This article is an abridged version of chapter 30 in the book 'Healthy Weight Loss --> Healthy Life: The essential guide to long-term healthy weight loss for Australian adults' (Read More....)
As I described here, to lose weight – you must consume less energy (kilojoules / Calories) than what you expend through physical activity. There are many ways you can do this of course, most of them not terribly healthy or realistically sustainable in the long run. (Read More....)
One of the more recent and faddish approaches to weight loss are low carbohydrate diets. Low carb diets certainly make you lose weight, though there is no secretive explanation as to why this is. Aproximately 50-60% of the energy most of us consume typycally comes from carbohydrate, so cutting back on our carbs simply resuts in energy restriction.
Dr Atkins Diet Revolution
Low carb diets originated with the book “Dr Atkins diet revolution”. Atkins claimed that obesity is the result of insulin resistance, which can only be combated by extreme carbohydrate restriction. He taught that people can lose more weight by following a carbohydrate restrictied diet than what you would by following a high carb diet of the same Caloric value, because of the difference on insulin function. (Whilst glycemic control was later found to be significant in weight management, his claims regarding the ratio of carbohydrates to other macronutients are simply incorrect.) His diet allows less than 20g of carbohydrate per day whilst providing large amounts of fat from meats and animal sources. This forces the body to utilize fat almost entirely as its energy source, consequently putting the body into a state of "ketosis".
Ketosis is a metabolic state that occurs when your body has run out of carbohydrate so instead starts converting fat into ketones to use for energy. Whilst this sounds fine in theory, other than the other problems it will have with your brain function, burning fat will make no difference to your body weight unless it is coming from your body fat. Eating a high fat diet such as the Atkins diet may indeed force your body to use fat for energy, but a lot of it will simply be coming from the fat in your diet – not necessarily the fat in your body stores. (Ketosis has since become a trendy term typycal of low carb fads, as anyone can go to a health food store and purchase ‘keto strips' which test for ketones in your urine.)
A recent clinical trial published in the May 2006 issue of the American Journal of Clinical Nutrition set out to test this theory.  Half the subjects in the study were given a diet of which only 5% of its energy came from carbohydrate (and 60% from fat) whilst the other half were given a diet of which 40% of the energy came from carbohydrate (and only 30% as fat). Both diets contained precisely the same number of total Calories, though the very low carb diet caused the subjects to enter a state of ketosis.
After 6 weeks, both groups showed improvement in insulin function though there was no significant difference in the amounts of weight lost between the two groups. The low carb, 'ketogenic' dieters reported feeling greater emotional discomfort (less 'vigour') however than the non-ketogenic diet group. Moreover, the ketogenic group also exhibited higher levels of inflammation when their blood was tested. The bottom line is that if someone is telling you that you need to be in a state of ketosis in order to successfully lose weight, they are lying.
The other thing about low carbohydrate diets is that they do tend to produce more rapid weight loss in the first few weeks or so, though this is largely due to fluid loss. Because carbohydrate stores in the body help maintain fluid levels, depleting it of carbohydrate will consequently results in dehydration. Studies have shown that after a couple of months or so, low carb diets do not produce and greater weight loss than do other diets of the same Caloric value. The initial impressive results (as reflected by the bathroom scales) however tend to be quite impressive to the dieter who will think that they have lost fat quickly, and usually continue along the low carb path, completely unnecessarily. 
The most disturbing thing about the Atkins diet is that it contains enormous amounts of saturated animals fats, very unhealthy amounts of meat, no fruit, very few vegetables and extremely low levels of fibre. We now know (from more recent population studies) that diets high in fatty meats and low in plant foods tend to increase the risk of heart disease and cancer.
Subsequent low carb diets however have promoted less unhealthy versions of the low carb fad, though are still associated with short term problems – most significantly in regards to mood and cognitive performance.
Studies have found that people perform better in memory tests after being given a slow-release energy low GL carbohydrate meal than if they are given a high GL meal or a meal with no carbohydrate at all. Although the precise biochemical mechanisms are not well understood, avoiding carbohydrates in the diet seems to be more likely to result in feeling depressed or angry.  That doesn't mean that filling up on carbs will necessarily make you feel better, but low carb dieting will more than likely make you feel worse than you need to.
 3. Anderson GH, Woodend D. Effect of glycemic carbohydrates on short-term satiety and food intake. Nutr Rev. 2003 May;61(5 Pt 2):S17-26
 Johnston CS, Tjonn SL, Swan PD, White A, Hutchins H, Sears B. Ketogenic low-carbohydrate diets have no metabolic advantage over nonketogenic low-carbohydrate diets. Am J Clin Nutr. 2006 May;83(5):1055-61.
 Bilsborough SA, Crowe TC. Low-carbohydrate diets: what are the potential short- and longterm health implications? Asia Pac J Clin Nutr. 2003;12(4):396-404.
 Benton D, Nabb S. Carbohydrate, memory, and mood. Nutr Rev. 2003 May;61(5 Pt 2):S61-7