A Brief Explanation of Protein, Carbohydrate and Fat Metabolism

(Thanks to http://www.elmhurst.edu/~chm/vchembook/630proteinmet.html). The Citric Acid cycle is also known as the Krebs cycle. The diagram shows that Fat is not metabolised into Glucose 6 Phosphate (blood sugar). It has a different metabolic pathway into Acetyl Coenzyme A. There is no pathway which takes it to glucose. Carbohydrates are metabolised into Glucose 6 Phosphate and Proteins can be metabolised into Glucose 6 Phosphate through gluconeogenesis via Pyruvic acid (this process occurs in the liver). The net result of all this is neatly explained in the following table...

http://www.ncbi.nlm.nih.gov/pubmed/1991575
http://en.wikipedia.org/wiki/Insulin_resistance

So from the table above it is easy to see that we want to avoid carbs and be careful with fat since it increases apparent insulin resistance. One has to burn either carbs or fat - a protein only diet is not possible from natural foods. However there is no point in replacing carbs with protein, since your metabolism will still be gluconeogenic, making glucose 6 phosphate newly from protein by gluco-neo-genesis rather than breaking carbs down to release it by catabolism. I suppose one could say that if eating 100gm of most carbs at noon is the same as eating 100 gm of sugar at 12:45 pm for a diabetic, then eating 100 grams of protein at noon is the same as eating  25-75 grams of sugar at 1 pm for someone on a low carb or calorie diet due to partial gluconeogenesis of the amino acids in the protein (some goes to building up the body, some goes directly to Acetyl Coenzyme A and some goes to glucose through gluconeogenesis in the liver). 

So however counter-intuitive it may seem, the idea in a Ketogenic diet is to replace the excess carbs with fat, not with protein. 

However for diabetics and indeed for everyone else there are two caveats. Firstly eating more fat will increase your CVD risk and your insulin resistance. So you must counterbalance these with a significant dose of Omega 3 fats. Specifically you must take in at least 4 grams per day of DHA or EPA Omega 3 fatty acids from fish oil. Secondly the brain uses 20% of your 1500 kcal resting metabolic rate energy consumption which is 300 kcal. That is the energy from 75 grams of sugar. The brain will do anything it can to get that sugar including telling your body to eat itself if necessary. So we must take in 70-80 grams of carb per day to keep the brain from becoming destructive of our muscle mass. This is important for diabetics. Perhaps not so important for obese people who are not diabetic.

So ideally, to reduce blood sugar, we reduce the carbs, we increase the protein and we increase the fat a bit but counterbalance that increase with extra Omega 3. 

Some proteins are more likely to be turned into glucose than others. Crustacean protein (Shrimps, prawns, crab, lobster etc.) seems to be especially gluconeogenic - because they have no fat - they are almost all protein - so to get any energy from them at all they have to be melted down into sugar by gluconeogenesis! Gluconeogenesis is protein type dependent see http://en.wikipedia.org/wiki/Gluconeogenesis. Do not eat protein without fat - it is almost that same as having sugar.

Carbs are metabolized first into glucose in preference to fat or protein metabolism. So we must eliminate them. Fats are metabolized second in preference to proteins, so we must make sure that the body has sufficient fat not to indulge in too much gluconeogenesis of protein which is metabolized last, partly into Acetyl Coenzyme A and partly into glucose 6 phosphate. The metabolic priority of carbs then fats then proteins is nicely described on http://www.virtualmedicalcentre.com/anatomy/metabolism/26 as follows...

"Carbohydrates form the main energy source for the body. They are the most efficient at producing ATP or energy (meaning they produce lots more ATP per amount of the fuel broken down). The body preferentially breaks down carbohydrates first, and then fats and finally proteins only if the other two fuels are depleted. This is important as proteins are generally less efficient at generating energy. In addition, proteins perform several important functions so if they were broken down several systems could fail.

For example, in the event of starvation, the body has fewer carbohydrates available, so will start to breakdown the fat stores in the body. Once all the available carbohydrate and fat stores have been depleted, the body will start to break down proteins to provide energy."

There are products, some of them natural, which block the enzyme (amylase) which metabolizes carbs - see http://www.lef.org/magazine/mag2010/feb2010_Block-Absorption-of-Killer-Carbohydrates_01.htm. These can be effective for weight loss and type 2 diabetes. The trouble is that when you come off the carb blocker you will reverse all the advantages you have seen, unless you change your eating habits. And why change them whilst you are on the carb blocker? So we do not recommend that route.