Originally Posted by tandoorichicken (Post 16397)

Watch who you accuse of wiki-ing. Fat is utilized as energy in three different ways.

Direct quotes from Human Physiology, 10th Ed. by Widmaier, Raff, & Strang, Pub. 2006, "Ch. 16 - Regulation of Organic Metabolism and Energy Balance:"

Gluconeogenesis
"The 180 g of glucose per day produced by gluconeogenesis in the liver (and kidneys) during fasting supplies 720kcal of energy. Normal total energy expenditure for and average adult is 1500 - 3000 kcal/day. Therefore gluconeogenesis cannot supply all the body's energy needs."

Which leads us to...

Fatty Acid Metabolism
"Now we focus on the liberated fatty acids, which circulate bound to plasma albumin. (Despite this binding to protein, they are known as free fatty acids because they are "free" of glycerol). The circulating fatty acids are taken up and metabolized by almost all tissues, excluding the nervous system. They provide energy in two ways: (1) they first undergo beta-oxidation to yield hydrogen atoms, which go on to oxidative phosphorylation, and (2) acetyl-CoA, which enters the Krebs cycle and is catabolized to CO2 and water."

"The liver is unique, however, in that most of the acetyl-CoA it forms from fatty acids does not enter the Krebs cycle but is processed into three compounds collectively called ketones. Ketones are released into the blood and provide an important energy source for the many tissues, including the brain, capable of oxidizing them via the Krebs cycle."

"The net result of fatty acid and ketone utilization during fasting is the provision of energy for the body and sparing of glucose for the brain. Moreover, as just emphasized, the brain can use ketones for an energy source, and it does so increasingly as ketones build up in the blood during the first few days of a fast. The survival value of this phenomenon is significant: when the brain reduces its glucose requirement by utilizing ketones, much less protein breakdown is required to supply amino acids for gluconeogenesis. Accordingly, the protein stores will last longer, and the ability to withstand a long fast without serious tissue damage is enhanced."

A substantial caloric deficit during fat loss can be interpreted by a body as a fast or partial fast, throwing it into ketogenesis. This process is a survival mechanism, and as I said before, unless you have some serious, uncontrolled pathological condition that would prevent the above from happening, you aren't in any serious danger from your subsequent rapid weight loss.

Sorry for the long physiology lesson, but this should help everyone all around get a better picture of how ketogenesis really works in the body.

Originally Posted by farr1022 (Post 1828)

I started Fitday and my diet/excercise routine on 1/6 and weighed in at 261. This morning I weighed 248.4. I am not complaining but it seems too fast and I really hope I can sustain it.

Anyone else take off a big chunk of weight when they first started?

Originally Posted by midwestj (Post 16611)

I also read in numerous places that large initial losses can be attributed to burning up your glycogen stores as glycogen is made up of mostly water. As much as 10 to 15 lbs could be attributed to the lost glycogen, so no real body fat has been lost yet. Once an exercise routine is halted the body will restore the lost glycogen and the weight will be gained back. This is what many short term commitment dieters experience and can be disheartening when the weight so quickly comes back.

Originally Posted by joshuam168 (Post 16250)

I would be careful about very rapid weight loss. If it is all fat you are losing it can be bad. The reason the recommended weight loss is 2 lbs. per week. is because the chemical breakdown of fat causes ketones to be released in your body. Now, with a small amount of ketones your body can excrete them without harm. But, if you breakdown a lot of fat, and thereby release many ketones, they can build up in your bloodstream and cause ketoacidosis, which is basically your blood Ph has become too acidic. Ketoacidosis is not a good thing, which is why we all need to be careful. However, some people can lose lots of fat and have no problem, everyones different. And if you do have ketoacidosis you will know, your gona feel like crap. So don't worry that you have it;)