Predicting metabolisable energy in commercial rat diets: physiological fuel values may be misleading

Abstract

Knowledge about metabolisable energy (ME) intake is crucial for various experimental settings in rodent studies. ME considers faecal and renal energy losses. In particular, faecal energy excretion can vary considerably between differentially composed diets. Thus determination of faecal energy losses, i.e. apparent energy digestibility, is the most important experimental approach to determine ME. Predictive equations for ME such as Atwater factors or an equation for pigs, which are frequently employed for rodent feed, consider an average energy digestibility for nutrients and average renal losses for protein. Both equations, however, were never validated for rat feed. We therefore determined experimentally the digestibility of energy (experimentally determined digestible energy 2 5·2 kJ/g digestible protein) and nutrients of eleven natural and five purified rat diets and compared the present results with the predicted values. Compared with natural diets, digestibility of gross energy (GE) and nutrients was higher by about 20 % in the purified diets (P, 0·0001). Mean GE digestibility in natural diets amounted to 71·4 % (range 53·3 -83·5 %; n 11). Atwater factors predicted ME with satisfactory accuracy in purified diets. In contrast, for natural diets, only the equation for pig feed gave acceptable estimates of ME. Choosing an inappropriate predictive equation for ME resulted in considerable error. For prediction of ME in mixed rat feed, we propose to use the equation for pig feed for natural diets and Atwater factors for purified diets. If the equation for pig feed cannot be applied we suggest using the lower modified Atwater factors instead of the 'original' Atwater factors to estimate the ME of a diet.Rodent feed: Commercial diets: Digestibility of energy: Digestibility of nutrients: Metabolisable energy: Rats Energy intake and utilisation are crucial preconditions for a variety of different experimental settings. In particular, rodent studies which are not focused on mainly nutritional aspects frequently declare energy contents of employed diets as gross energy (GE). The GE of rodent feed is routinely determined during the commercial production process and the food manufacturer usually provides this information along with the diet. However, for many studies more precise information about the actually 'available' energy would be desirable and expedient. This 'available' energy is termed digestible energy (DE) or metabolisable energy (ME). Subtraction of non-digested energy in faeces from the GE content of a diet gives diet-specific values for DE. Further subtraction of the remaining energy in urine or energy loss by microbial gas production from the DE value then provides the ME content. Of note, predictive equations for ME, such as the Atwater factors, usually neglect losses by microbial gas production.In general, rodent diets can be formulated as natural or as purified diets. Natural diets are composed of mixed ingredients such as different grains or soyabean meal. In contrast, purified diets use constant manu...

Effect of High Sugar Intake on Glucose Transporter and Weight Regulating Hormones in Mice and Humans

“…We here use values from the literature referring to Southgate and Durnin who showed that with bomb calorimetry determined and on the other hand calculated values are in good agreement [49]. Another report using a rodent model showed that Atwater factors predicted metabolisable energy with satisfactory accuracy in purified diets as we used here [50]. However, it is of note, that data available for our article was measured in humans or chicken [24], [51].…”

Isoenergetic Feeding of Low Carbohydrate-High Fat Diets Does Not Increase Brown Adipose Tissue Thermogenic Capacity in Rats

“…gastrointestinal uptake) is similar in all diets. To confirm the Atwater-formula based calculation of ME and assure similar digestibility of macronutrients in each diet, we had previously determined the ME content of each diet experimentally by digestion trials [9]. All diets were fed in a pair-feeding setting, since we explicitly wanted to exclude any effects which could potentially be attributed to differences in energy intake.…”

“…However, these potential mechanisms and the effectiveness of low carbohydrate diets have been questioned [8]. Moreover, we have recently shown that the loss of energy via excretion of ketone bodies is quantitatively negligible in rats fed a ketogenic diet and does not contribute substantially to overall energy expenditure [9]. In addition, ketosis was not associated with weight loss in humans [1].…”

Induction of ketosis in rats fed low-carbohydrate, high-fat diets depends on the relative abundance of dietary fat and protein

“…However, this theory has since been widely rejected (4,24). Also, we have previously shown that ketogenic LC-HF diet-induced energy loss via ketone bodies does not quantitatively contribute to the overall energy balance (10). In regard to energy expenditure, two recent investigations have shown that mice fed a ketogenic diet show increased energy expenditure, which could potentially explain LC-HF diet-induced weight loss (25,26).…”