The Opposite of Good
Even before their sustainability benefits were widely spoken about, it was well known that eating more beans, pulses and legumes was something we should be doing for the good of our health. We know this because records show that back in 1988, a well-meaning hospital catering team cooked up a vegetarian dish based on kidney beans for its staff, as part of a lunchtime healthy eating event.
This is a matter of record because the event did not go quite as planned. At around 3pm that afternoon, a couple of hours after the lunch, one of the surgical registrars became violently ill in theatre. Before long, staff across the hospital were dropping like flies, suffering from projectile vomiting and explosive diarrhoea. The onset was so rapid that nurses and doctors were throwing up whilst still on the wards, plunging the hospital into crisis.
Although everyone affected had fully recovered within 24 hours, probably helped by becoming ill in exactly the right place, this minor news story stuck with me over the years. Occasionally repeated for its ironic drama, it represents a recurring nightmare shared by all well-meaning chefs looking to innovate, and a reminder that there are occasionally hidden dangers, even in the most benign seeming foods.
In the investigation that followed, all common causes of food poisoning were ruled out. It was only when the kidney beans were analysed, and abnormally high levels of the plant lectin phytohaemagglutinin (PHA) were found, that the mystery was solved. PHA, a protein that binds to glycan receptors in the small intestine, can form large glycoconjugates (carbohydrates bound to other biological compounds) that the body really wants to eject. PHA would usually be inactivated by adequate cooking of dried kidney beans, but in this unusual case, it appears that the beans were cooked at a low temperature for a long time, without the rapid boil stage important in inactivating the toxin. Ever since, kitchen risk assessments across the UK have contained expanded sections on dried bean and legume preparation, and sales of catering sized tins of kidney beans (precooked and free from PHA) have grown enormously.
Lectins in beans can be dangerous, but cases of poisoning are extremely rare, with about 5 incidents per year in the UK between 1979 and 1989 and no fatalities. But such tales are an important reminder that even the healthiest seeming plant foods can carry huge risks. As we transition to more plant-based diets over the next few years, we are going to be developing and eating many new ingredients, concentrating proteins and other nutrients in novel ways. Up to 10% of the proteins in legumes are lectins, and as we concentrate them as ingredients for plant-based meat and dairy replacements, it is important that we take care not to cause harm.
Crucially, it is perhaps not as simple as making sure our new food products don’t result in projectile vomiting. Plants contain a variety of substances, sometimes classed as anti-nutrients, that can impact on digestion. Lectins affect carbohydrate digestion, but some studies show that they can also impact on absorption of essential minerals by binding to cells in the digestive tract. Oxalates, a class of substances found in many leafy vegetables, can mess with calcium absorption. Phytates, a group of substances found in most plant foods, can impair absorption of calcium, iron and zinc.
There are many more. The goitrogens found in brassicas can interfere with normal thyroid function. Tannins, present in almost all plant foods, can reduce absorption of iron and other minerals. And trypsin inhibitors, often found in legume-based drinks such as soya milk, can interfere with protein absorption.
Although most of these are not likely to cause explosive diarrhoea or vomiting as part of a normal diet, there may well be more creeping harms if we allow them into our food system unchecked. Reduced mineral or protein absorption can have devastating consequences. This is especially the case in developing countries, but also of concern in the UK, especially in women, teenage girls and the elderly. Iron, iodine, calcium and selenium deficiency is already rife in many segments of the population. As we rapidly transition to more plant-based diets, often concentrating down proteins and extracts to form more convenient products, we need to be cautious that we do not cause slow, creeping harm. Unlike the effects of undercooked kidney beans, mineral and protein deficiencies may take years to show up in the population, causing a good deal of suffering before they do.
When I was a restaurant chef, all I had to worry about in my risk assessments was salmonella, campylobacter and the occasional sliced finger. But now, working in large scale manufacturing and focused on the transition to more plant-based diets, there are things I never thought I would need to consider. In developing new plant based ingredients, and transitioning more and more people away from meat and dairy, are we storing up harm?
The conventional answer is, of course, to strip out and inactivate these substances as much as possible, using a selection of processing techniques. Heat application is effective in inactivating lectins. Oxalates, goitrogens, tannins and trypsin inhibitors are also inactivated by most conventional cooking temperatures. Phytates are a little trickier, requiring higher levels of heat up to 150C, but can also be inactivated by phytase enzymes.
It can also help to physically sort plant ingredients in advance of heat or enzyme treatment, for instance by removing the skin or outer segments of legumes where most tannins are contained. Fermenting or sprouting grains and legumes can also reduce anti-nutrient levels. In a few rare cases, it may be necessary to supplement products with minerals to help compensate for the anti-nutrient effects.
Thankfully, this means that we are headed for a modern food system where all the harmful anti-nutrients can be stripped away, leaving only delicious, healthy plant proteins and nothing else. Perfect. Well done science and industry. Progress in action.
But there’s a problem. As usual, this overly simplistic narrative of harmful plant anti-nutrients, familiar to anyone who has encountered the low carb carnivore diet space, is potentially harmful. Because even though it is easy to flash a bit of cherry picked ‘evidence’ showing that ‘anti-nutrients’ such as tannins, lectins, phytates, goitrogens and oxalates are modern day toxic threats, that’s not the whole picture. The term ‘anti-nutrient’ is perhaps one of the most unhelpful in the whole nutrition space, something illustrated by the fact that the term also encompasses plant fibre, known to have an array of health benefits. Many of the substances contained in plant-based ingredients that we are working so hard to strip away and discard may in fact be of great value.
Even lectins, famous for their rapid onset poisoning, are thought to have positive health impacts in small quantities, helping with cell adhesion and growth. Although phytates bind to useful minerals preventing absorption, they are also thought to have strong antioxidant activity, anti-cancer properties, anti-cardiovascular disease properties and may potentially help prevent kidney stone formation. Goitrogens are thought to have useful anti-cancer and detoxification properties. And tannins are thought to have an array of potential health benefits, with known detoxification, anti-oxidant, immunomodulatory and cardio-protective effects.
Fruit and vegetable consumption is well known to have a positive impact on health, yet the mechanistic reasons for this are complex and poorly understood. It is not just a case of them having a bit of fibre and a low energy density. Plant foods contain an array of useful substances that can improve our wellbeing. But these effects are complex, and some of the useful substances may have negative impacts if consumption gets too high. We need, as ever, a bit of balance.
And that’s why the only sensible way to navigate anti-nutrients as part of a normal life is to forget about them. Forget you even came across this blog post. Try and eat a wide variety of fruits and vegetables, without idolising certain ingredients or compounds. There is overwhelming evidence that doing this has a positive impact on health, but the reality is that we may never know exactly why. It is likely that many of the complex array of substances in plant foods, several of which are unhelpfully termed anti-nutrients, are good for us. But the best way to benefit is by eating a varied diet with lots of vegetables, wholegrains and legumes. That’s the same boring message people like me have been shouting about for years.
When people eat diets largely composed of highly refined foods, made from plant sugars, fats or starches that have been stripped of many of these ‘anti-nutrient’ compounds, they tend to have poorer health outcomes. Although correlation is not causation, this is surely something worthy of consideration. The benefits of a plant-based diet to not extend to people who only eat Frazzles and sugar cubes.
So, as we transition to more plant protein in our diets, we should probably keep this in mind. It often makes sense to remove some antinutrients, especially if we are preparing kidney beans, but we should perhaps worry about a drive to remove them all. Currently, little attention is paid to what we remove when we concentrate the large amounts of soya, pea and fava bean used in plant-based burgers and sausages. Although left-over starch, oil and fibre does get recycled back into the food system, in removing ‘anti-nutrients’, are we discarding huge quantities of the most healthful plant ingredients of all? In banishing tannins, phytates, oxalates and lectins, are we throwing precious metals onto the slag heap, in search of refined iron?
The food system is changing rapidly as we transition diets from animal to plant protein. This is likely to be as dramatic a shift as the changes seen in the late nineteenth century, when many of our modern food processing techniques were established. This included the large-scale refining of starches, oils and sugars, and the wholesale removal of fibre, which is, probably rightly, blamed for many of the issues seen within our current food system. We must be careful not to repeat the mistakes of the past, over refining for function and palatability, without considering broader the broader implications for health and nutrition.
With our increased understanding, we may one day be able to separate some protein and mineral consumption from anti-nutrient uptake, allowing better digestion, but retaining the broader health benefits. But we should also remember that as much as we can, we should be aiming for less processing and refining, using our technology and skill to make delicious food without all its complexity stripped away.
