What is a "low carb" diet?
All weight reduction diets reduce calories so that the dieters take in fewer calories than they burn. Some diets are low fat, others restrict all groups of food except those that are extremely low in calories.
Low carbohydrate diets usually restrict carbohydrates severely, and increase calories from protein and fat. They reduce the calories from carbohydrates to 20-40% of calories per day.
Examples:
Atkins diet
Sugar Busters
South Beach diet (later stages)
Zone Diet
Carbohydrate Addict Diet
The Atkins diet does not use the "glycemic index". The other three do.
Most diets start with extreme carbohydrate restriction; less than 20 gm/day.
 

How do low carb diets work so fast?
When you take in fewer calories than you need your body uses fat and protein as fuel. You burn your fat and muscle. When you burn fat you produce ketones. Ketones are the chemicals that give you a fishy smell in your breath and urine. Ketones may make you less hungry but can also make you feel light headed. The usual way of getting rid of ketones is for the liver to combine them with sugar and using them for energy: in a low carb diet ketones buid up in the blood because sugar is limited, and the kidneys pass them out. Some pass out in sweat so you may smell fishy too!
When your body produces sugar from protein or fat the side effect is the production of water. This excess water passes out in urine.
In the early stages of the diets there is a marked weight loss from loss of water. (Diuresis). This is encouraging to the dieter and increases the likelihood of staying on the diet at least in the early stages. Most low cab diets increase protein more than fat. Protein satisfies hunger better than a low carb high fat diet, which also helps the dieter to stick to the diet in the early stages.
 

What is "Glycemic Index"?
Some but not all low carb diets use the "glycemic index" concept. When you eat carbohydrate it is broken down by saliva and stomach acid into sugar. Your blodd sugar rises, "glycemia - glyc=sugar; emia=blood". Hypoglycemia is low blood sugar, hyperglycemia is high blood sugar found in diabetes. The glycemic index rates foods by how much they raise blood sugar compared to a standard e.g., a slice of white bread. If a item of food raises the blood sugar only a little and not for long it has a low glycemic index. If it raises blood sugar a lot and for a longer time it has a high glycemic index.
 

What is the effect of low carb diet on cholesterol? Diabetes?
These diets do not raise cholesterol even though they substitute ft for some of the missing carb calories. Indeed the dieter's lipid profile (cholesterol, HDL-LDL, triglycerides - see "all about cholesterol") may improve. Not enough studies have been done to see the effect on diabetes. The diet puts a stress on the insulin system, which is bad, but lowers the degree of obesity which is good. (See metabolic syndrome).
 

Are low carb diets safe?
In general low fat diets are reported safe. They are often lacking in fibre, minerals and some vitamins so supplements are required: or consult with  a dietician who can modify the diet to make up for deficiencies.
Gout may be worsened, as may the tendency to kidney stones. Women at risk for osteoporosis need to pay special attention to the requirements of their skeleton.
Constipation can be a real problem.
Any increase in fat intake and decrease in fibre could put a person at risk for breast or bowel cancer (see breast cancer) so the diets that substitute fat for carbs have a theoretical danger but no studies have been published as yet.