What are the disadvantages of plant sources of protein?

November 27, 2025 •

3 min read

Research suggests that plant-based protein has lower digestibility than animal protein, leading to less efficient absorption by the body. Understanding the disadvantages of plant sources of protein is essential for individuals structuring a nutritionally complete and balanced diet, especially when transitioning to or maintaining a vegetarian or vegan lifestyle.

Quick Summary

Plant protein sources may offer lower bioavailability, contain incomplete essential amino acid profiles, and harbor antinutrients that inhibit mineral absorption. This can potentially lead to deficiencies if not properly managed through a varied diet, proper food preparation, or supplementation.

Key Points

Incomplete Amino Acids: Many plant proteins lack one or more essential amino acids, requiring a varied diet for a complete profile.
Lower Digestibility: Antinutrients and tough plant cell walls contribute to the lower absorption rate of plant-based protein compared to animal sources.
Mineral Absorption Inhibitors: Compounds like phytates in plants can bind to minerals such as iron and zinc, reducing their bioavailability.
Vitamin B12 Deficiency Risk: A strict plant-based diet is typically devoid of Vitamin B12, necessitating supplementation or fortified foods.
Processing is Key: Simple methods like soaking, sprouting, and fermentation can significantly reduce antinutrient content and improve nutrient absorption.

Incomplete Amino Acid Profiles and Lower Protein Quality

Unlike most animal-based protein sources, which are considered 'complete' because they contain all nine essential amino acids (EAAs), most plant proteins are 'incomplete,' lacking one or more EAAs. While this once led to the belief that specific plant proteins must be paired at each meal, nutrition experts now confirm that consuming a variety of plant protein sources throughout the day is sufficient to meet EAA needs. However, this requires careful dietary planning to ensure a full spectrum of amino acids is consumed, a step not necessary with most animal proteins. Some plant sources, such as soy and quinoa, are notable exceptions and are considered complete proteins on their own.

Bioavailability Issues and Antinutrients

A significant drawback of relying heavily on plant-based protein is its typically lower bioavailability compared to animal protein. This means a smaller percentage of the protein consumed from plant sources is digested and absorbed by the body. A key reason for this is the presence of antinutrients.

The Impact of Antinutrients

Antinutrients are compounds in plants that interfere with the absorption of other nutrients, including protein and minerals. Some of the most common antinutrients found in plant protein sources include:

Phytates: Present in grains, nuts, and legumes, phytates bind to minerals like iron, zinc, magnesium, and calcium, forming insoluble compounds that the body cannot absorb.
Protease Inhibitors: Found in raw or undercooked cereals and legumes, these inhibit the digestive enzymes (like trypsin and chymotrypsin) needed to break down protein.
Tannins: Found in legumes and grains, tannins can bind to proteins and reduce their digestibility.
Lectins: Present in many plant foods, lectins can interfere with nutrient absorption and, in some cases, cause gastrointestinal distress.

Thankfully, many of these antinutrients can be significantly reduced through common food preparation techniques, such as:

Soaking
Sprouting
Fermentation
Cooking

Potential for Micronutrient Deficiencies

Transitioning to a plant-based diet without careful planning can increase the risk of specific micronutrient deficiencies.

Vitamin B12: This vitamin is synthesized by microorganisms and is not naturally found in foods of plant origin. A plant-based diet, especially a strict vegan one, can result in a deficiency, requiring fortification or supplementation.
Iron: The non-heme iron found in plants is less bioavailable than the heme iron in animal products, and its absorption is further inhibited by phytates and polyphenols.
Zinc: Similar to iron, zinc is less bioavailable from plant sources and its absorption is inhibited by phytates. While the body adapts over time, ensuring adequate intake can be a challenge.

Other Considerations: Taste, Texture, and Cost

For those relying on protein supplements, plant-based powders can present specific issues.

Taste and Texture: Many plant protein powders, especially those made from brown rice or pea protein, can have a chalky or gritty texture that is difficult to mix smoothly. The flavor can also be less palatable than whey protein for some individuals.
Cost: Some plant-based protein powders and supplements can be more expensive than their animal-derived counterparts, like whey.

Plant Protein vs. Animal Protein: A Comparison


Feature	Animal Protein	Plant Protein	Notes
Amino Acid Profile	Complete (contains all EAAs)	Often incomplete (lacks one or more EAAs)	Requires combining various plant sources throughout the day, except for a few exceptions like soy and quinoa.
Bioavailability	High (90-95%)	Generally lower (75-80%)	Attributed to plant cell walls and antinutrients.
Digestibility	Very high	Lower	Antinutrients like protease inhibitors can hinder protein breakdown.
Micronutrients	Rich in heme iron, B12, and zinc	Lower in B12 and less bioavailable iron and zinc	Requires careful planning, fortification, or supplementation for essential nutrients.
Antinutrient Content	Generally free of antinutrients	Contains phytates, tannins, and lectins	Processing methods like soaking and fermenting can reduce antinutrient levels.

Conclusion

While plant sources of protein come with several disadvantages, including potentially incomplete amino acid profiles, lower digestibility due to antinutrients, and challenges with certain micronutrients like vitamin B12, iron, and zinc, they can still form the foundation of a healthy diet. These issues can be largely mitigated through a varied and well-planned diet that incorporates a wide range of protein sources, combined with proper food preparation techniques like soaking and sprouting. For strict vegans or those with higher protein needs, supplementation may be a necessary and effective strategy to ensure all nutritional requirements are met. Overall, the key to a successful plant-based diet lies in informed choices and diligent planning rather than relying on a single source. More research is needed to further understand and optimize plant-based protein consumption.

Frequently Asked Questions

Plant protein is not necessarily inferior, but it does come with certain disadvantages, such as generally lower bioavailability and the presence of antinutrients. However, with careful planning and preparation, a plant-based diet can meet all protein and nutrient needs.

By consuming a varied diet of different plant protein sources throughout the day, the body can acquire all nine essential amino acids. Combining complementary proteins, like rice and beans, within the same day also helps ensure a complete amino acid intake.

Yes, Vitamin B12 is crucial for neurological function and red blood cell production but is not naturally found in plant foods. It is highly recommended for vegans and strict vegetarians to take supplements or consume fortified foods.

Antinutrients are compounds in plants, such as phytates and tannins, that interfere with the digestion and absorption of nutrients, including protein and minerals like iron and zinc. They reduce the overall bioavailability of plant protein.

Traditional food preparation methods such as soaking, sprouting, fermenting, and cooking can significantly reduce the levels of antinutrients, thereby improving the digestibility and absorption of plant proteins.

Yes, some plant-based proteins are considered complete, meaning they contain all nine essential amino acids. Examples include soy (tofu, tempeh) and quinoa.

Yes, the non-heme iron in plants is less easily absorbed than the heme iron from animal sources. This absorption is further hindered by phytates and other compounds in plants, making conscious effort needed to ensure adequate iron intake.