Secondary plant compounds: substances with many unknowns

Posted on by Barbara H. Kozak

Just a few years ago, nutritional science assessed phytochemicals as insignificant or even harmful. Today, research suggests that these substances prevent disease and are a necessary component of nutrition for the long-term maintenance of human health.

Since the early 1990s, nutrition research has been increasingly concerned with phytochemicals. This interest is twofold justified. On the one hand, it is believed that the beneficial effects of a plant-based diet rich in vegetables, fruits, legumes, cereals and nuts are largely due to their phytochemicals content. On the other hand, the pharmaceutical and food industries hope to be able to produce profitable nutritional supplements and functional foods with once identified and isolated individual substances.

While phytochemicals actually have the task of repelling pests and attracting insects, they have numerous health-promoting effects in the human organism (Table 1). Several epidemiological studies show that high consumption of vegetables and fruits significantly reduces the risk of certain diseases. Thus, cardiovascular diseases, cancer, obesity, rheumatoid arthritis, asthma, osteoporosis, neurological diseases and certain eye diseases (macular degeneration) occur significantly less frequently in the case of high vegetable and fruit consumption. Although there are certain explanations for the mechanisms of why plant food positively influences human health, these are basically scientifically unclear. In short, we still do not know why eating vegetables and fruits is healthy.

  • Hardly comparable in animal foods

Carbohydrates, fiber, proteins and fat are among the primary plant compounds. They are of vital importance for the metabolism of the plant. Secondary plant compounds are neither produced in the energy metabolism nor are they absolutely necessary for the maintenance of life. They represent a heterogeneous group of substances, occur in very different amounts in plants and are often specific to them. Their functions are manifold: they take on, for example, signal and pigment functions, act as defense poisons against fungi and pests, protect against UV light and negative environmental influences or form colorings, fragrances and aromas. In the human organism, they have an antioxidant, tumor preventive effect and trap free radicals. Remarkably, that there are virtually no substances in animal foodstuffs that are comparable in their action to phytochemicals; apart from the Zoochemicals omega-3 fatty acids in fish and lactic acid in acidified milk products.

  • Unexplored diversity

With more than 250,000 higher plants on earth, it can be assumed that there are at least as many phytochemicals. So far, about 80,000 of them have been identified. In human food 5,000 to 10,000 phytochemicals are suspected. As 90 percent of global calorie needs are met by consuming about 30 plants, scientific research focuses on these few food crops. Only a limited number of phytochemicals are found in the individual plant foods, for example in onions about 70-100, in apples 200-300 or in tomatoes 300-350. Vegetables – as with vitamins – usually have higher contents than fruits.

The mainly studied secondary plant substances belong to the classes of carotenoids, phytosterols, glucosinolates, polyphenols (flavonoids, phenolic acids) and phytoestrogens (isoflavonoids, lignans). Few studies are available on saponins, monoterpenes, sulfides, chlorophyll and phytic acid. Flavonoids are the most widely used phytochemicals. So far, more than 6,500 different compounds have been identified. For other classes of compounds there is still a great need for research, as most of the phytochemicals in our diet have not yet been identified (Table 2).

  • Feed recommendation unknown

With the usual mixed diet, humans take several grams of phytochemicals daily. For vegetarians, this amount is often much higher. However, little is known about the bioavailability of phytochemicals from foods. While carotenoids, glucosinolates, phytoestrogens, monoterpenes and sulfides are relatively well absorbed by the body at more than 15 percent, flavonoids have some good but some poorly available compounds.

The preparation of food can increase the bioavailability of certain substances. Thus, beta-carotene and lycopene from heated carrots or tomatoes are better absorbed by the body. On the other hand, when cooking in water certain phytochemicals can be lost. Thus, glucosinolates, phenolic acids and protease inhibitors are sensitive to heat, as are the oxygenated carotenoids lutein and zeaxantine. Saponins and glucosinolates are partially leached. By contrast, other substances such as phytoestrogens or polyphenols survive various preparation processes, industrial processing steps and longer storage times.

Secondary plant compounds are found in vegetables and fruits mostly in the skins, outer layers and leaves. Studies have shown that part of the phytochemicals reaches the colon and is metabolised differently depending on the composition of the intestinal flora. About the extent of these different metabolism is not yet known. Due to the microbial remodeling, some compounds become bioactive or more potent. Most of the phytochemicals currently lack reliable data on bioavailability, metabolism, and mechanisms of action. This explains that so far no scientifically justified intake recommendations exist. Only for beta-carotene, the German-speaking nutrition companies give an estimate of 2-4 milligrams for daily intake. This amount is easily reached by eating vegetables such as kale, carrots, savoy cabbage and red pepper.

  • polyphenols prolong life?

From a nutritional point of view, polyphenols are among the most interesting phytochemicals. In vitro, ie in a test tube, they develop one of the strongest antioxidant effects of food components and activate genes in the organism that code for radical-scavenging enzymes. Polyphenols are produced by the plant to ward off stress and injuries. The more than 10,000 substances are divided into several classes, the great variety makes a nomenclature difficult. Significantly fewer compounds are relevant to human nutrition.

A diet rich in polyphenols reduces the risk of chronic diseases, especially cardiovascular disease. Protective effects are also observed with stress, smoking, inflammation and UV radiation. There is growing evidence that regular intake of polyphenols is needed to reach the full lifespan while minimizing the risk of chronic disease. Because of this very important function of polyphenols, especially for age, scientists have defined these as lifespan essentials.

  • Deficiency increases disease risk

Epidemiological studies indicate that dietary phytochemicals are preventive. However, it can not be said whether these favorable effects are predominantly based on individual compounds or on the complex spectrum of nutrients found in plants.

Health-promoting effects of phytochemicals on the vascular and nervous systems have been proven. A long-term low intake of plant foods – especially vegetables and fruits – is now regarded as a potential disease-causing factor. For cardiovascular diseases, stroke and hypertension, the preventive effect of phytochemicals with “convincing evidence” is scientifically proven. In cancer, the evidence is likely to be possible in dementia, obesity, rheumatoid arthritis, asthma, osteoporosis and macular degeneration. To date, it has not been demonstrated for any single phytochemical that it alone reduces the risk of disease in a physiologically relevant concentration.

  • Food Synergy: the key to a healthy diet

The term food synergy first appeared in the scientific literature in 2001 . In doing so, American nutrition researcher David Jacobs of the University of Minnesota’s School of Public Health is pursuing a new approach to understanding food effects. The reason behind the development of this concept of food synergy was the disappointing results of studies based on a simplistic approach. In particular, the many intervention studies with vitamin supplements have brought sobering results.

According to Jacobs, the food ingredients in food are coordinated, that is coordinated, and develop in their intact total additive and synergistic effects. In this interaction of the substances, Jacobs sees the health-promoting active principle of a plant-stressed, low-processed diet. He symbolically speaks of the “hypothesis of orchestrated food synergy”. The concept of food synergy also emphasizes that foods have a buffering effect during digestion, releasing nutrients more slowly and, to some extent, ingesting them to a lesser extent. Nutrients from natural foods have different effects than those that have been subjected to a technological process.

According to Jacobs, research should therefore focus more on the effects of complete diets. The Hippocratic approach of having to regard food as a medicine is increasingly confirmed. The nutrition physiologist Prof. Andreas Hahn, who is intensively studying the effect of food as a drug at the University of Hannover, sums up: “Foods have much more extensive effects than previously assumed. They not only contribute to nutrient supply in the narrower sense, but also have preventive and dietary-therapeutic effects. ”

  • Vegetables and fruits are safest

Numerous studies show that it is never too late for a change in diet. Thus, the risk of developing chronic diseases is significantly reduced by increased consumption of vegetables and fruits in a relatively short period of just five years. The beneficial effects of a plant-based diet are already visible on the skin’s appearance within a few weeks and can be measured by changes in risk factors such as weight reduction, lower blood pressure or improved blood lipid levels.

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