Declining Nutrition

The Shrinking Nutrient Content of Food

You look at your watch: it’s approaching a quarter past twelve and your stomach is beginning to growl. As you close your laptop, a thought suddenly appears: you remember that leftover salad sitting in the fridge since last night. You go and get it, mouth salivating. But before sinking your teeth into that familiar munch, you pause. Beans, rice, chopped veg and sauce. All seems normal, yet something’s missing. What?

Ask a nutritionist and the answer might surprise you. In 2015, the New York Times published a short column called “A Decline in the Nutritional Value of Crops”, citing evidence that demonstrated a significant decline in the nutritional content of the food we eat [1]. The bulk of the article can be summarised as follows: you may now need to eat three oranges to get the same amount of Vitamin C that your grandma used to get from one.

Is Food Less Nutritious?

Anne-Marie Berenice Mayer, a nutrition consultant, has studied changes in crop micronutrient content for more than two decades. In a 1997 landmark study, using data published in the UK Government’s Composition of Foods tables, she compared the mineral content of 20 fruits and 20 vegetables grown in the 1930s and the 1980s, and showed several marked reductions in mineral content [2]. More recently, her team published a new analysis of long-term trends of the mineral content of fruits and vegetables from three editions of the UK’s Composition of Foods Tables (1940, 1991 and 2019). The analyses showed that for all minerals, apart from phosphorus, concentrations declined between 1940 and 2019, and the greatest overall reductions during the 80-year period were sodium (52 percent), iron (50 percent), copper (49 percent) and magnesium (10 percent) [3].

Similar findings have been reported in the US. In 2004, Donald Davis and fellow researchers from the University of Texas [4] found important nutrients in some garden crops to be up to 38 percent lower than 70 years previously. The team tracked changes in 13 nutrients based on data collected from vegetables between 1950 and 1999 and found a decline in six key nutrients: protein, calcium, phosphorus, iron, riboflavin and ascorbic acid. Alarmingly, the trend seems to be truly global. In 2017, scientists at the National Institute of Nutrition (NIN) of Hyderabad released data on 151 nutrients from 528 food items collected from markets across six geographical regions in India, revealing a significant decrease in nutrient value [5].

Digging Up the Root Causes

The authors of these studies chalk up the declining nutritional content to the preponderance of agricultural practices designed to improve species’ traits, such as crop size, growth rate and pest resistance, rather than nutritional content. Healthy soil is a prime source of minerals, providing essential support for growing crops. Traditionally, farmers used the crop rotation technique to preserve soil fertility, which allowed it to ‘rest’ for a season and recover from the effects of farming. Nowadays, as demand for crops continues to increase, this isn’t always viable. These mass production techniques, the theory goes, are depleting minerals faster than the microorganisms in the soil can replenish them. Essentially, as the yield went up, nutrient levels went down. This is referred to as the ‘dilution effect’ [6].

What the studies are telling us is that the increased use of chemical fertilisers and pesticides  has contributed to the altered composition of micronutrients in the soil, which, in turn, affects the quality of the food we eat. Combined, these factors mean that over the past few decades, new techniques developed to increase efficiency and to make food look juicier and more appealing, have lowered the nutritional content. On its website, the European Environment Agency reports that 60-75 percent of EU agricultural soils have excessive nutrient inputs, costing the EU more than €50 billion per year [7].

More Carbon, Fewer Nutrients

Could there be anything else to it? A 180-year study on wheat seems to suggest so. In the autumn of 1843, agricultural scientist John Bennet Lawes and chemist Joseph Henry Gilbert sowed the first crop of wheat on a field named Broadbalk. Every year since then, researchers from the institute have sown winter wheat on all or some parts of the field to compare crop yields grown using inorganic fertilisers with those grown using organic or farmyard manure (FYM). Nearly 200 years after the first crop of wheat was sowed, researchers continue to dig up interesting findings from this plot of land. The latest concerns the impact of rising levels of carbon dioxide (CO₂) in the environment on wheat’s nutrients. Recent studies have shown that, although the concentration of key minerals, such as iron, zinc, copper and magnesium, remained stable between 1845 and the mid-1960s, they’ve decreased significantly since [8]. 

A 2014 Nature paper seems to confirm these findings. The study compared the nutrient content of wheat grown in present-day conditions with wheat grown in an atmosphere with an elevated level of CO₂, similar to what’s predicted by 2050. The researchers found that wheat grown in high CO₂ levels had 9.3 percent less zinc, 5.1 percent less iron and 6.3 percent less protein. Rice grown in such conditions had 5.2 percent less iron, 3.3 percent less zinc and 7.8 percent less protein. The protein decline is particularly important: plants are a key source of protein for many people in developing regions, and one study estimated that by 2050, 150 million people could be at risk of protein deficiency, particularly in countries like Bangladesh or India [9]. The exact reasons for the protein decline are unclear, but growing evidence suggests that elevated CO₂ levels reduce a plant’s ability to take up nitrogen, which in turn affects the protein content of food. A 2015 study showed that the effect persisted even after the crops were grown with nitrogen-rich fertilisers, ruling out the possibility that the lower protein content is due to limited access to nitrogen in the soil [10]. Could it be that with a greater amount of CO₂ in the atmosphere, there’s less “room” for other elements like nitrogen? Or is CO₂ in some way interfering with plants’ abilities to take up nitrogen?

Further evidence for the declining nutrition of our crops can be found in regenerative agriculture, a method of farming that might also offer a solution. A 2022 paper published in PeerJ – Life and Environment highlights several independent comparisons that have shown that regenerative farming practices enhance the nutritional profiles of both crops and livestock compared to conventional practices. Specifically, conservation agriculture – a style of regenerative farming that combines no-till, cover crops and diverse rotations – produced crops with higher levels of certain micro- and phytonutrients. The authors also compared the fatty acid profiles of beef and pork raised on these farms to supermarket produce and found higher levels of omega-3 fats [11]. Soil health is likely an under-appreciated influence on nutrient density, positively influencing vitamin, mineral, phytonutrient and fatty acid profile, and this will have relevance for the prevention of chronic disease.

Methodological Caveats

While the nutrient-content data are compelling, several crucial caveats should be highlighted. First, most studies are based on country-by-country food tables, which represent compendia of historical information on the nutritional composition of foods. However, government bodies have correctly pointed out that these tables differ from each other due to several factors, such as variations in sample size and the degree of a crop’s ripeness. For instance, in order to extend shelf life, today’s fruits and vegetables are often harvested before they have fully ripened, which means they may not have had sufficient time to develop their full nutrient content at the time at which the measurement is taken. For these reasons, some researchers have argued against the use of these tables for historical comparisons of food nutritional content [12].

The second caveat has to do with the study methodologies. The research conducted by Dr Mayer and colleagues has been contested by the food and farming industries, who argue that the differences in historical comparisons reflect changes in how nutrients in food are measured. Older studies didn’t have access to the latest technologies to measure the nutritional content of food, which makes historical comparisons more open to interpretation. Additional changes to the way food is stored, shipped and consumed also make direct comparisons tricky. For example, in 2003, Spanish researchers discovered that broccoli kept in supermarket conditions lost 70 percent of a protective compound called glucosalinate and 60 percent of its antioxidant flavonoids over 10 days [13].

A third caveat stems from cooking practices. A long-standing notion in nutritional science is that certain cooking methods significantly reduce the nutritional content of food. Broccoli, spinach, and cabbage, for instance, are excellent sources of vitamin C. However, these vegetables may lose 50 percent or more of their vitamin C content when boiled, especially if the cooking vessel has no cover. B-vitamins are similarly heat sensitive: up to 70 percent of these vitamins may be lost when meat is grilled or baked [14].

Filling the Gaps

Despite the methodological caveats, the broad evidence of nutritional decline is hard to dismiss. What’s more, the findings from these studies are significant because, although getting sufficient protein, fat and carbs is important, it’s equally critical that our diet provides adequate amounts of micronutrients. These substances help keep the body running and protect against both non-communicable diseases, such as cancer, diabetes and heart disease, and infectious diseases. A reduced intake might translate to a reduced ability to fight off potential pathogens and protect against chronic disorders.

The declining crop nutrient content is a particular concern for those residing in developing regions. It’s estimated that more than one-third of people globally are micronutrient malnourished, a figure that includes Western nations [15], and over half of the world’s children under five years old and over two-thirds of non-pregnant women of reproductive age are deficient in at least one essential vitamin or mineral [16]. Fruits and vegetables can be key dietary contributors for many micronutrients, so declining nutrient concentrations are concerning. As one paper summarised: “Past and ongoing efforts to increase yields, combined with apparent broad tradeoffs between yield and the concentrations of perhaps half of all essential nutrients, work against recent efforts to increase one or a few micronutrients in individual foods” [17].

For sure, much more research is needed to fully dissect the causes of this nutritional decline. In the meantime, greater awareness about our food’s declining nutritional content has intensified explorations for new solutions. Fortified crops could be one way of tackling this problem, particularly in regions where people are more likely to be affected. Supplements have also been widely publicised – not to mention, marketed – as tools to fill gaps in nutrient intake.

For most of us, how big a deal is this? When it comes to minerals, the authors of a 2005 paper note that as “horticultural products in general, and fruits and nuts in particular, are relatively small contributors of minerals to the average UK diet, historical changes in mineral composition are unlikely to be significant in overall dietary terms” [18]. Could it be that, instead of focusing on the pounds, we’re fixating on the pennies? Today’s fruit and veg consumption is far from ideal in most Western countries. A study published in 2021 showed that, in the UK, if everyone ate the recommended 400 grams per day fruit and vegetable intake, healthy life expectancy would increase by an average of 7-8 months [19]. As well as this, many highly processed junk foods lose much of their valuable micronutrient content during production. While keeping tabs on the changes in the nutritional value of food is important, let’s not forget that modern fruits and vegetables continue to be nutrition gems with key health attributes. Making them a regular and plentiful inclusion in your diet is a fundamentally good idea.

Postscript

This is another article co-written with Marco Travaglio. Marco holds a PhD in Neuroscience and is passionate about health and nutrition.

References:
1. Ray, C. C. (2015) ‘A Decline in the Nutritional Value of Crops’, The New York Times, 12 September. Available at: https://www.nytimes.com/2015/09/15/science/a-decline-in-the-nutritional-value-of-crops.html (Accessed: 14 June 2024).
2. Mayer, A. (1997) ‘Historical Changes in the Mineral Content of Fruits and Vegetables’, British Food Journal, 99(6), 207-11.
3. Mayer, A.-M. B. et al. (2021) ‘Historical Changes in the Mineral Content of Fruit and Vegetables in the UK from 1940 to 2019: A Concern for Human Nutrition and Agriculture’, International Journal of Food Sciences and Nutrition, 73(3), 315-26.
4. Davis, D. R. et al. (2004) ‘Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999’, Journal of the American College of Nutrition, 23(6), 669-82.
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