Lipids play important roles in the nutrition, biochemistry, and physiology of animals. Lipids have multiple nutritional functions. They are an economical source of energy, but they also supply phospholipids that participate in cell membranes and are used as carriers for multiple liposoluble nutrients (vitamins, cholesterols, etc…). There is an increased interest in maximizing the use of fat supplements in the diets in recent years because of the general increase of energy cost but as well the increase of the dietary energy density of the diets to meet the requirements of fast-growing animals.

Vietnamese Burmese

The process of lipid digestion is quite complex. It involves emulsification and the formation of micelles. Emulsification is essential to create an interface oil-water and increase the surface of action for lipase. We start from triglycerides (ester of three fatty acids) and lipase will break the ester bonds to release 2 fatty acids on one side and the remaining monoglycerides on the other side. But these components are still liposoluble and cannot pass the gut wall. For short-chain fatty acids, their water solubility side is dominant, and they can pass through the mucus and will be absorbed directly by the enterocytes. But medium and long-chain fatty acids, some vitamins, and monoglycerides are too liposoluble to do so. They will therefore need to join to form small micelles. As such, they will be able to go through the mucus and penetrate the enterocyte's membranes.

Inside the cell, they will be re-esterified to form again triglycerides that can be either stored as fat deposits, used by the liver as energy, or building blocks for lipoproteins and membranes phospholipids.

But this complex digestion process leads to a lot of variabilities affecting the absorption of lipids, depending on the age of the animals, the source of lipids, and even the interactions with other nutrients namely Calcium.

Indeed, some studies show that the production of bile acids starts low in young chicks and increases 8-10 times between 4- and 21-days post-hatch. Without a proper level of bile acids acting as natural emulsifiers, chicks below 10 days old are not equipped to digest properly the lipids.

The table below illustrates well the difficulties of chicks to digest lipids during the first week of age. This is critical for nutritionists to take this element into account and define a different AME in their matrix for the one-week chick.

The source of the lipids can create as well significant variations on their nutritional value as their retention depends on the nature of their lipids. Unsaturated fatty acids are better absorbed than saturated ones. For example, contrarily to animal fat or palm oil, soybean oil is made of 83% of unsaturated lipids namely C18:1 Oleic and C18:2 Linoleic which explains his high absorption rate comparable fat which contains a higher portion of saturated fatty acids.

The molecular structure of the lipids impacts as well their absorption. Short-chain fatty acids will get into enterocytes faster than medium and long-chain fatty acids because of their small size and their higher affinity for water.

Calcium as well significantly influenced the bioavailability of lipids. It has been described that Calcium will bind long-chain fatty acids to create ‘soaps’ often referred to as being insoluble. These soaps make up most of the fecal lipid in infants fed high calcium diets and are responsible for increased fat malabsorption in diets with a high level of calcium.

The study illustrated below shows that when we are increasing the contribution of animal fat in the diets, the interactions with Calcium and its negative effect on the bioavailability of fat become an issue.

This negative interaction is difficult to incorporate into the AME of fat in the matrix as it depends both on the level of fat and the level of calcium but on the other side, it cannot be ignored if we want to ensure a minimum level of fat bioavailability.

All these interactions between the need for effective emulsification, the different composition of fat sources, the gut immaturity of young chicks, and the interactions with Calcium make the predictability of AME of a broiler starter diet very difficult. Nutritionists have different options to handle this situation.

They can develop a very detailed matrix to associate a different AME value to each fat source depending on each situation but that would bring much complexity to a matrix that is already very complex.

They could work with dietary emulsifiers as lysophospholipids. These technologies help for the emulsification required for the action of lipase but these lysophospholipids act as well as carriers for the formation of micelles and the penetration through the enterocyte’s membranes.

Finally, we may need to review the level of calcium in chicks and piglets’ diets as they already have difficulties digesting fat. Being a cheap ingredient, Calcium is often used in excess in diets. But when we consider its negative effect on phytase, on amino-acid digestibility, and on fat absorption, it may be necessary to review calcium content in diets. Some additives are available like pidolate (intracellular precursor of arginine required for the synthesis of Calcium transporter) to focus on improving calcium absorption enabling a reduction in its contents in the diet and limiting the anti-nutritional factors mentioned in this nutricle.