If your study involves in vivo imaging using fluorescence technologies, it’s very important that the ‘background’ or ‘noise’ signal be as low as possible. Unfortunately, many commonly used grain-based chow diets contain chlorophyll, which fluoresces at 680 nm and can therefore interfere with the detection of fluorophores. The result is that the GI tract of the animal will fluoresce and your signal of interest may be hard or impossible to quantify.
On the surface, it seems that the logical solution to this problem is to remove the chlorophyll from the diet. However, with chow diets, this is easier said than done. The reason is that chow diets are made from grains and other plant materials (ground corn, soy, wheat, oats, alfalfa) which are the source of not only the chlorophyll, but of nutrients as well. So, removing any chlorophyll-containing ingredients also means eliminating needed nutrients.
The problem of diet-induced background fluorescence can be avoided altogether by feeding what are called purified ingredient diets. Also called ’defined’ diets, they are made from ingredients such as casein, corn starch, sucrose, cellulose, soybean oil, vitamins and minerals. Since each ingredient essentially contains one nutrient, it’s possible to easily add or remove a single nutrient while keeping everything else the same. This is why, in the early days of lab animal nutrition research, purified ingredient diets were crucial in helping define the nutrient requirements of different species. Nowadays, researchers employ purified ingredient diets in many different research areas, including metabolic disease.
The reason these diets are so helpful in the field of in vivo imaging is also tied to the fact that the ingredients are so refined. Non-nutrient materials like chlorophyll are removed as the ingredients are refined and so they are absent from purified ingredient diets. As a result, the level of background fluorescence due to diet is essentially zero. Once an animal is switched from a chow to a purified ingredient diet, it takes about 7 days to wash out the chlorophyll signal in the GI tract.
Purified diets also have other advantages over chow diets. Aside from lacking chlorophyll, purified ingredient diets also do not contain phytogestrogens or heavy metals (which are found in chow diets at levels that can affect the animal’s phenotype). Since the ingredients in purified diets are very refined, there is minimal variation from batch to batch of diet. And finally, purified ingredient diet formulas are very easy to modify, meaning customized diets are readily available to the researcher.