Are Eggs Made Out of Calcium? The Truth About Eggshell Composition

An average chicken eggshell is composed of approximately 95% calcium carbonate, a chemical compound familiar as chalk and limestone. Despite this, the rest of the egg's interior does not share this high calcium content, making the truth about eggs and calcium more nuanced than it first appears.

Quick Summary

The vast majority of an egg's shell is calcium carbonate, but the edible parts—the white and yolk—contain relatively low amounts of calcium. Laying hens require significant dietary calcium, from sources like oyster shells, to produce strong, mineralized shells.

Key Points

• Eggshells are Mostly Calcium Carbonate: The hard outer shell of an egg is composed of roughly 95% calcium carbonate, which is the same mineral found in limestone and chalk.

• Inner Egg Has Low Calcium: In contrast to the shell, the edible part of the egg—the yolk and white—contains only a moderate to low amount of calcium.

• Hens Need Dietary Calcium: A laying hen requires a significant dietary intake of calcium, often supplemented with materials like oyster shells, to form strong eggshells.

• Bodily Reserves are Used: If a hen's dietary calcium is insufficient, she will draw upon her own bone reserves to mineralize the eggshell, which can lead to health issues.

• Eggshell Powder Can Be a Supplement: Properly prepared eggshell powder is a viable, budget-friendly calcium supplement for humans and can be absorbed effectively by the body.

Eggshells are Made of Calcium, But the Inside Is Not

One of the most common misconceptions about eggs is that the entire thing is full of calcium. This belief stems from the fact that eggshells are indeed almost entirely composed of calcium carbonate (CaCO$_3$). However, the nutrient profile of the egg's edible components—the yolk and the white—is vastly different. While the shell acts as a protective, mineralized barrier and a calcium reserve for a developing embryo, the yolk and white provide a different set of nutritional components.

The Science Behind the Eggshell's Mineralization

The formation of the eggshell is one of nature's fastest biomineralization processes. It begins in the shell gland of the hen's oviduct, where the mineral layer is deposited over the inner and outer membranes. Here's a quick breakdown of the process and structure:

• Organic Matrix: The process starts with an organic matrix, a network of collagen fibers and other proteins, that forms the eggshell membranes.
• Calcium Deposition: Calcium ions (from the hen's diet) and bicarbonate ions are secreted into the uterine fluid. From this supersaturated solution, calcite crystals—the mineral form of calcium carbonate—precipitate onto the membrane fibers.
• Crystallization: The calcite crystals grow, forming columnar structures that interlock to create the shell's hard, rigid surface.
• Role of Proteins: Specific matrix proteins embedded within the shell regulate this crystallization, influencing shell strength and structure. These proteins ensure the shell is tough enough to withstand impact yet pliable enough for the chick to hatch.

Where Do Hens Get All That Calcium?

Laying hens have an immense need for calcium. For example, a single egg can contain about 2 grams of calcium, a significant amount that is deposited over approximately 20 hours. Hens acquire this vital mineral primarily from their diet.

• Dietary Sources: Commercial poultry feed for layers contains high levels of calcium, often supplemented with sources like oyster shell grit, limestone, or crushed eggshells. The larger particle size of these supplements ensures they remain in the gizzard longer, providing a slow, steady release of calcium into the bloodstream.
• Bodily Reserves: If a hen's dietary calcium is insufficient, her body will draw calcium from its medullary bone reserves to produce the eggshell. This can lead to weakened bones and conditions like cage layer fatigue over time.

Nutritional Comparison: Eggshell vs. Inner Egg

To understand why the statement "eggs are made out of calcium" is a misnomer, it's essential to compare the mineral content of the shell and the rest of the egg.

| Feature | Eggshell | Egg Yolk | Egg White | | --- | --- | --- | --- | | Primary Composition | ~95% Calcium Carbonate (mineralized calcite) | Water, protein, fat, vitamins (A, D), minerals (iron) | ~90% Water, protein (ovalbumin), minerals | | Calcium Content | Extremely high (~40% elemental calcium) | Moderate (~129 mg/100g) | Low (~7 mg/100g) | | Purpose | Protection for embryo, gas exchange, calcium reserve | Nutrient supply for embryo, emulsification | Protein source, hydration, protects yolk |

Can Humans Use Eggshells for Calcium?

Because of their high calcium content, processed eggshells can be used as a calcium supplement. Homemade eggshell powder is an inexpensive alternative to commercial supplements and has been shown to be effectively absorbed by the body. For individuals with a calcium deficiency or osteoporosis, this can be a viable option, but it comes with caveats. The shells must be properly cleaned and sterilized (e.g., by boiling) to eliminate harmful bacteria like Salmonella before being ground into a fine powder.

In conclusion, while the robust shell of an egg is a testament to the incredible efficiency of calcium biomineralization, it is important to distinguish this from the composition of the edible interior. The yolk and white offer a wide array of proteins, fats, vitamins, and other minerals, but they are not the primary source of calcium within the egg itself. The shell, and the hen's diet, hold the key to the egg's high calcium content.

For more in-depth information on the structure of an egg, you can refer to the detailed diagram and explanation provided by the Exploratorium.

Frequently Asked Questions

Does eating eggs give me calcium?

While eggs contain some calcium, especially in the yolk, the majority is concentrated in the inedible shell. A significant amount of dietary calcium is found in the shell, not the interior you eat.

What is the primary component of an eggshell?

The primary component of an eggshell is calcium carbonate (CaCO$_3$), which accounts for about 95% of its total weight. This mineral provides the shell with its strength and hardness.

How do chickens get enough calcium to make eggshells?

Laying hens obtain calcium from their diet. This is often supplemented with sources rich in calcium, such as oyster shells or limestone, which are ground into the feed.

Is it safe for humans to eat eggshells for calcium?

Yes, properly prepared eggshell powder is a safe and effective source of calcium for humans. The shells must be sterilized (e.g., by boiling) and ground into a fine powder to avoid bacteria and physical harm.

How much calcium is in the yolk versus the white?

The yolk contains a moderate amount of calcium (approx. 129 mg per 100g), whereas the egg white has a very low amount (approx. 7 mg per 100g).

Why does a hen lay soft-shelled or shell-less eggs?

A hen will lay soft-shelled or shell-less eggs if she does not receive enough calcium in her diet. The eggshell requires a large amount of calcium that cannot be supplied if her diet is deficient.

What is the nutritional difference between the eggshell and the egg's contents?

The shell is almost pure calcium carbonate. The contents, however, are a mix of proteins, fats, vitamins (including D), and other minerals, with relatively low calcium content compared to the shell.