Laboratory animal diets basically fall into two categories: chows and purified ingredient OpenSource Diets. Chow diets have been used since the 1940s as the ‘background', ‘maintenance' or ‘control' diet in experiments. They are relatively inexpensive to produce and provide complete and adequate nutrition. Referred to as grain or cereal based, these diets typically contain ingredients such as ground corn, ground oats, alfalfa meal, soybean meal and ground wheat. Vitamins, minerals and fat are added to ensure nutritional adequacy.Chow formulas are generally ‘closed' formulas, meaning that the exact amount of each ingredient added is kept secret by the manufacturer.
Variability from batch to batch
An important point to remember is that each of the plant materials in chows contains many compounds, each inseparable from the next. Some of these are nutritive (protein, carbohydrate, fat, vitamin, minerals and fiber) and some are non-nutritive (for example, plant derived compounds collectively termed phytochemicals) components. Because the nutritional content of these plant materials will naturally vary with harvest location and across growing seasons, this means that chow diets will have variability from batch to batch.
For example, the soybean meal used in a chow today may not have the same percentage of protein (arguably the nutritional standard by which the ingredient is judged) as the soybean meal used 6 months ago. So when making a chow, one is left with two choices – to use the same amount of soybean meal every time the chow is made, or to account for nutritional differences by adding more or less soybean meal to ‘correct' for differences in the protein levels.
Actually, chows are made using both methods and each has disadvantages. If soybean meal levels are always kept constant, then the protein levels of the diet will vary with the protein levels of the soybean meal. With the second method, overall protein levels can be roughly maintained by varying the amount of soybean meal used in a particular batch of chow.
Variability in data over time
However, this raises a new issue – in keeping dietary protein levels constant by changing the level of soybean meal, what has happened to the levels of non-nutritive components of that soybean meal? Soybean meal (and other plant-derived ingredients) contains many varied and interesting phytochemicals, numbering in the hundreds. A subclass of phytochemicals are the phytoestrogens. These phytoestrogens can bind to estrogen receptors in the animal and have either pro- or anti-estrogenic effects. Since the progression of disease states such as atherosclerosis and cancer can be affected by such estrogenic or antiestrogenic activity, it may be advisable to use a diet without phytoestrogens altogether. Secondly, if soybean meal levels are varied across batches to account for differences in protein levels, it follows that the levels of phytoestrogens will vary from batch to batch. Such variability in phytoestrogens may translate into variability in data over time, leading to cost increases due to either repeating studies or having to use to larger numbers of animals per study. Neither of these outcomes is cost-efficient nor desirable.
Is it easy to report a chow?
One can give the name of the chow being used, but is it really the same as what was fed last year, especially down to the non-nutritive components? Arguably the answer is no given the variability in the ingredients used. Plus, since most chow formulas are closed, one can never truly know how much of each ingredient was used in this particular batch. Is it easy to repeat a chow? Using the same argument about ingredient variability, the answer here is also no.
Can a chow be modified as research progresses?
Modifications can mean removing something from or adding something to a diet. Given that each plant ingredient in chow can contain a dozen (or more) nutrients, removing a nutrient from the chow is not possible. For example, one could not study the effects of a very low iron diet using chow. There is just no way to remove the iron from any or all of the plant materials - it is like trying to remove the sugar from a baked apple pie.
This restricts chow modifications to additions. However, there are limitations here as well. As an example, let's examine high-fat diets. Given the increasing population of obese and diabetic people in Westernized cultures, research in these related areas has increased greatly in the last decade. Laboratory animals are fed high-fat diets in order to test the ability of therapeutic compounds to prevent or reverse obesity. While it is possible to make a high fat chow by mixing fat with powdered chow and either feeding it as such or pelleting the mixture, this should be done with caution, because as fat is added, the nutrient concentrations in the chow are diluted.
In this example, 20% fat has been added to a chow (800 gm chow plus 200 gm lard).
|Ingredient||Chow||Chow w/ 20% Fat|
While this effectively increases the fat from 12% to 48% of calories, it has also diluted the level of protein from 28% to 17% of calories. Thus the protein calories and all other nutritive and non-nutritive components have been reduced by 40%. This can be problematic for two reasons. First, such overzealous addition of fat can dilute the diet enough as to make it protein deficient, clearly not the intention when studying the effects of a high-fat diet. Secondly, this dilution effect makes comparisons to the control diet (presumably the unmodified chow) difficult. Not only will the experimental group be eating a higher fat diet, but per calorie of food they will also be eating less protein, vitamins, minerals and fiber relative to the control group. Hence when comparing data between the groups, it will be impossible to determine if differences in phenotype were due to changes in any one nutrient.
Since chows are not easy to report, repeat , and revise, what choice does the researcher have?
Purified Ingredient OpenSource Diets.
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