Why does meat have more protein than vegetables?
The differing biological needs of plants and animals contribute to the disparity in protein content between the two. Typically, individuals who are vegetarian or vegan need to consider carefully their sources of protein, as even popular plant-based substitutes often lack the protein density present in animal products. Tofu, for instance, delivers about 8 grams of protein per 100 grams, while chicken breast contains approximately 31 grams for the same amount.
According to Christi Calhoun at the American Meat Science Association, the cellular requirements of plants and animals lead to distinct compositions. Animal cells are rich in proteins due to functions like muscle movement, energy metabolism, and cellular repair, which require enzymes and hormones—both specialized proteins—as well as other proteins such as actin and myosin for muscle fibers. Conversely, plants rely more on carbohydrates for energy storage and structure, naturally resulting in less protein content.
Proteins in both plant and animal sources serve diverse functions within their respective organisms but are distinguished at a molecular level by differences in amino acid profiles. As Kinga Balogh from JM Nutrition describes, proteins can be thought of as "beaded necklaces," where each bead represents an amino acid. There are 20 amino acid types, with nine classified as essential because our bodies can't synthesize them; these must come from the diet.
Animal proteins deliver all essential amino acids, categorizing them as "complete" proteins, and they provide higher bioavailability. In contrast, plant-derived proteins often miss one or more essential amino acids, thus deemed "incomplete." Plant proteins also contain materials like fiber, making them harder for the human body to break down and absorb.
In 1993, the PDCAAS (Protein Digestibility Corrected Amino Acid Score) was created by the U.S. Food and Drug Administration and the World Health Organization to rate protein sources based on amino acid profile and bioavailability, with higher scores representing higher quality. Beef and eggs score between 0.9 and 1, whereas plants like black beans score 0.75, and peanuts 0.52. Soy products, such as tofu and tempeh, achieve a respectable 0.92.
While raw protein content isn't the sole factor to consider when comparing plants and animal products, both offer enough diversity and combinations to cater to dietary needs. Vegetarians and vegans can pair different plant proteins to absolutely cover essential amino acids—such as combining beans with rice.
Balogh further advises against focusing intensely on protein alone, recommending a varied diet to meet nutritional needs efficiently. Our bodies benefit from consuming appropriate quantities of protein along with energy, carbohydrates, and fats.
Overall, animal and plant proteins provide differently in terms of bioavailability and nutritional requirements, yet both are essential components of a balanced diet.