Greens, especially leafy greens, play an essential nutritional role in the natural human diet. Most animals eat green plant matter. Even obligate carnivores do! All frugivorous apes and primates consume a substantial amount of green plant foods, especially leaves. Some lesser primates are categorized as folivores because the main part of their diet is leaves, while apes are highly frugivorous with fruits as their main source of energy and nutrition. Besides a nutritional source, many greens are used as herbs for self-medication by primates, indicating that the use of functional greens is part of a natural diet.
Nutrients and anti-nutrients in greens
Greens contain substantial amounts of a variety of minerals and proteins because those are all needed for photosynthesis:
However, the vegetative parts of plants protect themselves from unfriendly chemicals towards whoever wants to eat them, which is why some leafy greens – especially the dark and bitter ones, have high levels of anti-nutrients, like the so-called “superfoods” kale. Tender, young, and succulent leaves generally contain less anti-nutrients!
- Tannins, which can be very bitter
- Lectins impede the digestive system
- Trypsin inhibitors, impede protein digestion
- Saponins are bitter and impede lipid absorption
- Alkaloids, of which many are toxic to animals
Toxins and anti-nutrients in plants are the results of an evolutionary rat race (except for fruits). And they are not entirely bad. First of all, animals build up an adaptive tolerance toward “their” foods. Moreover, many compounds help combat parasites and act as antimicrobial agents! Polyphenols are another example of how some compounds can be beneficial in species that have evolved to eat them.
If you like to learn more about nutrients and anti-nutrients in greens and which types of greens are the healthiest for humans, read our in-depth article on how to do greens right!
Many nutrients found in greens are also found in fruits. Because fruits have a higher water content, the nutrient quantity per weight is often lower in fruits than in greens, BUT fruits can be consumed in much larger quantities than greens! For example, iron: While greens are rich in iron, fruits are too and provide a substantial amount in the quantity we have evolved to eat. However, having access to ripe tropical fruits plays a crucial role in getting the nutrients we need! To understand human nutrition, we need to see the bigger picture: not only absolute nutrient content matters, but also how much we can consume of a food type, how bioavailable the nutrients are within the food matrix (i.e., anti-nutrient content), and if it’s our species natural food source.
Humans are Frugivores, not herbivores!
As frugivores, humans have not evolved to extract nutrients effectively from greens, which is why many raw leafy greens are hard to digest. In contrast, herbivores have a specialized digestive system to get the nutrients they need from green plant matter – for example, ruminants like cows and goats, but also non-ruminant animals like gorillas. Additionally, many leaves contain harsh (or even toxic) plant compounds, some of which serve the purpose of chemical self-defense against plant-eating animals.
Herbivores have a higher tolerance for these compounds than frugivores. Herbivores are actually mastering greens as an energy source: They are specialized in digesting a large amount of green matter and extract energy from cellulose with fermenting bacteria (i.e.,Fibrobacter succinogenes, Ruminococcus flavefaciens, and Ruminococcus albus). Their microbiome also produces phytase enzyme that helps overcome the anti-nutrient effects of phytates.
In contrast, frugivorous apes, which includes humans, are not specialized for large amounts of greens but are within a range of ca. 5-20% of the diet. An average chimpanzee’s diet consists of around 70% tropical fruits and figs, 24% other plant-based foods (flowers, nuts, leaves, sprouts, seeds, barks, tubers), 6% animal-based (of which 4 % are insects, and 1-2 % are meat).
Unlike ruminant herbivores, which eat mostly grass and ferment it in a specialized foregut, apes have only one stomach. Apes, including humans, are monogastric hindgut fermenters. The overall structure of gut morphology is highly similar in all apes and primates, with some differences in proportions, which actually are influenced by the diet itself rather than pure genetics (K. Milton, 1999). This means that the diet we eat shapes the proportion of our gut. This plasticity is likely an effective adaptation that helps us digest different types of foods. After all, we are frugivorous omnivores and able to survive on different types of food matter.
What types of greens are biologically appropriate for humans?
This question is not so easy to answer, and I am just starting to dive in. But as a hint, we might learn from chimpanzees and from the traditions of people living in the tropics! Why the tropics? Because humans are a tropical species. Humans have evolved in tropical forests, as newer anthropological research suggests. And, like all apes, our adaptations speak loud and clear: we are frugivores! Our evolutionary foods are tropical fruits, greens, and nuts. No wonder most wild tropical fruits are edible and are a superior food source to fruits from temperate climates!
It, thus, comes as no surprise that the favorite leaves eaten by chimpanzees are also some of the most miraculous herbal extracts used by people!
“Many fruits and unconventional foods are available in the tropics and represent valuable minerals sources. Green leaves appear as outstanding mineral sources, especially in calcium and iron, followed by nuts.”Leterme et al. 2006
Leaves in the chimpanzees diet
Chimpanzees consume leaves, buds, or young shoots with a preference for young leaves or types of leaves with low tannin content (Takemoto, 2003). Even during food scarcity, they avoid tannin-rich leaves. When we compare these preferences to humans, we can see that they are the same: Bitter or hard leafy greens, like dark lettuce or kale, are naturally not appealing to us without dressing or processing like steaming, just on their own.
“Young leaves from tropical trees are far more nutritious than I realized. In fact, the young tips have the same profile of essential amino acids as meat, although in lower concentrations,” said Milton. She said that leaf protein is perfectly good and clearly satisfies all the protein needs of the monkeys” (source)
Just like apes, humans are naturally most attracted to succulent, slightly sweet, tender greens. However, we do not live in our natural habitat like apes do – with consequences on our food availability!
How similar are chimpanzees and humans in terms of nutritional requirements? They are known to be highly similar! The nutrient requirements are nearly identical, which is why the RDAs for humans are used for dietary guidelines for chimpanzees in captivity. Vice versa, studying the natural chimpanzee diet can be just as useful to understand the human natural diet.
Chimpanzees spend a substantial part of their time foraging for fruits in the trees and also consume the leaves of the trees they are foraging. In general, they eat the leaves and fruits of a high number of different wild and cultivated plant species. The food choice of our closest relatives strongly depends on temporal and spatial availability.
For example, in Bossou, chimpanzees consume the fruits and leaves of wild tropical trees and cultivated trees, which are also used by local people:
- Fig tree leaves (Ficus exasperata); humans use leaf extract for pain relief and others. Read more here.
- Monkey fruit tree leaves (also known as giant yellow mulberry, Myrianthus arboreus); humans use the young leaves as food and other parts as medicine. Read more here.
- Corkwood tree leaves (Musanga cecropioides); human use the leaves as “popular medicine, especially to treat a range of feminine complaints” Read more here.
- Papaya leaves: papaya leaf tea is well-known in ethnobotanical uses. Read an overview here.
- Okra leaves; Okra leaves are a common food source for humans. Read more here.
- Avocado leaves; Avocado leaves can be used as a food source and have ethnobotanical uses. Read more here.
Those are just a few examples, and there are thousands of beneficial tropical leaves in the tropics, which are also a food source for primates. If we do not consume any of those greens, as chimpanzees do, we miss out on many anti-parasitic and anti-inflammatory bioactive components, which have been proven highly beneficial for humans! Therefore, integrating a few of them as herbs could be a great idea!
Tropical leaves used by humans
What types of tropical leaves are edible and have proven benefits? Here is a list:
- Yoshimura, H., Hirata, S. and Kinoshita, K. (2021) ‘Plant‐eating carnivores: Multispecies analysis on factors influencing the frequency of plant occurrence in obligate carnivores’, Ecology and Evolution, 11(16), pp. 10968–10983. doi:10.1002/ece3.7885.
- Kaczmarek, B. (2020) ‘Tannic acid with antiviral and antibacterial activity as a promising component of biomaterials—a Minireview’, Materials, 13(14), p. 3224. doi:10.3390/ma13143224.
- Leterme, P. et al. (2006) “Mineral content of tropical fruits and unconventional foods of the Andes and the Rain Forest of Colombia,” Food Chemistry, 95(4), pp. 644–652. Available at: https://doi.org/10.1016/j.foodchem.2005.02.003.
- Milton, K. (1999) ‘A hypothesis to explain the role of meat-eating in human evolution’, Evolutionary Anthropology: Issues, News, and Reviews, 8(1), pp. 11–21. doi:10.1002/(sici)1520-6505(1999)8:1<11::aid-evan6>3.0.co;2-m.
- Takemoto, H. Phytochemical Determination for Leaf Food Choice by Wild Chimpanzees in Guinea, Bossou. J Chem Ecol 29, 2551–2573 (2003). https://doi.org/10.1023/A:1026366119705
- Abotsi WM, Woode E, Ainooson GK, Amo-Barimah AK, Boakye-Gyasi E. Antiarthritic and antioxidant effects of the leaf extract of Ficus exasperata P. Beauv. (Moraceae). Pharmacognosy Res. 2010 Mar;2(2):89-97. doi: 10.4103/0974-8490.62958. PMID: 21808547; PMCID: PMC3140113.
- Orwa C, A Mutua, Kindt R , Jamnadass R, S Anthony. 2009 Agroforestree Database: A tree reference and selection guide version 4.0 (http://www.worldagroforestry.org/sites/treedbs/treedatabases.asp)
- Author unknown (No date) Musanga cecropioides. Useful tropical plants Available at: https://tropical.theferns.info/viewtropical.php?id=Musanga%2Bcecropioides (Accessed: 30 October 2023).
- Hill, A. (2020) 7 emerging benefits and uses of Papaya Leaf, Healthline. Available at: https://www.healthline.com/nutrition/papaya-leaf#2.-May-promote-balanced-blood-sugar (Accessed: 30 October 2023).
- Okra leaves Information, Recipes and Facts. Specialty Produce. Available at: https://specialtyproduce.com/produce/Okra_Leaves_8953.php (Accessed: 30 October 2023).