka, "These are amazing results, since they provide a non-pharmacological way of treating diseases associated with blood vessel dysfunction."

The investigation was undertaken after results from a clinical trial in 2013 indicated eating ½ cup a day of legumes, specifically a mixture of beans, peas, lentils and chickpeas, can improve blood flow to the legs of persons with peripheral artery disease, a condition that is closely linked to heart disease and diseases of the blood vessels.6

"The most notable finding of the latest study was the fact that lentils could alter the physical properties of blood vessels so that they resembled the vessels found in healthy animals," says
Dr. Zahradka. While human studies will still be needed to confirm these findings it does provide an interesting insight into how legumes may help prevent heart disease.

The Grains & Legumes Nutrition Council (GLNC) has commissioned the University of Wollongong to analyse data from a large Australian-based study to find out if there is a link between eating legumes and risk of heart disease in an Australian population. Results are expected later this year.

GLNC recommends Australians enjoy legumes such as lentils at least 2 - 3 times every week. Both dried and canned legumes are great additions to many meals. To reduce the salt in canned legumes, drain the canning liquid and rinse the legumes well.

For meal and snack ideas using legumes visit www.glnc.org.au

References:
  1. Bazzano LA et al. Legume Consumption and Risk of Coronary Heart Disease in US Men and Women. Arch Internal Medicine. 2001;161(21):2573-8
  2.  Bazzano LA et al. Non-soy legume consumption lowers cholesterol levels: A meta-analysis of randomized controlled trials. Nutrition, Metabolism and Cardiovascular Diseases. 2011;21(2):94-103
  3. Jayalath VH, et al. Effect of Dietary Pulses on Blood Pressure: A Systematic Review and Meta-analysis of Controlled Feeding Trial. American Journal of Hypertension. 2014;27(1):56-64
  4. Heart Foundation of Australia. Guide to management of hypertension, 2009.
  5. Hanson MG, Zahradka P, Taylor CG. Lentil-based diets attenuate hypertension and large-artery remodelling in spontaneously hypertensive rats. British Journal of Nutrition. DOI: http://dx.doi.org/10.1017/S0007114513002997
  6. Zahradka P et al. Daily non-soy legume consumption reverses vascular impairment due to peripheral artery disease. Atherosclerosis. 2013;230(2): 310–314
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