Plant-based diets: Are they nutritionally complete?

Speaking at the recent conference Nutrition in Medicine: A Focus on Plant-based Nutrition event, Dr. Conor Kerley, lecturer at Dublin Institute of Technology and member of the Irish Nutrition and Dietetic Institute (INDI) explained what may be lacking in a plant-based diet, identified the nutritional concerns of pursuing such a diet, and outlined the nutrients we get in abundance form plant-based foods.

Speaking at the event Kerley advised that those switching to a plant-based based diet should focus on vegetables, fruit, legumes, whole grains.​

"Add some nuts and seeds, always ensure a steady, reliable course of vitamin B12," ​he said - adding that it would also be sensible to consider omega-3, vitamin D and iodine supplementation.

Nutrients lacking​

Kerley outlined four main nutrients as being lacking in plant-based diets: vitamins D, B12 and A, plus cholesterol.

With the exception of certain types of sun-dried or UV irradiated mushrooms, a vegan diet will be almost absent of vitamin D, explained Kerley.

Even in people who consume fish, eggs, fortified milks and cereals, deficiency can be commonplace, he added.

“You would need to eat a big portion of salmon every single day for the rest of your life,”​ said Kerley - noting that it is nearly impossible to achieve adequate vitamin D from dietary sources and that in the absence of sun during the winter, there is no real alternative to supplementation.

Vitamin B12 is found in abundance in meat but according to Kerley, there is “no reliable ‘natural’ plant source of vitamin B12.”​

Previous research has indicated deficiency rates of 50-60% of vegans in the US and UK.

“You can get vitamin B12 really easily from fortified foods and supplements,"​ he said - adding that deficiencies in vitamin B12 ​are "easy to prevent, but not so easy to correct."​

While the retinol form of vitamin A (found in animal liver and cod liver oil) is lacking in vegan diets, there is an abundance of the vitamin in the beta-carotene form in carrots, sweet potatoes and other vegetables, noted the expert.

Consumption of these vegetables can prevent overall deficiency, and has the benefit of avoiding retinol overload from supplements or excessive liver consumption by omnivores, Kerley added.  

The absence of cholesterol in plant-based diets is a benefit rather than a concern, suggested Kerley.

The association between high levels of LDL-cholesterol and heart disease, and the fact that frequent meat-eaters tend to have higher LDL-C level than vegans, reinforced this view, he said.

Other nutrients of concern​

According to the Irish expert there are several other micronutrients present in plant-based diets - but at possibly inadequate levels. These include calcium, iodine, iron, zinc, omega-3 fatty acids and protein, said Kerley.

The presence of calcium in green vegetables, pulses, soy products, dried fruits, as well as fortified plant ‘milks’ and cereals, meant that risk of deficiency among vegans is not a major concern, he said.

This view is supported by evidence that vegans may in-fact require lower dietary calcium to achieve adequate bone health, he added.

Iodine deficiency in vegans is a concern - especially in pre-, peri- and post-natal women - because of its importance for brain development and cognitive function in infants.

The major source of dietary iodine in the UK is currently from cow’s milk, said Kerley, who added that iodine supplements are 'the preferred solution.

Although other sources are iodised salt, and seaweed and kelp supplements, all other options can raise concerns over sodium intake and possible iodine toxicity from excessive amounts present in the latter, he suggested.

Kerley contrasted the types of iron derived from red meat (‘haem’-iron) and plant (‘non-haem-iron’) sources. 

He added that high haem-iron intakes have been associated with an increased risk of several cancers including pancreatic and lung in addition to an increased risk of type-2 diabetes and coronary heart disease.

Studies have found body iron stores in meat consumers significantly exceeding levels needed to maintain health, he said - adding that lower absorption levels using non-haem iron might therefore be beneficial.

As with iron, zinc absorption levels in vegetarians and vegans have been an area of focus in recent years. Although pulses, grains, nuts and seeds, are good sources of zinc, absorption levels can be hampered by the presence of phytates in some plant-based foods, he said.

Avoiding consumption of coffee and tea with foods and ensuring adequate vitamin C intake can improve absorption, added Kerley.

Long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFAs), EPA and DHA are often lower in vegans than non-vegans.

Although humans are able to produce LC omega-3 PUFAS from short-chain, conversion capability is limited.

Vegans should therefore consider supplementing with algal-based DHA and EPA, suggested Kerley.

Quantity and quality of protein intake in vegetarians and vegans has often been questioned. However, The American Dietetic Association has taken the view that protein needs can be met completely through an intake of an assortment of plants foods. Plant protein can meet requirements when a variety of plant foods is consumed and energy needs are met.

Research also indicates that an assortment of plant foods eaten over the course of a day can provide all essential amino acids and ensure adequate nitrogen retention and use in healthy adults.

Soy, quinoa and a number of legumes provide a wide variety of amino acids and may constitute complete or near-complete plant proteins, Kerley added. 

Abundant plant-based nutrients​

The rich antioxidant and phytochemical contents of plant-based foods have numerous health-benefits including cancer risk reduction, lower inflammation and reduced risk of other chronic diseases. The abundance of potassium, magnesium and dietary nitrate, may be of particular benefit in fighting the epidemic of high blood pressure, Kerley suggested.

The natural form of folate, prevalent in plant-based foods may be preferential to synthetic folic acid in pregnancy, while the dietary fibre content of plant-based foods promotes the multiple health benefits of maintaining a healthy microbiome, he added.