The other pathway uses ketone bodies, derived from body or dietary fat, in the "fatty acid spiral" to power cells. As you can see if you look at this link, the path from source to energy is much more direct using ketone bodies than it is using glucose. Also, one of the nice things about the fatty acid spiral is that it has completely absorbable by-products of CO2 and H20, whereas the krebs cycle produces lactic acid, which can cause muscle knotting and soreness. This can be of great benefits to anyone who works out, works hard or even occasionally has to stress his or her body more than usual.
A final point before the nitty-gritty. If you are looking at this because you need to lose weight, remember that you did not gain your weight overnight. Often a low carber will loose a dramatic amount of weight and be disappointed when that level of loss is not maintained. As you get smaller your calorie needs will be reduced. When they meet in the middle you'll be on a maintenance diet, not a loss diet. It may be necessary at that point to make a conscious reduction in calories consumed, as outlined in this table, which is based on a theoretical me (female, 41 year old, 5'2" at various weights, no exercise assumed). Since one pound of weight gain equals 3500 kcal(the "correct" term, hereafter simple "cal"), a 500 (3500/7) calorie per day reduction is anticipated to reduce weight by one pound/week.
You will notice that for a sedentary person (lazy me) the difference in maintenance calories at 20 lb differences in weight is on the order of 100 calories per day. Activity multiplies this difference. My theoretical me, exercising at 180 lbs can have 2101 calories/day. My lazy theoretical me can only have 1724. That's over 377 calories difference at 180, but it drops to 329 at 140. I used this online calculator to derive these numbers. According to it, at age 31, and weighing 220 lbs, I could maintain at 2002 cal/day with no exercise, but I have to say there's no way I ate that much, yet I still could not lose.
So ballpark it at 100 cal/day per 20 lbs body weight, in this range, so when you've lost 60 lbs your body will need 300 fewer calories per day, and if your deficit was only about that you'll suddenly be in equilibrium. The larger you are to start, the bigger that difference and the longer it takes to get there. My 220 lb lazy self could have lost weight at 1502 cal/day, but my 140 lb lazy self would be maintaining at that level.
| Weight in pounds | Maintain weight | Lose 1lb /week | Maintence with 5 days Exercise | Loss with 5 days Exercise |
| 220 | 2002 | 1502 | 2440 | 1940 |
| 180 | 1724 | 1224 | 2101 | 1601 |
| 160 | 1615 | 1115 | 1968 | 1468 |
| 140 | 1506 | 1006 | 1835 | 1335 |
| 120 | 1396 | 896 | 1702 | 1202 |
The Tradeoffs page addresses changes your body will undergo if you elect to go low carb, and suggests ways to deal with the ones that aren't as popular. Since your body doesn't like change, you may find you experience digestive problems, lethargy, sleep changes and other differences as your metabolism re-tools. Some of these may be problematic in the short term, but in the long term they will resolve themselves. Most people take two or three weeks to adjust to a lower daily intake of carbohydrates. This can be altered by how drastically the diet is changed to begin with. The links below may be of some assistance:
Why "low carb" isn't always, and gets a bad rap.
Protein Power web site, with blog by Dr. Mike and lots of food porn by Dr. Mary Dan. This is a commercial site where you can buy stuff, if you need it. Or not. Dr. Mike's blog is full of great information, both on the benefits of low carb and on the frustrations the low carb world deals with, both as eaters and as medical professionals.