After summarizing years of research into carbohydrate structure and metabolism, I had occasion to turn back to fat metabolism this week after running into a very knowledgeable Paleo enthusiast.
I discovered a couple of interesting branches of research on the Google Safaris that followed. One suggests that it is not caloric restriction in general that promotes longevity, but restriction of protein, and the other, restriction of carbohydrate. Both of these findings have been published in just the last 5 years.
What was more interesting to me though, was to understand how fat, or fatty acids in particular - as opposed to the glycerol that's also part of triglycerides - is actually metabolized by cells (Cellular Uptake of Fatty Acids), both adipose cells that store fat, and muscle cells that power the large skeletal muscles, through a pathway that requires zero insulin.
Also of interest is that 18 of the 20 amino acids in proteins can be converted to glucose via gluconeogenesis, and are the major source of energy in large skeletal muscles when catabolic muscle destruction takes place. When you eat too much protein, or incomplete protein, it's burned as carbs. By contrast, only the glycerin part of triglycerides can be converted to glucose.
Here's the exciting thing for cyclists and other endurance athletes. Fatty acids, which supply up to half of total energy needed for endurance athletes, have a completely separate, and parallel metabolic pathway from glucose, and can transition through the cell wall and into the cell mitochondria WITHOUT the aid of insulin. The implications of this are so far-reaching that I am still coming to grips with it. But already a few things are clear.
First, super-surges in strength almost certainly occur when energy demands are fed both through fatty acids AND high glucose levels while insulin levels are very high. This condition would occur after 5-10 minutes of relative rest, say to 50-60% of max HR, when liver glycogen, and ride fuel digestion products, have been added together to create high blood glucose levels, and then the 6-minute insulin release cycle starts by dumping huge amounts of insulin into the blood. If this were to occur when maximal fatty acids are available to the muscle cell, and sufficient oxygen is available, the muscle cells have as much fuel and oxidizer as they can ever hope to use.
Second, this explains why so many are suffering with metabolic syndrome, obesity, and type II diabetes. When you eat either carbs, or protein (via gluconeogenesis) your body cannot immediately use, you use the carbohydrate, small intestine, insulin pathway to fuel the cell's mitochondria. Worse, there is some good evidence from research at UCSF that insulin is the primary cause of aging via the "Grim Reaper" gene.
When you eat, or use fat stored in adipose tissue, only the small percentage of the triglycerides that is glycerol can be converted to glucose, and digested via the aqueous environment of the small intestine. Almost all of the energy in fat, in the form of fatty acids, is digested into the bloodstream by the liver with the help of bile salts, and those fatty acids are available to the cell directly, in a completely separate pathway, where insulin plays no role. Fatty acids are always a ready fuel, ready to go, delivered by their own private digestion pathway, and they never become toxic like glucose does in hyperglycemia.
Since the energy density of fatty acids is much greater than glucose, and elevated fatty acid levels are not toxic like glucose, the fat-based energy cycle doesn't have to be as tightly controlled as the glucose-based cycle. The glucose-based cycle is regulated against excess by adipose tissue mopping up glucose when there is no immediate demand from muscle, so sedentary consumption of carbs must result in storing fat to prevent hyperglycemia. The required speed of adipose mitigation of glucose levels depends of the GI of the carb.
By contrast the liver digests fat into the bloodstream, pulls it back out when VLDLs get too high, can convert stored glycogen into glucose, can convert fat's glycerin to glucose, and can absorb up to 2,000 calories worth of glucose in the form of glycogen. In short, it's a much more complete and flexible organ for regulating energy levels than is the pancreas, but you have to give it some fat to work with.
The push to move people to high carb, low fat diets has led us to run almost an entirely insulin-driven metabolism, both for powering muscle, and for storing fat. This is in contrast to our historical diet of mostly fat and protein driven diets. As a result our pancreases are dangerously overworked for most of our lives, are failing in many people, and the muscle and fat cells in our bodies are forced to consume glucose and insulin instead of primarily fatty acids - with only small amounts of glucose from liver glycogen, converted amino acids, and glycerine to support exclusively glucose-fueled organs like the brain. Wild swings in energy levels has created an epidemic of ADD, ADHD, and other behavioral problems in the "bargain".
In short, we're trying to run a marathon on one leg! It just doesn't work.
I'm always cautious when incorporating new information in my daily life, but I have been eating more omelets and have switched from skim milk to 1%. The effect on hunger is amazing. Tentatively, I intend to save carbs to fuel rides, and move more and more towards fat as part of the nutrient rich diet that I eat off the bike, sans carbo loading, and recovery.
On a personal note, I'm just stunned at the utter stupidity of the nutritionists who suggested this, or somehow believed a low fat diet was going to work out well. As Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology in this excellent video shows, there were some strong dissenters from the stampede to carbs and away from fats.
When the entire population of the US is 25lbs heavier than they were 25 yrs ago, and we have an epidemic of childhood obesity in 6 month old children, it's hard to argue that obesity is caused by lazy slobs who just can't be bothered to get off their butts and exercise. Something more profound is going on, and this video lays it out pretty well.
Give your pancreas a rest. Eat some fat, and live a longer and healthier life.
4 months ago