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All About… Legumes

ALL ABOUT LEGUMES

Legumes are an interesting food group, in that many of those who dismiss the rationale behind Paleo cannot conceive of why this perfectly humble selection of beans, pulses and peas (and peanuts) would be eliminated on a diet purporting to be healthy.

For us at Paleo in the UK, Legumes make up an interesting group quite simply because every compound that we have discussed as potentially ‘damaging’ or ‘toxic’ in other areas on this site – i.e. lectins, saponins, glycoalkaloids, phytates – can be found in legumes.

 

When looked at in terms of ‘nutrients vs. anti-nutrients’ therefore, Legumes fair incredibly poorly.

 

But first – let us take a step back:

 

The Hunter-Gatherer Diet Debate

 

Legumes are plants of a particular family – and this includes all beans and pulses. Sometimes classified as ‘seeds within a pod’, this definition is often confusing because we consume so many Legumes out of their pod… So to be clear, we are talking chickpeas, peanuts, soy, broad beans, peas, lentils, along with all the beans and pulses you can think of.

In order to be at all edible, most Legumes must be cooked, or at least soaked for a long time – possibly even sprouted. Heat renders the tough parts of the plant digestible… (just how digestible will be covered below). This is not strictly true, in that some Legumes have ‘edible pods’ (think green beans and peas). These Legumes will be more easily digested and can also be eaten raw. This is in drastic contrast to, for example, kidney beans which are positively toxic (i.e. literally life threatening) if not cooked properly.

The need for cooking and lengthy preparation methods suggests that Legumes – certainly those that are most ‘damaging’ to humans – were not consumed prior to the advent of the Agricultural Revolution. In fact, discussions of agriculture are apt here because Legumes contain nitrogen-fixing bacteria in their root nodules which actually makes them excellent when used as part of a crop rotation because they can replenish the soil. It is this use which has popularised the growing and harvesting of Legumes within modern farming.

All of this discussion of utilisation for farming, however, should illuminate the fact that these were hardly part of an ancient hunter-gatherer diet… and yet, this is actually relatively difficult to prove. Unlike more modern ‘invented’ or ‘cultivated’ foodstuffs, Legumes are actually pretty close today to their early forms. Whether hunter-gatherers consumed Legumes is akin to a discussion of the early consumption of Grains – it becomes a question of anthropological debate – and one that does not particularly lead to solid conclusions.

So beyond the arguments about the hunter-gatherer societies, what about modern Legumes and modern human digestive systems? Are these plants, renowned for being rich in fibre, carbohydrates, some proteins and other vital nutrients and vitamins, really suited to human digestion today?

 

Legumes’ Anti-Nutrients

 

Legumes are NOT Paleo (and certainly not AIP). This is because whilst it is true that they contain a spread of nutritional benefits (the above-mentioned fibre, carbohydrates, proteins and vitamins/minerals), they also contain not just one – but a handful – of anti-nutrients which have been proven to have adverse effects on the human gut, the microbiome, the intestinal barrier and the immune system.

 

  • Phytates are a primary compound within Legumes – and one of their main ‘anti-nutrients’. Phytates have become such a myth-laden topic within modern ‘nutritionism’ that we have dedicated a whole page to them, which you can read all about on our All About Phytates page.

 

  • Legumes are also rich in Lectins – and for all the information about the potential damage a toxic Lectin can cause, check out our All About Gluten Page under the heading ‘LECTINS’. (The Lectin in Legumes is an Agglutinin – and Wheat Germ Agglutinin is discussed at length in the context of wheat toxicity on the Gluten page.)

 

  • Then there’s Saponins, which you can read about on our All About Potatoes page which discussed Glycoalkaloids, a form of Saponin. However, we will detail more about Legumes, Saponins and the intestinal permeability potential below.

 

  • And, just for good measure, there’s Protease Inhibitors which effectively block the break down and assimilation of proteins. Legumes (along with grains and pseudo-grains such as couscous) are rich sources of these too.

 

 

The Saponin of Legumes

 

It is easy to remember the function of Saponins because they act almost like soap inside the human body. By this, we mean that they have a dissolving effect – though not (as would be customary for soap) on grease and dirt. In the case of Saponins in foods, these can dissolve the cell membrane of microbes and insects – and also the cell membranes of our digestive tract as these compounds reach the delicate intestinal wall.

The chemistry that makes Saponins soap-like is to do with the combination of both water-soluble and a fat-soluble molecule. Each cell membrane in our body is made using cholesterol – and the interaction of these molecules can create literal holes in the surface membranes of cells. Because the same occurs with microbes and insects we know that these Saponins are designed as part of the plant protection mechanism. Yet again, we discover that these ‘defence’ systems are actually anti-nutrients for humans – creating and perpetuating damage.

 

In changing the cholesterol molecules on the surface of gut cells (enterocytes) Saponins create a porous cell wall. In so doing, a variety of substances can get across into the cell. This sounds horrendous – and has led many advocates of Legume avoidance to castigate these plants for their potential to wreak havoc. And yet… At Paleo in the UK, we prefer a more balanced and practical view.

 

Obviously we’re not all full of holes – and not everyone suffers similarly with the consumption of Saponins (and, to be honest, we have several other anti-nutrients in Legumes as well and many people are fine eating them so this Saponin part can’t be completely catastrophic, can it?)

In truth it is believed that this detergent-like effect of Saponins on cells can be healed relatively quickly.

 

The question, of course – and with everything within the ‘anti-nutrient’ category – is one of degree, quantity and the current health status of the individual consuming these foods. Different Saponins have different levels of impact – creating smaller pores. In fact, some of this ‘detergent’ activity may actually be vital for enabling the absorption of nutrients from the Saponin-containing plants – without overtly damaging the enterocytes.

 

Larger pores, however, aren’t easy to fix. They can both allow more toxic substances into the cell itself, causing cell damage and death OR the cell that has been made porous will simply lose its ability to survive the insult and invasion and dies. Either way, Intestinal Permeability may be the potential result. One concern that many have about this whole process is that Legumes are notorious for being slow-digesting foods. Simply due to the way we digest and are able to process these compounds they do stay around within the digestive tract for longer. This means that there is the potential for more damage to be done to the enterocytes than with other anti-nutrients which are quicker to pass through the digestive system.

And whenever Intestinal Permeability is of concern, so too is the nutrient absorption potential of the enterocytes lining the gut. The health of each of these gut cells is essential to allow for maximum nutrient absorption. Anything which impairs this nutrient absorption is not considered optimal for human health, simply because it makes everything to do with being ‘healthy’ that much harder.

 

Beyond Anti-Nutrients… to Protease Inhibitors

 

Legumes are complex – and so if you’ve read about Lectins in wheat, All About Phytates, Saponins above and the Glycoalkaloids in Potatoes and you’ve also read about the Lectins in Nightshades then you’ll be completely clear as to why we at Paleo in the UK are in the “no-legume” camp – there’s just too many anti-nutrients to be considered optimal.

But there are more concerns when it comes to Legumes. These revolve less around potential damage to the cells of the digestive tract (as with all the plant compounds mentioned above) and are instead factors which literally affect the digestion of these Legume plants themselves.

As you may be able to interpret, given that these plants all seem to come with very potent anti-nutrients to damage anything (or anyone) who eats them – these plants really don’t want to be digested. As such, grains, pseudo-grains and Legumes all contain Protease Inhibitors which are essentially there to prevent their proteins from being digested at all. These Protease Inhibitors do as the name would suggest – they impair the function of the enzymes that break down the proteins in the plants into amino acids.

In fact, it is highly likely that were Protease Inhibitors not part of these plants the human digestive system would adequately cleave them up into toxins and amino acids and deal with each component incredibly effectively. As it is, the inhibition of Protease enzymes handicaps the human digestive system. Not only does it means we can’t break up the amino acid portions of these plants – but we also end up with a high proportion of a different digestive enzyme, Trypsin, in comparison to Protease. Trypsin is remarkably useful … but in excess (even just proportional excess – because everything about gut function is founded on concentration gradients) it can weaken the connections between the enterocytes of the gut… creating… increased Intestinal Permeability.

This, coupled with improperly broken down strings of amino acids, is a recipe for inflammation as a result of Legume consumption, increased immune activity and the triggering of a cascade of negative health consequences.

Legumes, therefore, are not eliminated because hunter-gatherers didn’t eat them. They’re eliminated because even in the healthiest of individuals there are so many complications with the consumption of Legumes that it is highly likely that the modern digestive system would be better not eating them.

 

Fibre, FODMAPs, Digestion and Wind

 

You won’t necessarily feel the impact of eating anti-nutrients and Protease Inhibitors. But everyone has had the experience of eating a meal heavy in legumes (beans, peas, pulses, peanuts) and experienced the fibrous effects of gas and sometimes bloating. Even the healthiest digestion can suffer from an overload of these foods.

What’s happening here is not to do with anti-nutrients – instead it is down to fermentation of the fibre inherent in Legumes themselves. Fibre proves a field-day for the microbiome – which is often a good thing as feeding the microbiome creates a wonderfully rich and diverse gut flora which is essential for overall health. When it comes to fibre, though, there is always the potential for fermentation – and digestive gas is the result.

 

We are, in no way, ruling legumes out because of their fibre content and the digestive disruption that comes from excess fibre. If anything, some in the Paleo-sphere campaign for fibre to be made an essential micronutrient – it really does carry an enormous amount of health benefits.

 

And yet, fibre can cause issues. However, the degree of bloating and gas created when ingesting fibre has a lot to do with the health of your GI tract – from the bacterial balance to quantity and diversity of the strains within your internal flora. This means that those with any form of dysbiosis or microbiota disruption are going to experience the negative effects of consuming Legumes. As digestion and microbiome health is regularly impaired for those with any health concerns whatsoever, the fermentability of Legumes (on top of their anti-nutrient content) make them an unwise choice for consumption.

But can these beans and pulses that many people have eaten for centuries really not be made digestible through any means whatsoever?

 

Sprouting, Soaking, Washing, Cooking – Can We Eat Legumes Differently?

 

Certain plant compounds can be rendered digestible by various methods of either soaking and sprouting, cooking, roasting, pulverising etc. Soaking actually does do quite a lot to alter the Phytate content of Legumes – but not the Lectin content. Cooking further diminishing some of the Lectins, but as is the case with Wheat Germ Agglutinin, certain Lectins cannot be degraded even at high temperatures. Sometimes fermentation is recommended, or consuming the young seedlings or shoots of things that become beans, or ‘sprouted’ beans…

All of these methods will perhaps minimise the potential harm that a Legume can cause – but they are never going to add to the nutrient profile of the Legume itself.

When it comes to fibre, protein, carbohydrate, vitamins and minerals there are, quite frankly, better sources of all of these essential ingredients of human nutrition. The Legume has become a staple because it is quick and easy to grow, simple to cook (i.e. not much effort or preparation involved) and these can also grow in fairly nutrient-poor soils – making it a great crop to help alleviate starvation in areas where droughts are common.

 

Being a cheap and easy food makes Legumes suitable for commercial mass production – but it does not in any way make Legumes a positive contribution to human health outcomes when there are other foods available. Much like candy in a time of famine – eating Legumes is unlikely to do immediate and irrevocable damage. And yet, when it comes to nutrient density and energy availability there are far healthier, far more nutrient-dense foods available. And for those with pre-existing health conditions, consuming relatively cheap Legumes may prove much more costly to health outcomes in the longterm.

References:

Gupta, Y. P., Antinutritional and toxic factors in food legumes: a review, Plant Foods Hum Nutr. 1987;37:201-228

Lajolo, F. M. And Genovese, M. I., Nutritional significance of lectins and enzyme inhibitors from legumes, J Agric Food Chef. 2001;50(22):6592-8

Shi, J., et al., Saponins from edible legumes: chemistry, processing, and health benefits, J Med Food. 2004 Spring;7(1):67-78

Tsai, C. Y., et al., Effect of soy saponin on the growth of human colon cancer cells, World J Gastroenterol. 2010;16(27):3371-6

Van Damme, E. J. M., et al., Handbook of Plant Lectins: Properties and Biomedical Applications, West Sussex: John Wiley and Sons, 1998.

Wnag, Q., et al., Identification of intact peanut lectin in peripheral venous blood, Lacet. 1998;352:1831-2

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