Comment: Dig a bit deeper and you'll find that some super foods aren't that super - but don't go blaming farmers, writes Dr Jacqueline Rowarth.

Super foods appear regularly in articles on "how to be healthy". Lists of the "top 10 (or however many) super foods for the year" over the last few years have promoted such foods as beetroot, broccoli, cauliflower, kale, turmeric and tiger nuts.

Beetroot and kale have considerable quantities of nitrate and are linked to heart health (though you do have to eat quite a lot).

Broccoli and cauliflower are linked to glucosilonates which have in turn been linked to anti-cancer (but again, there are big questions about quantity and effectiveness), and turmeric has been the subject of considerable debate, again because of dose.

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Tiger nuts have attracted attention rather more, one suspects, because of the name than anything else.

They are an underground storage organ and though they can make a gluten-free flour, so can many other sources.

Despite the hype about being a super food the nutrient profile is similar to other tubers like potato. And the health benefits were debunked last year by medical doctors concerned about the claims being made. In an article on Healthybutsmart.com (the website of a social enterprise committed to helping people make evidence-based decisions on matters of health), 19 scientific studies were examined, and nine claims reviewed.

The conclusions were as follows:

• It is a more expensive source of fibre than regular bran

• It is marketed as being rich in magnesium, potassium, and oleic acid, but we don't need to supplement any of these.

• It poses a very significant risk of infection.

• It has been shown to have aphrodisiac potential in rodents, but this is unproven in humans. It has also been shown to be contaminated with Fifty Shades of Faeces. The very thought is an anti-aphrodisiac.

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Moving on from tiger nuts and super foods (the term has been prohibited since 2007 in the EU unless accompanied by credible scientific evidence), more recently, nutrient dense foods have been promoted.

The basic concept is the same - some foods have more nutrients in them per calorie or kg than others, so by eating those with dense nutrients you will be able to eat less and be healthier.

This is all part of the wellness and "putting back what modern-day life has taken from you" health approach.

In the case of food, the conspiracists imply that modern day farming has robbed the consumer of nutrients, though quite why they would do that is something of a mystery: farmers also eat modern-day food.

It is also important to remember that no natural food product contains all essential nutrients and nutrient density does not give information on which ones are missing.

Dr Jacqueline Rowarth. Photo / Supplied
Dr Jacqueline Rowarth. Photo / Supplied

This means that even a diet incorporating lots of high-density foods could still be lacking in several essential nutrients.

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This doesn't mean that farmers and growers have somehow reduced the nutrient quality of food.

Food on the farm, orchard or market-garden has a biological purpose connected with growth and reproduction.

Milk is the perfect food for the young of the species (and changes during lactation according to the stage of the progeny).

Meat is muscle and moves the animal around. Lettuce leaves are green and spread for capturing the sun's energy and photosynthesising. Peas and corn contain the nutrients required for the new seedling.

Good growing conditions (which include temperature, moisture and available nutrients) mean more milk, heavier animals more quickly, lettuces reaching harvest in a shorter time and more peas and corn in the pod or on the cob.

There have been some minor changes in nutrient content during the years, but these have been found to be very small when the analyses are done on "products" from the same season in the same region.

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Listen to Jamie Mackay interview Dr Jacqueline Rowarth on The Country below:

Cultivar, season and geographical location make the difference, and cultivars have been bred over the last few decades to be crisper, sweeter, juicier - and give more choice.

New Zealand's green and gold kiwifruit, and now the types that have different sizes and colours, are a terrific example.

There is evidence to suggest that increased carbon dioxide in the air has resulted in an increased carbohydrate content in plants without increasing mineral nutrient content.

This applies to all plants whatever the production system and the change is not significant within the context of modern diets.

Meeting consumer demand for price has also resulted in selection for yield and post-harvest quality.

This has enabled food prices to increase more slowly than income (so that food is now a lower proportion of household income than in the past) and to stay fresh (in the case of fruit and vegetables) for longer than in the past, reducing waste.

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The real problem with healthy food choices is not what the farmers and growers do, but the decisions made in the supermarket and fast food outlets. The addition of sugar to oats (biscuits and bars) and oil to potatoes (chips) changes the nutrient density.

Debates about how wellness can be achieved are likely to continue, but the solution doesn't lie in modern farming.

And it certainly doesn't lie in tiger nuts with the affiliated contamination (when bought at the roadside) – safe food is another benefit of modern farming.

• Dr Jacqueline Rowarth CNZM CRSNZ HFNZIAHS has held Professorial positions in Pastoral Agriculture (Massey University) and Agribusiness (University of Waikato). She is a farmer-elected director on the boards of DairyNZ and Ravensdown. The analysis and conclusions above are her own. jsrowarth@gmail.com