High fat intake lowers hepatic fatty acid synthesis and raises fatty acid oxidation in aerobic muscle in Shetland ponies.

The research article explores how higher fat intake, replacing the same amount of energy from maize starch and glucose, impacts fat and sugar metabolism in the bodies of Shetland ponies, with a specific focus on the liver and muscles.

Research Design and Participants

• The study comprised twelve adult Shetland ponies divided into two dietary groups — a control group and a high-fat diet group.
• The control group was provided with a diet containing 15g of fat per kg of dry matter (DM), while the high-fat group received a diet with 118g of fat per kg of DM. The difference in energy from the reduction in carbohydrates was compensated by the increased fat content.
• This dietary routine was carried out for 45 days.

Results of the Research

• Switching to a high-fat diet led to lower levels of plasma triacylglycerol (TAG), the primary constituent of body fat in ponies, by 55%.
• A significant reduction in glycogen, the stored form of glucose used for energy, occurred in the masseter (a jaw muscle), gluteus (a major muscle of the hip), and semitendinosus muscles (a muscle of the thigh).
• Conversely, the TAG content of the semitendinosus muscle increased by 80% under a high-fat diet, indicating redistribution of fat storage.
• Activity levels of two key enzymatic components of fatty acid (building blocks of fat) synthesis in the liver, acetyl-CoA carboxylase and fatty acid synthase, diminished by 53% and 56%, respectively.
• Diacylglycerol acyltransferase activity, involved in TAG formation, remained steady irrespective of diet.
• A high-fat diet reduced the sensitivity of carnitine palmitoyltransferase-I (CPT-I), an enzyme crucial to fatty acid metabolism, to its inhibitor malonyl-CoA in the liver. This could enhance the transport of fatty acids to mitochondria for oxidation, thereby increasing fat burning.
• Notably, CPT-I activity increased in the masseter muscle with a high-fat diet, along with citrate synthase (an enzyme indicating cellular energy output), suggesting higher aerobic metabolic capacity in these muscles.

Conclusion

• The researchers concluded that feeding Shetland ponies a higher-fat diet appears to increase the transport and oxidation of fatty acids in highly-aerobic muscles, such as the masseter. This higher oxidative ability, combined with a reduced rate of new fatty acid synthesis in the liver, could contribute to a decrease in plasma TAG concentration observed under the high-fat diet.
• These findings could have implications for equine dietary management and the understanding of equine metabolic adaptations to high-fat diets.