If we use the most useful definition of “nutritious” – “having a macronutrient profile that is conducive to health, and a high ratio of micronutrients to calories” – the short answer is yes, and quite severely.
Macronutrients include carbohydrates, protein, fat and fibre, and the distribution of these nutrients, and quality of each, strongly influences overall metabolic health. Micronutrients primarily include vitamins, which are organic molecules produced by living organisms, and minerals, which are chemical elements or molecules of those elements that are necessary for biological function. This is an important distinction, because a plant can generally produce vitamins from the same basic building blocks it uses for growth, but if a particular element is not present in soil then it simply can’t end up in a plant grown in that soil.
Overall, it has been shown that food is definitely less nutritious than in the past, for three main reasons: food choice and processing, farming methods, and topsoil mineral depletion.
Food choice is the most straightforward of these causes. People eat more highly-processed foods than in the past, resulting in less favourable macronutrient profiles and, generally, less intake of micronutrients, with negative effects on overall metabolic health. Increased consumption of processed foods has been found to correlate to worse outcomes on virtually all measures of health. In South Africa, the prevalence of diabetes almost tripled between 2010 and 2019, and South Africa was listed as the “least healthy country on Earth” by the Indigo Wellness Report, an independent index tracking a range of health markers. Reversing the trend toward processed foods is a priority for public healthcare.
But even ‘correct’ food choices may be becoming less nutritious.
This leads us to the second of the causes on our list. Farmers’ revenues depend on yield, appearance and flavour of their products, not on nutrient density, which is not something that customers can discern for themselves. Nutritional content of food is not highly studied but there is solid evidence of slowly worsening nutritiousness over time.
An aggregate of analyses of the nutritional content of UK and US fruit and vegetable crops between 1950 and 1999 revealed a consistent modest decline in micronutrient content of vegetables, along with a decline in both macronutrient and micronutrient quality in grains over an 80-year period. There are multiple causes behind this. The first is the ‘dilution’ effect, where increasing yields from chemical fertiliser result in a reduction of other nutrients.
Another cause is prioritising yield rather than nutrient density when selecting what to grow and propagate. Elevated atmospheric CO2 may also make a modest contribution; it has been shown that higher levels of carbon dioxide result in faster growth and higher yield but a decrease in micronutrient density. Tilling practices, particularly ploughing, have also been shown to disrupt the soil microbiomes that enable mineral uptake in crops, resulting in lower nutrient density.
These same practices are also a primary driving force behind the third and most insidious cause on our list – mineral depletion. This means that the same food item, even if grown in precisely the same way, may become less nutritious over time. Soil sampling for mineral content is not common practice, so this phenomenon is not thoroughly tracked and the true extent of it can only be loosely estimated.
The studies we referred to earlier looked at the mineral content of plants, not of the soils in which they grow, and it may be that depletion in the soil is not directly reflected in the plant until it becomes critical. Furthermore, those studies looked only at a limited set of minerals, namely calcium, iron and phosphor in a study by Davis, and calcium, magnesium, iron, copper, sodium, potassium, and phosphor in a study by Mayer. These are the minerals that are needed in the largest quantities and are considered most important to health, but this is by no means a complete list of required minerals. Other minerals considered essential are chloride, chromium, iodine, manganese, molybdenum,selenium, fluoride and zinc, many of which are needed only in small amounts and whose effects on health are not well known.
A potentially worrying indicator from the studies is that the mineral that showed the sharpest decline was copper, which has the lowest concentrations in soil of the minerals studied, and the lowest amount needed in the diet. If it is a general trend that the low-concentration trace elements are the most severely depleted, that would suggest that the minerals we have not been measuring may be depleting faster than the ones we are measuring. This means that essential minerals may be rapidly disappearing from our diets but we can’t easily find out how fast, because we didn’t measure them in the past and we aren’t measuring them now.
There are also trace minerals which have not been identified as essential or which have no known biological role, but which preliminary animal experiments suggest may be necessary for health or even for survival.
This situation represents a startling degree of ignorance. We are depleting these minerals at a rate we are not even tracking, before we have even determined what the consequences for our health might be. The health effects may still turn out to be trivial, but there is also a possibility that at some stage in the future we start to see unexplained, widespread declines in health that we can’t predict or treat.
There have already been some observable health effects of mineral depletion among the better-known minerals. Magnesium deficiency arising partly from declining levels in crops has been described as a public health crisis demanding urgent attention. Magnesium, fortunately, can be easily taken in supplements. But under-studied minerals, like the ones mentioned above, can’t be, because of the uncertainties surrounding their biological role and optimal dosage. From a public health standpoint, then, it is important to explore ways to bring biosphere mineral content back into equilibrium and arrest the decline.
The scale of this issue, unfortunately, almost defies belief. The amount of nitrogen extracted from the atmosphere and converted into fertiliser by industry is now about double the amount that is extracted by all plants and microbes on the surface of the Earth. By the end of the decade, human nitrogen fixation will exceed all natural extraction, including that in the ocean. Annual mining of phosphate rock, the main source of mineral fertiliser, exceeds 200 million tons per year and despite that, phosphate depletion has been identified as a major threat to agricultural productivity.
Human activity on this scale is bound to profoundly disrupt the normal equilibrium of the biosphere, creating excesses of some substances and deficiencies of others. This threatens not just our own health, but also the stability of most natural ecosystems.
This crisis demands urgent action and would require sweeping global reform of farming practices and application of concepts like closed-loop agriculture to cut down on nutrient losses. Such reforms are, unfortunately, unlikely to take place in the near term. With high global food prices, restrictions on fertiliser usage would be poorly received. In fact, Dutch farmers recently protested against laws aiming to limit fertiliser usage to reduce environmental impact.
For people with micronutrient deficiencies, the only avenue for the moment is to prioritise nutrient-dense foods and to take supplements of the few micronutrients which have been proved to be beneficial. Nutritional supplements, unfortunately, are a bit of a minefield of junk science, so each one of those is an article in its own right.