• Rabbit_Models

Diet-Induced Rabbit Models for the Study of Metabolic Syndrome

by Conrad Strain, David Hill, Neill Gardiner

Introduction

Metabolic Syndrome is an umbrella term to describe an array of health conditions which can result from an unsuitable diet. Metabolic Syndrome (MetS) consists of diseases such as Obesity, Hypertension, Cardiovascular disease, Dyslipidaemia, and Type 2 Diabetes.

In contemporary society, metabolic syndrome has become a growing public health problem. Rates of cardiovascular disease, type 2 diabetes, Obesity and other metabolic conditions have been rapidly increasing over the last half century. This is due to a changes in the standard diet among the general population in many western countries.

When studying metabolic diseases in humans, rabbits have been an ideal candidate as an animal model for carrying out such research. The reason for using rabbits is because rabbits are phylogenetically closer to humans than other experimental animals such as rodents. Rabbits have very similar lipid, lipoprotein and metabolic features to humans, when compared to other laboratory animals such as mice, as seen in table 1 below (Fan et al., 2018) -

Rabbits have the same basic nutritional requirements as humans. For optimal health, rabbits need appropriate amounts of proteins, carbohydrates, lipids, fibre, minerals, water, and vitamins. If these nutrients are consumed in inappropriate ratios or amounts, serious metabolic conditions can develop.

All the constituents of a basic rabbit diet should take into account the age, gestation, lactation, breed, activity levels of the rabbit or any agricultural purposes for the animal.

The MSD Veterinary Manual recommend appropriate macronutrient ratio guidelines that are needed for rabbits in different conditions (E.g. Pregnancy). A brief summary of recommended guidelines are displayed in table 2 (Nutrition of Rabbits - Exotic and Laboratory Animals - MSD Veterinary Manual, 2022).

Obesity

Obesity is a complex disease which reduces the lifespan and quality of life of animals. According to the journal of Exotic Pet Medicine there are 6 potential factors for rabbit obesity these include, age, sex and neutered status, diet, housing and owner characteristic (Adji, Pedersen and Agyekum, 2022).

Obesity in rabbits has countless negative health consequences such as atherosclerosis, insulin resistance, hepatic lipidosis, and arthritis. It also increases the risk of post anaesthetic complications.

In an case control study carried out by Oscar Julian Arias-Mutis et al (Arias-Mutis et al., 2018), a group of New Zealand white rabbits were given a high fat and high sucrose diet and compare to a control group after 14 weeks and 28 weeks. The experimental group’s high fat diet consisted of lard (5%) and hydrogenated coconut oil (10%), and their drinking water was 15% sucrose solution.

During and after the study there were very noticeable changes to the Bodyweight (Kg) and Body mass index (Kg/meter squared) of the rabbits. Before the study, both the control group and the experimental group had very similar weights and BMI scores. The control group had an average weight of 4.35(0.15) and an average BMI score of 32.8(1.9). The group that was going to be experimented on also had similar scores with an average weight of 4.43(0.14) and an average BMI score of 32.9(1.9).

Half way through the study (after 14 weeks), significant changes could already be seen between the control group and the experimental group. The control group’s average weight had increased but not drastically as their weight was 4.49(0.12). Their average BMI score however did not change as it remained at 32.8(1.2). On the other hand the experimental group experienced significant weight change over the 14 weeks as the average weight was 5.42(0.17) and the average BMI score was 36.8(1.9).

At the end of the study (after 28 weeks), there was great disparities between the control and experimental group. The control group remained the same through out the study maintaining an average weight of 4.51(0.13) and scoring an average BMI score of 32.6(2.1).

4 In comparison the experimental group after the 28 weeks put on a very significant amount of body fat. Their average weight increased to 5.75(0.6) and the average BMI score was 39.3(6.0). The high fat and high sugar diet resulted in the rabbits becoming obese which is consistent with results that are seen when humans consume similar diets.

Another study was carried out by Admed Bilal Waqar et al (Waqar et al., 2010), Japanese White rabbits were placed into three different groups. Two groups on high fat diets, and one group a diet consisting of normal rabbit chow. The groups that were on the high fat diet were fed a diet of consisting of coconut on top of their standard rabbit chow over the course of 22 weeks. One of the high fat groups had a diet consisting of 3% coconut oil. The other high fat group had a diet consisting of 10% coconut oil.

The rabbits that were on a diet that consisted of 3% coconut oil did not develop obesity, however they did develop an array of other diseases that are associated with metabolic syndrome such a Hyperglycaemia, Hypertension and Dyslipidaemia. Rabbits in the study that were fed a diet that consisted of 10% coconut oil did develop obesity as well the listed diseases found in the 3% coconut group.

Hypertension

Hypertension is a metabolic syndrome which is high blood pressure in the arteries (the vessels from the heart to the rest of the body). Although hypertension shows no symptoms, in the long run it can cause stroke, coronary heart disease and heart failure.

Metabolic diseases induced by diet are observed by PG Burstyn and DR Husbands (Burstyn and Husbands, 1980). In this study rabbits were fed coconut, palm or safflower oil. In some of these diets they were also given cellulose. The blood pressure was measured over the duration of two months in a non-invasive method. It is seen that the blood pressure was always increasing with the fat enriched diet and with the addition of cellulose it was seen to have diminished its effect on the blood pressure. However it was then seen that the cellulose had no effect on the blood pressure with the absence of fat. The rabbits used for this experiment were weaning New Zealand Whites weighing in around one kg and one and a half kg.

The rabbits were weaned onto a controlled diet for 3 weeks where their blood pressure was taken using a non-invasive method. This non-invasive method was carried out by using a thin, transparent balloon to occlude the blood flow in the ear from the unanaesthetised rabbit. The blood pressure becomes constant after about 6 or 10 days. There were three fats used. Coconut oil which is saturated, palm oil a solid at room temperature is half saturated and safflower oil largely polyunsaturated. The study was divided into two experiments. In the first experiment twenty-nine rabbits of both sexes were divided up into four groups containing different amounts of cellulose and coconut oil carried out over 8 weeks.

The groups and diets were as follows: 9 animals with 20% coconut oil, 9 animals with 20% coconut oil and 20% cellulose, 5 animals with 5% coconut oil, 5 animals with 5% coconut oil and 20% cellulose. The rabbits weighed in at around 3 to 3.5 kg after the study, gaining an average of 2kg throughout the experiment. In the 2 groups of 5 containing 5% coconut oil the blood pressure did not change that much whether or not they had cellulose in their diet or not. However the groups containing 20% coconut oil showed increasing blood pressure which became clearly different from that of the control. The maximum increase in the blood pressure was seen in the cellulose poor group at an increase of 17.2% compared to the cellulose rich group was seen to have an increase of 6.7%. It is evident that hypertension is increased with a fat enriched diet and relieved with the addition of cellulose into the diet.

In the second experiment fifteen rabbits were taken of both sexes and divided up into three groups. They received diets consisting of 20% palm oil with no cellulose, 20% palm oil with 20% cellulose and 20% safflower oil with no cellulose, over the course of 9 weeks. There was an increase in blood pressure in the groups fed diets containing 20% palm oil and 20% safflower oil. The results show very interesting factors where the maximum increase was seen in the cellulose poor 20% palm oil diet which was 30.2% increase and 21.2% increase in the cellulose rich palm oil diet. The safflower oil diet there was a 20.0% increase in blood pressure.

Among the 3 low fibre groups the palm oil appeared to have the largest effect on the blood pressure, safflower being less effective and coconut oil being the least potent. The results somewhat suggest that fats containing longer or longer fatty acids like palm oil and safflower oil are more hypertensive than that of the coconut oil. The palm oil is seen to increase the blood pressure more as it is more saturated than the unsaturated safflower oil. The discussion of this study is clear as It was seen that with the addition of cellulose to the diet with palm oil there was a delayed hypertension effect.

Hyperglycaemia

Hyperglycaemia is when there is excess glucose in the bloodstream and is often associated with diabetes mellitus. Hyperglycaemia can become more frequent when the rabbits are fed a high-fat diet. In three different studies the effect of four different diets can be observed.

The first study observed was carried out by Waqar et al (Waqar et al., 2010), on Japanese White male rabbits. The rabbits were fed firstly on a diet containing 3% coconut oil for a duration of 22 weeks, not only did the results show that the rabbits had acquired hyperglycaemia but hypertension and dyslipidaemia also.

In the second part of the study the rabbits were fed on a diet consisting of 10% coconut oil, which showed that not only did it show the same finding as the first part of the study but the rabbits had become obese along with the three other conditions mentioned above.

The second study was carried out by Carroll et al (Carroll, King and Cohen, 2004) on New Zealand White female rabbits where the rabbits were fed for a duration of 12 weeks on a diet of 15% fat, 10% corn oil and 5% lard. The oils used in these studies differ in structure as the corn oil which was used in the study involving the New Zealand White species can be categorized as a polyunsaturated fat while on the other hand the coconut oil used in the study involving the Japanese White species can be categorized as a predominantly saturated fat.

The studies suggest that a low and high concentration of the coconut oil both cause Hyperglycaemia but only a high concentration causes obesity.

The third and final study was carried out by Alacron et al (Alarcon et al., 2018) and was based on the Hybrid Flanders rabbit (Cross between the Flemish Giant, Champagne D’ Argent and Californian breeds). These particular rabbits were fed on a diet similar to the Carroll et al study, just a higher concentration and for a shorter period of time.

This diet was comprised of 18% fat, 10% corn oil and 8% lard for a 6 week time period, the results observed were very similar to that seen in the study by Waqar et al where the Japanese White species was fed on a 3% coconut oil diet where the rabbits did not become obese.

Atherosclerosis

Atherosclerosis is the thickening of the walls of arteries. Deposits of fatty substances such as cholesterol build up in the inner lining of arteries. The deposits then harden to become plaque. The plaque build up eventually narrows the lumen of artery.

The history of using rabbits for studying atherosclerosis dates back to the beginning of the 20th century when a Russian scientist named Nikolai Anichlov carried out many experiments on rabbits. His studies consisted of feeding rabbits very high cholesterol diets. All the rabbits on the high cholesterol diet developed atherosclerosis, and all the rabbits that were fed a diet of hay and vegetables did not develop atherosclerosis (Konstantinov et al, 2006).

An experiment of Kolodgie et al carried out an experiment on seventy male new Zealand white rabbits on a diet of standard chow. The experimental group then had cholesterol added to their diet while the control group were kept on the standard chow. After eighteen weeks, the results found that the rabbits that were fed standard chow remained healthy. However the rabbits that were fed the diet that included cholesterol had developed coronary atherosclerosis (Kolodgie et al., 1996).

Conclusion

In conclusion rabbits are a suitable animal to study metabolic syndrome. These dietary experiments can be very important to further understand metabolic disorders in humans. Metabolic disorders such as Obesity, Hypertension, Hyperglycemia, and atherosclerosis are an ever growing public health problem. As more of the global population develop metabolic syndrome, it is ever important to carry out such experiments in order develop a better understanding of what causes such diseases.

Reference List

Adji, A., Pedersen, A. and Agyekum, A., (2022). Obesity in pet rabbits (Oryctolagus cuniculus): A narrative review. Journal of Exotic Pet Medicine, 41, pp.30-37.

Alarcon, G., Roco, J., Medina, M., Medina, A., Peral, M. and Jerez, S., (2018). High fat dietinduced metabolically obese and normal weight rabbit model shows early vascular dysfunction: mechanisms involved. International Journal of Obesity, 42(9), pp.1535-1543.

Arias-Mutis, Ó., Genovés, P., Calvo, C., Díaz, A., Parra, G., Such-Miquel, L., Such, L., Alberola, A., Chorro, F. and Zarzoso, M., (2018). An Experimental Model of Diet-Induced Metabolic Syndrome in Rabbit: Methodological Considerations, Development, and Assessment. Journal of Visualized Experiments, (134).

BURSTYN, P. and HUSBANDS, D., (1980). Fat induced hypertension in rabbits. Effects of dietary fibre on blood pressure and blood lipid concentration. Cardiovascular Research, 14(4), pp.185-191.

Carroll, J., King, J. and Cohen, J., (2004). Hydralazine treatment alters body composition in the rabbit model of obesity. Acta Physiologica Scandinavica, 181(2), pp.183-191.

Fan, J., Chen, Y., Yan, H., Niimi, M., Wang, Y. and Liang, J., (2018). Principles and Applications of Rabbit Models for Atherosclerosis Research. Journal of Atherosclerosis and Thrombosis, 25(3), pp.213-220.

Kolodgie, F., Katocs, A., Largis, E., Wrenn, S., Cornhill, J., Herderick, E., Lee, S. and Virmani, R., (1996). Hypercholesterolemia in the Rabbit Induced by Feeding Graded Amounts of Low-Level Cholesterol. Arteriosclerosis, Thrombosis, and Vascular Biology, 16(12), pp.1454-1464.

Konstantinov, I. E., Mejevoi, N., & Anichkov, N. M. (2006). Nikolai N. Anichkov and his theory of atherosclerosis. Texas Heart Institute journal, 33(4), 417–423.

MSD Veterinary Manual. (2022). Nutrition of Rabbits - Exotic and Laboratory Animals - MSD Veterinary Manual. [online] Available at: <https://www.msdvetmanual.com/exotic-and-laboratory-animals/rabbits/nutritionof-rabbits?fbclid=IwAR1aaLn8XC_YMF6RSZR2oXg7Bw9OPBUy4mvAw793Es8mBuoQOK4lnFLKWo> [Accessed 19 March 2022].

Waqar, A., Koike, T., Yu, Y., Inoue, T., Aoki, T., Liu, E. and Fan, J., (2010). High-fat diet without excess calories induces metabolic disorders and enhances atherosclerosis in rabbits. Atherosclerosis, 213(1), pp.148-155.