Eating tree leaves is not a common part of the modern human diet, but in times of famine or survival situations, turning to foliage from trees as a food source has helped humans sustain themselves until other options became available. Looking at the nutritional qualities, toxins, and digestibility of various tree leaves can provide insight into how viable they are as a food source.
Nutritional value of tree leaves
Tree leaves can potentially provide nutrients, but often not in the ideal ratios or quantities to sustain humans long-term. Here is an overview of some of the key nutrients found in tree leaves:
- Protein – Tree leaves can contain moderate amounts of protein, ranging from 2-7% of dry weight for most species. This is lower than nutritional requirements for humans.
- Carbohydrates – Leaves contain carbohydrates as fiber and sugars. The fiber is often indigestible. The sugars, such as glucose and fructose, may offer an energy source.
- Vitamins and minerals – Tree leaves offer varying amounts of vitamins A, C, E, K, and B vitamins. Mineral content can include calcium, magnesium, and potassium.
- Fat – The fat content is very low, under 5% dry weight.
While tree leaves can contribute valuable nutrients, the proportions are not ideal compared to meats, fruits, vegetables and grains. Eating tree leaves alone would likely lead to nutritional deficiencies over time without other food sources.
Toxicity dangers
Some tree species contain toxins that can be extremely dangerous for humans if ingested. Examples include:
- Yew trees – Contain taxine, a potent neurotoxin and cardiotoxin. Can be fatal if eaten.
- Oaks – Leaves and acorns contain tannic acid, which can cause kidney damage. Especially toxic when fresh.
- Eucalyptus – Oil is toxic to humans and can cause nausea, vomiting, and diarrhea.
- Cherry – Leaves contain cyanide and can be deadly if eaten in excess.
Many trees have adapted chemical defenses in their leaves to deter predators from eating them. Even if not acutely toxic, ingesting these compounds places metabolic stress on the body and can cause complications after prolonged exposure.
Digestibility issues
Leaves from trees and shrubs tend to be more difficult for humans to digest than cultivated fruits, vegetables and grains. Some of the factors impacting digestibility include:
- Tough cell walls – The structural components of leaves like cellulose and lignin are very fibrous and resistant to breakdown in the human gut.
- Indigestible components – Components like waxes, tannins, silica and cutin found on leaf surfaces tend to pass through the GI tract without being digested.
- Toxic compounds – As mentioned above, various chemical toxins interfere with digestion and nutrient absorption.
The overall effect is that a significant portion of the biomass from tree leaves ends up indigestible for humans. We may lack the appropriate digestive enzymes and specialized gut microbes to fully break down the leaves.
Trees that offer relatively safe leaves for human consumption
While no tree leaves should be considered a practical staple food, some species offer relatively better nutrition and safety profiles if consumed in moderation. Examples include:
- Birch – Leaves offer vitamin C and can be dried and steeped into an edible tea beverage.
- Pine – Pine needles are high in vitamin C and make a tea or edible mush when cooked.
- Banana – High in potassium and magnesium, the large tender leaves are edible raw or cooked.
- Basswood – Young leaves are edible raw or cooked and were used as food by some Native Americans.
- Maple – Leaves can be boiled and eaten or dried and made into tea.
It’s still vital to properly identify any tree species before consuming its leaves. Only very young, tender leaves should be considered since older leaves grow tougher. Introducing leaves gradually aids digestibility as the gut adapts.
Preparing tree leaves for food
To best extract nutrients and energy from tree leaves, while avoiding potential toxicity, several preparation methods can help increase digestibility and palatability as food:
- Cooked – Heat helps break down tough cell walls, denature toxins, and kill pathogens.
- Dried – Removing moisture makes leaves easier to chew and digest.
- Tea – Steeping leaves extracts some vitamins, minerals, sugars, and beneficial plant compounds.
- Fermented – Microbial fermentation helps digest some components and neutralize antinutrients.
- Ground/mashed – Mechanically breaking leaves helps increase digestibility.
Ideally, a combination of cooking, drying, and grinding helps transform the biomass in leaves into a form humans can partially draw nutrition from. Multi-step processing methods used in cuisines around the world reflect traditional wisdom on enhancing the palatability and sustenance drawn from foliar foods.
Table: Nutritional value of select tree leaves per 100 grams of dry weight
| Tree Species | Calories | Protein | Fat | Carbs | Calcium | Iron | Vitamin A | Vitamin C |
|---|---|---|---|---|---|---|---|---|
| Oak | 205 | 5 g | 5 g | 41 g | 400 mg | 3 mg | 167 IU | 7 mg |
| Maple | 186 | 4 g | 4 g | 44 g | 256 mg | 3 mg | 120 IU | 35 mg |
| Basswood | 164 | 3 g | 2 g | 39 g | 379 mg | 1 mg | 42 IU | 18 mg |
This table helps compare the nutritional attributes of a few tree leaves. While they offer modest amounts of protein, carbs, vitamins and minerals, the ratios are different than more conventional foods and could lead to deficiency over time if heavily relied on. The figures also depend heavily on season, soil, light exposure, and other factors.
Historical use of tree leaves as human food
While not ideal as a steady food source, tree leaves have offered life-saving sustenance during famines, wars, and survival situations. Some examples of using leaves and other unusual foods out of desperation include:
- Netherlands famine of 1944-45 during WWII – People ate tulip bulbs and leaves.
- Great Chinese Famine, 1958-62 – Leaves from trees, shrubs and bamboo were consumed.
- Famine in Ethiopia, 1983-85 – People foraged leaves from wild plants.
- Yemen famine, 2016-present – Yemenis have eaten leaves from local trees to survive.
During crises like these, gathering leaves, grasses, bark, and anything even modestly edible can temporarily stave off starvation. The human body is remarkably adaptable when pressed to survive.
Table: Toxicity levels of select tree species
| Tree | Toxic Parts | Toxin | Toxicity Level |
|---|---|---|---|
| Cherry | Leaves, seeds | Cyanide | Highly toxic, potentially fatal |
| Yew | All parts | Taxine alkaloids | Extremely toxic, fatal in low doses |
| Oak | Leaves, acorns | Tannins | Can be toxic, especially to kidneys |
| Eucalyptus | Oil in leaves | Cineole | Toxic if ingested; can cause nausea |
This table summarizes toxicity data on a few tree species. In general, tree leaves tend to be less toxic than their seeds, bark or other parts, but caution is still warranted, especially with trees known to be poisonous.
Are leaves with insects or damage safe to eat?
Finding insects, disease, fungi, or physical damage on tree leaves naturally raises concerns about their safety and palatability as food. Here are some general guidelines:
- Insects – Should be removed if possible. Cooked thoroughly.
- Disease/fungi – Avoid leaves with heavy discoloration or decay.
- Holes/tears – Remove severely damaged sections of leaves.
- Wilting – Only use fresh, healthy leaves.
- Packaged leaves – Inspect carefully for cleanliness and freshness.
Light insect or physical damage is common and can be mitigated through washing and cooking thoroughly. However, heavily infested or rotten leaves may be unsafe to eat due to pathogens or accumulation of toxins. When in doubt, do not consume questionably deteriorated foliage.
Foraging ethics when collecting wild tree leaves
Harvesting tree leaves directly from parks, public lands, or private property requires ethical considerations:
- Know the laws and regulations on foraging in your area.
- Obtain permission from landowners whenever possible.
- Never harvest in protected wilderness areas.
- Take leaves sparingly from each tree.
- Prioritize abundant, weedy species for collection.
- Avoid rare, slow-growing trees.
- Consider the wildlife that depend on each tree.
Sustainable foraging aims to gather leaves in ways that still preserve the tree’s health and ecosystem contributions. Be aware some urban areas purposefully spray trees with chemicals which can linger. Foraging ethically helps ensure the privilege is not abused.
Benefits of eating processed versis raw tree leaves
Weighing the tradeoffs between processed and raw tree leaves:
Benefits of processed leaves
- Increased digestibility from cooking, drying, fermenting
- Neutralizes anti-nutrients like tannins and phytic acid
- Destroys bacteria, viruses, parasites
- Reduces certain toxins
- Converts starches to more digestible sugars
- Can taste better with seasoning, spices
Benefits of raw leaves
- Retains some vitamins vulnerable to heat
- Preserves live enzymes and antioxidants
- Faster to prepare
- More fiber and bulk for digestion
- No contaminants from processing
- Natural taste provides sensory variety
In most cases, cooking leaves makes more nutrients bioavailable to humans by breaking down indigestible compounds. But occasionally eating tender young leaves raw offers different organic compounds and texture.
Conclusion
While tree leaves can technically be eaten by humans, they are far from an ideal food source. Leaves often lack complete nutrition, contain toxins, and resist digestion compared to cultivated plants. However, with careful preparation and avoidance of poisonous species, tree leaves have offered sustenance during famines and survival situations throughout history. With proper ethical foraging, tree leaves could again serve as emergency food but should not become a long-term diet.
