Diet- Induced Disease Models

Modifications

It is important that when modifications are made, the remainder
of the diet should be identical to the unmodified control diet.This makes comparisons across experimental groups easy to make, since only one diet component is changing at a time. This concept is quite simple to understand when it comes to removing or adding components that do not have caloric content – vitamins and minerals for example. So when vitamin B6 is removed from a diet, no calories are removed – just the vitamin. Hence the experimental and control diet are different only in presence or absence of this vitamin.

What about changing dietary components that contain calories?

Protein, carbohydrate and fat? At this point, it is necessary to introduce a concept called the nutrient-to-calorie ratio . Not to be confused with the caloric density (the number of calories per gram of diet), this ratio compares the level of a particular nutrient (or nutrient group) per calorie of diet. Taking a look at the formula for the AIN-76A rodent diet (Figure below) we see which ingredients have caloric content. Using the standard Atwater physiological fuel values of 4, 4 and 9 kilocalories (kcal) per gram for protein, carbohydrate and fat, respectively, the 500 grams of sucrose, for example, contributes 2000 kcal to the diet. We now have the information we need to calculate the nutrient to calorie ratio for any nutrient. For example, this diet contains 10 grams of vitamin mix and 50 grams of cellulose per 3902 kcal.

Now that we calculate this ratio, why is it important?

The answer lies in the fact that animals will for the most part, eat for calories, not weight of food, in an effort to consume the same amount of calories over the long term. This means that if an animal is used to eating a low-fat diet and they are switched to a higher-fat diet which (because fat is such an energy-dense nutrient) contains more kcal per gram of food, they will (after a period of adjustment) spontaneously eat fewer grams of food. They do this in order to continue eating the same number of calories (not grams) of food as they were when eating the low-fat diet. The reverse is true if switched from a high- to a low-fat diet. Similarly, rodents will eat more grams of food when the levels of dietary fiber (which has no caloric content) are increased, thereby lowering the caloric density of the diet. (In reality the ability to eat for calories does not always hold true – some species/strains will not regulate feeding and will overeat when exposed to a very high-fat diet for example).

Knowing that the animals will generally eat for calories explains why diets of different caloric densities (high- and low-fat diets for example) should be formulated to have similar nutrient to calorie ratios. This ensures that per calorie of food consumed (but not per gram), animals consuming diets of different caloric densities will receive the same absolute amount of nutrients (except those changed by design).