Fat metabolism and the ketogenic diet

Fats provide 20 to 40% of the calories in an average daily American diet.

More than 95% of dietary fat is in the form of triglycerides which are a combination of three fatty acid (FA) molecules and a single glycerol molecule. The other 5% is cholesterol and phospholipids. Triglycerides are an important energy source while the cholesterol and phospholipids are important as building blocks for cell growth.

In the small intestine, triglycerides are cleaved into their component molecules – glycerol and fatty acids. The fatty acid molecules are then transported through the intestinal lining cells into the blood. As they circulate, they diffuse through cell membranes throughout the body where they are either directly metabolized as an energy source or reconstituted into triglycerides for storage for future use.

The bulk of the body's 50,000 – 60,000 stored fat calories are in fat cells (lipocytes) with 2,000-3,000 additional calories stored in liver and muscle cells.

Both fatty acids and glycogen (the storage form of carbohydrates) are metabolized into the same energy containing compound, Adenosine Triphosphate (ATP), to provide energy for the body's cells.

Decades of research suggested that a diet low in fat, with the bulk of daily calories from carbohydrates, was the healthiest diet. But in the past few years that recommendation has come into question.

Instead it's been found that replacing carbohydrate calories with fat calories may be the healthier approach, actually increasing HDL (good cholesterol) which suggests less long term cardiovascular disease.

 And dieting with 33% of daily calories from fat was actually a more effective long term weight loss strategy than a diet with fats limited to only 20% of daily calories.

If that's the case, why all the interest in a ketogenic diet?

The basis of both the Atkins low carbohydrate and the more severely carbohydrate restricted ketogenic diet was speculation that the combination of carbohydrate restriction, combined with calorie restriction, would force the body to turn to fatty acid metabolism for energy and in turn increase the rate of fat (weight) loss.

When there are inadequate carbohydrate calories available, we know cells use fat to power cells with the metabolic end product being ketones. This occurs overnight, when fasting, and in diabetics where carbohydrates cannot get into the cells. The cells turn to fat for fuel, and the patient becomes ketotic with an acetone (a ketone) smell on their breath.

Although logic supports the ketogenic argument, the ultimate outcome is not what we would predict.

Initially, a ketogenic diet gives dramatic results. On a standard calorie restricted diet, a study group lost about a pound a week. Switched to a ketogenic diet this increased to 3 ½ pounds in the first week.

But further investigation of their body composition found that the rate of fat loss had decreased by 50% with the bulk of the extra weight loss from water weight. It appeared that the with carbohydrate restriction, the body began cannibalizing its own protein for energy instead of using fat. The group lost less fat and more protein (and water).

In a real life scenario, a group of CrossFit trainees switched to a ketogenic diet based on the assumption that a switch to a fat based metabolism would help their endurance performance. They found their leg muscles shrank by 8%. Not the trade off they were expecting.

The moral to the story? Fats, protein, and carbohydrates all have a place in a healthy diet. Your body evolved to use carbohydrates to fuel muscle activity. While excess carbohydrates beyond those needed for daily activities are converted into fat, too few are also a problem as the body scavenges needed energy from the protein of its own cells.

References:

http://www.cptips.com/fat.htm

https://journals.sagepub.com/doi/pdf/10.1177/0310057X1804600506  Ketosis with fasting

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5309297/   Ketones and exercise, with good set of references.

https://onlinelibrary.wiley.com/doi/full/10.1002/(SICI)1520-7560(199911/12)15:6%3C412::AID-DMRR72%3E3.0.CO;2-8?sid=nlm%3Apubmed  Ketone metabolism.