What Do Egg Shells Contain? A Comprehensive Guide to Its Components

December 23, 2025 •

3 min read

An estimated 11 billion eggs are produced commercially in the United States every year, resulting in an enormous amount of eggshell waste. Far from just waste, the humble eggshell is a nutritional powerhouse, composed almost entirely of calcium carbonate, alongside a complex matrix of proteins and other vital minerals. This composition gives it a range of potential uses, from dietary supplements and joint health remedies to garden fertilizer.

Quick Summary

Eggshells are primarily composed of calcium carbonate, with smaller amounts of protein and other minerals like magnesium, phosphorus, and strontium. A fibrous membrane found on the inside is rich in collagen and other beneficial compounds. While a potential calcium source, proper sterilization is crucial to prevent contamination.

Key Points

Calcium Carbonate (CaCO3): Makes up approximately 95% of a dry eggshell's weight, providing its signature hardness and acting as a dense source of calcium.
Eggshell Membrane: A thin layer between the shell and the egg white that is rich in proteins such as collagen, elastin, glucosamine, and chondroitin sulfate.
Trace Minerals: Eggshells contain small but valuable amounts of other minerals, including magnesium, phosphorus, fluoride, strontium, and selenium.
Safe Preparation is Crucial: For human consumption, eggshells must be boiled and finely ground into a powder to eliminate the risk of Salmonella contamination and prevent injury from sharp fragments.
Highly Bioavailable Calcium: Studies show that the calcium from eggshell powder is readily absorbed by the body, making it an effective and natural supplement for bone health.

The Primary Composition: Mostly Calcium Carbonate

Approximately 95% of a chicken egg's dry shell weight is calcium carbonate ($CaCO_3$), the same compound that makes up limestone and seashells. This mineral is what gives the shell its incredible strength and hardness, creating a protective barrier for the developing chick inside. In fact, due to its shape and compressive strength, a whole egg can withstand surprising force. The source of this calcium is primarily the hen's diet, and inadequate levels can lead to weaker, thinner eggshells.

The Organic Matrix and its Crucial Role

Though calcium carbonate is the major component, a complex organic matrix binds the crystalline structure together and plays a vital role in shell formation and strength. This matrix consists of proteins and other organic compounds that not only contribute to the shell's structural integrity but may also enhance calcium absorption in the body.

The Inner Membrane: A Source of Proteins and More

Peeling a hard-boiled egg reveals the thin, fibrous membrane lining the shell. This membrane is not part of the hard outer shell but is a crucial component with its own unique nutritional profile. The eggshell membrane (ESM) contains significant amounts of beneficial proteins and other compounds, which are often separated and used in health supplements.

Key components of the eggshell membrane:

Collagen: This is the most abundant protein in the membrane, with types I, V, and X all present. Collagen is vital for healthy skin, hair, nails, and connective tissues.
Elastin: This protein provides elasticity to tissues throughout the body, including cartilage and skin.
Glycosaminoglycans (GAGs): This group includes compounds like glucosamine, chondroitin sulfate, and hyaluronic acid. These are well-known for their roles in supporting joint and cartilage health and are often found in supplements designed for joint relief.
Amino Acids: The membrane is rich in various amino acids that are important for tissue repair and regeneration.

A Host of Trace Minerals

In addition to the abundant calcium carbonate, eggshells contain a variety of trace minerals, which can vary depending on the hen's diet and environment. These minerals contribute to the eggshell's overall nutritional value. The following are some of the key trace minerals found in eggshells:

Strontium: This mineral has been shown to play a role in bone health.
Fluoride: Also important for bone strength, fluoride is present in small amounts.
Magnesium: This mineral is vital for numerous bodily functions, and eggshells contain a small percentage of it.
Selenium: Known for its antioxidant properties, selenium is another mineral found within the shell.
Phosphorus: A key mineral for bone structure and repair.

Potential Health Benefits and Safe Consumption

For those interested in leveraging the nutritional benefits of eggshells, proper preparation is essential to mitigate health risks like bacterial contamination, particularly from Salmonella. Grinding sterilized shells into a fine powder is the standard method for creating a safe supplement. The bioavailability of eggshell calcium is also high, making it an effective supplement for bone health.

Comparison: Eggshell vs. Oyster Shell Supplements

When considering natural calcium supplements, eggshells are often compared to oyster shells. Here is a comparison of their key aspects:


Feature	Eggshell Supplement	Oyster Shell Supplement
Calcium Source	Primarily calcium carbonate from hen eggs.	Calcium carbonate from mollusk oyster shells.
Contaminants	Low levels of toxic metals like lead and aluminum. Must be sterilized to remove Salmonella.	Historically associated with higher levels of contaminants like lead and aluminum.
Bioavailability	High; some studies suggest superior absorption due to associated proteins.	Good; often sold as purified calcium carbonate.
Added Nutrients	Contains beneficial membrane proteins like collagen, elastin, and GAGs.	May contain trace minerals but lacks the unique membrane compounds of eggs.
Waste Reduction	Excellent for reducing food waste.	Utilizes a different form of natural marine waste.

Conclusion

Eggshells are far more than a simple discardable container. They are a complex, multi-layered structure composed predominantly of calcium carbonate, reinforced by an organic matrix, and lined with a protein-rich membrane. The presence of trace minerals and the high bioavailability of their calcium make them a valuable and sustainable resource. While proper safety precautions are necessary for human consumption, their potential as a dietary supplement and fertilizer demonstrates that there is significant value in what many consider kitchen waste. By understanding the full spectrum of what egg shells contain, we can appreciate their potential and utilize this resource more effectively. Learn more about the potential of eggshell utilization in various industries from this ResearchGate study.

Frequently Asked Questions

The main component of an eggshell is calcium carbonate ($CaCO_3$), which accounts for about 95% of its dry weight and gives the shell its hard, protective quality.

Yes, you can eat eggshells for calcium, but they must be properly prepared. This involves sterilizing them by boiling and then grinding them into a very fine powder to avoid injury and bacterial contamination.

The eggshell membrane is the thin, fibrous lining inside the shell. It is rich in proteins like collagen (Types I, V, and X), as well as elastin, glucosamine, chondroitin sulfate, and hyaluronic acid.

Yes, eating raw eggshells carries health risks, including the potential for bacterial contamination, most notably Salmonella. The sharp edges of shell fragments can also cause injury to the throat and digestive tract.

In addition to calcium, eggshells contain small amounts of other essential minerals, including magnesium, phosphorus, strontium, selenium, and fluoride.

To make eggshell powder safely, first wash the shells, then boil them for about 15 minutes to sterilize them. Dry them thoroughly and grind them into a very fine, uniform powder using a coffee grinder or mortar and pestle.

Some studies suggest that the calcium from eggshells may be more bioavailable than purified calcium carbonate supplements due to the presence of other organic compounds. However, excessive intake should be moderated and discussed with a health professional.