Understanding the Human Digestive System and Wax Esters
Lipids, or fats, are an essential part of the human diet. The most common dietary fat is a triglyceride, which consists of a glycerol backbone with three fatty acid chains. Wax esters, in contrast, are formed from a long-chain fatty acid and a long-chain fatty alcohol. This structural difference is the key to understanding why human digestion of wax esters is so limited and inefficient, especially when compared to our efficient processing of triglycerides.
The Role of Enzymes and Bile
For fats to be digested, they must first be emulsified by bile salts in the small intestine, increasing their surface area. The pancreas then releases enzymes, primarily lipases, to break down the fat molecules. While the same pancreatic enzymes (lipases and carboxylesterases) that break down triglycerides also show activity towards wax esters, they are significantly less effective at it. The hydrophobic nature of wax esters makes them more difficult to emulsify and, therefore, more resistant to enzymatic attack.
Consequently, the digestion of wax esters is slow and often incomplete. For normal dietary intake—from sources like unrefined grains, seeds, and nuts—our bodies can usually manage to hydrolyze some of the wax esters. However, when faced with an overwhelming quantity, the digestive process fails, and the undigested wax esters continue through the gastrointestinal tract.
The Keriorrhea Effect: When Digestion Fails
Consuming large amounts of indigestible wax esters is the direct cause of keriorrhea. This condition has been linked to eating fish from the Gempylidae family, such as escolar and oilfish. These fish are known to contain significant levels of wax esters, sometimes comprising over 90% of their fat content.
The undigested wax esters, or gempylotoxins as they are sometimes called, act as a natural laxative, leading to the characteristic oily, orange discharge. This leakage occurs because the indigestible esters accumulate in the rectum, bypassing the absorption process entirely. It is important to note that the severity of keriorrhea depends heavily on the amount consumed and individual sensitivity. In Italy and Japan, the sale of these fish is restricted due to these effects.
Wax Esters vs. Triglycerides: A Comparison
| Feature | Wax Esters | Triglycerides |
|---|---|---|
| Chemical Structure | Long-chain fatty acid + long-chain alcohol | Glycerol backbone + three fatty acid chains |
| Primary Digestive Enzyme | Pancreatic carboxylesterases (less efficient) | Pancreatic lipase (highly efficient) |
| Emulsification | Difficult due to hydrophobic nature | Efficient with bile salts |
| Absorption in Humans | Slow and incomplete, especially in high doses | Rapid and efficient under normal conditions |
| Impact of High Intake | Can cause keriorrhea (oily diarrhea) | Digested and stored as body fat; no oily leakage |
How Wax Ester Digestion Differs Across Mammals
While humans struggle with high wax ester intake, other mammals and even some seabirds have evolved to digest them more efficiently. For example, studies on bowhead whales show that they possess the ability to digest and absorb wax esters, which make up a large portion of their prey's lipids. Some bird species can also achieve high assimilation efficiencies for wax esters, often with the help of adapted digestive mechanisms like retrograde duodenal movement. This highlights the evolutionary differences in digestive capabilities based on dietary needs. Our limited capacity is likely due to the relatively low historical intake of high-wax foods.
The Potential Benefits of Moderate Wax Ester Intake
It's a mistake to categorize all wax esters as 'bad.' When consumed in moderate amounts, the fatty acids and alcohols liberated during the slow digestion process can be absorbed. For example, Calanus oil, a dietary supplement derived from marine copepods, is rich in wax esters containing beneficial omega-3 fatty acids like EPA and DHA. Studies have shown that these fatty acids are bioavailable for human consumption, suggesting that the slow digestion may even provide a delayed release of beneficial nutrients. This shows that the context and quantity of wax ester intake are crucial.
Navigating the Wax Ester Landscape
For most people, consuming foods with naturally low levels of wax esters, such as whole grains, seeds, and nuts, poses no digestive issues. The problem arises from consuming species of fish that store exceptionally high concentrations of these lipids. The key takeaway is moderation and awareness. Being able to identify fish species like escolar and oilfish is important to avoid the unpleasant side effects of keriorrhea.
Conclusion
In summary, the human digestive system can process wax esters to a limited degree, especially in the small, natural amounts found in many foods. Our bodies rely on pancreatic enzymes that are far less efficient at breaking down wax esters than they are with common triglycerides. This inefficiency becomes problematic when large quantities are ingested, as is the case with certain oily fish like escolar, leading to the laxative effect of keriorrhea. Therefore, while humans can partially digest wax esters, our limitations mean that it is unwise to consume them in large volumes. The crucial distinction is between small, digestible amounts and the concentrated, indigestible load from specific seafood sources. For beneficial omega-3s, other sources or supplements from wax ester-rich oils can be consumed safely in controlled dosages.
For more in-depth information on the metabolism of long-chain fatty acids and alcohols from dietary waxes, see the review published in Experimental Biology and Medicine: Nutritional significance and metabolism of very long chain fatty alcohols and acids from dietary waxes.
