We’ve all heard that the healthiest way to diet is to cut out fat. Then again we’ve also heard that the best way to lose fat fast is to cut “carbs”. There are many die-hard fans out there of nearly every type of diet out there, but many of the most popular diets all have one thing in common: they are centered around manipulating the ratios of the macro-nutrients (a term that is not wholly useful in light of our current understanding of biochemistry, but that we’ll save for a later post), namely, carbohydrates, fats, and protein. Protein is necessary for life. A diet far too low or far too high in protein is incompatible with any organism surviving for very long. This leaves us with the option of manipulating the amount of carbohydrates and fats in the diet. Expert opinions abound where randomized control trials lack. I’ve compiled a small literature review using some of the best sources available to me to help determine whether low carbohydrate diets are better than high carbohydrate diets in the long term for the reduction and long term maintenance of body fat while taking into account general markers for health like mortality from heart attacks, stroke, or other diseases.
A low carbohydrate diet limits daily intake to 30-130 g/d. A low fat [or high carbohydrate] diet typically limits fat intake to 10%-15% of total calories. Obesity is defined as an increase in body weight beyond the limitation of skeletal and physical requirement, as a result of an excess of fat in the body. A high quality literature review was published in 2009 by the Cochrane Collaboration covering six randomized controlled trials (with a total of 202 participants) comparing low glycemic index (LGI) (load) diets with high glycemic diets or “other” control (C) diets. The decrease in body mass, total fat mass, and body mass index was modest but significantly greater with LGI diets compared to C diets. The decrease in total cholesterol and LDL cholesterol was also significantly greater in the LGI diets compared to C diets. No study reported adverse effects, mortality, or quality of life data. The authors found that in studies comparing ad libitum LGI diets to conventional restricted energy low-fat diets, participants fared “as well or better” even though they were allowed to eat as much as they wanted. In other words, if you’re on a low carb diet you can eat what you like and you’ll still probably lose more weight than restricting calories on a traditional low fat diet. The authors concluded that “lowering the glycemic load of the diet appears to be an effective method of promoting weight loss and improving lipid profiles and can be simply incorporated into a person’s lifestyle.” 
Another randomized trial published in JAMA in 2011 compared low glycemic load versus a low fat diet in 73 obese young adults. Two diets were used with the LGI group having macro-nutrient ratios approximating 40% carbohydrate, 35% fat, 25% protein while the low fat group used ratios of 55% carbohydrate, 20% fat, 25% protein. The outcome measures used included body weight, body fat percentage determined by dual ray x-ray absorptiometry, and cardiovascular disease risk factors. The change in body weight and body fat percentages did not differ between the two groups. In participants that had a higher insulin response than the median response to an oral glucose dose, a sign of metabolic syndrome, the LGI diet produced greater weight loss and body fat loss compared to the low fat diet – 5.8kg lost versus -1.2kg, and -2.6% bodyfat versus -.9%. Let me repeat that again for emphasis – participants with metabolic syndrome lost more weight and a higher percentage of body fat when on the low glycemic index diet. Plasma high-density lipoprotein cholesterol and triglyceride concentrations improved more on the LGI diet – probably two of the most important markers for cardiovascular disease risk. Comparatively, low-density lipoprotein cholesterol “improved” or was lowered in both diets but more on the low fat diet. The make up of the LDL cholesterol was not discussed. LDL is actually a molecule (a lipoprotein for the biochemistry nerds out there) that actually has different types. There is small dense LDL and large fluffy LDL and they probably don’t signal the same health status in people. Currently there is not sufficient evidence to determine whether particle size has an impact on cardiovascular disease risk. 
A literature review published in 2011 in Nutrition in Clinical Practice concluded that “reducing carbohydrate appears to create a metabolic milieu that can positively affect appetite and reduce fat storage as well as, or more effectively than other dietary strategies”. One study used in this review to come to that conclusion was conducted in 2007 by Gardener et al, with a total of 311 overweight pre-menopausal women randomized to one of four popular diets: Atkins (<20g CHO/d; no caloric restriction), Zone (40% CHO, calorie-restricted), LEARN (55%-60% CHO, calorie-restricted), and Ornish (< 10% fat; no caloric restriction). The mean weight loss at 12 months was -1.6 for the Zone diet, -2.2 kg for the LEARN diet, -2.6 kg for the Ornish diet, and -4.7 kg for the Atkins diet. The Atkins groups also experienced the most improve metabolic effects including improved HDL, TG, and systolic blood pressure. 
A more recent trial conducted with 322 moderately obese individuals were randomized to one of three diets: a VLC with no caloric restriction, a calorie-restricted Mediterranean diet (MED), or a caloricly restricted low fat diet. The mean weight change was again, the greatest in the restricted carbohydrate group at -4.7 kg with the MED diet coming in close second with -4.4 kg lost. The serum cholesterol ratio compared to the HDL decreased in all groups, with the VLC group showing the greatest improvement with a relative decrease of 20% compared to the MED diet with a decrease of a 12%. The participants of this study were given instructions on how to structure their own diets. A food journal was used to determine adherence to the experimental parameters.  The success of this experiment and others using models that allow participants to create their own diets with given parameters may speak to the relative clinical effectiveness of each diet.
Interestingly, a study published in the American Journal of Clinical Nutrition in 2000 and another published by the American Society for Nutritional Sciences in 2001 showed that when a patient was advised to eat a low fat diet for cardiovascular health reasons, the fat is usually replaced with carbohydrate. For most patients, this leads to an elevation of fasting serum triglyceride levels accompanied by a decrease in the HDL-C ratio, the ratio between HDL cholesterol and total cholesterol. Therefore, the authors concluded that reducing dietary carbohydrate reliably reduces serum triglycerides, increases HDL cholesterol, and can improve other aspects of the lipid profile. This change occurred even in studies that did not show a decrease in weight in subjects. [4,5].
Additionally, in a randomized control trial published in 2006 by JAMA with a total of 48,835 post menopausal women aged 50-79 years of diverse backgrounds and ethnicities women were split into two groups, dietary intervention group and a control group. The mean follow up time was 8.1 years. This large study found that over 8.1 years, the experimental dietary intervention that reduced total fat intake and increased the intake of fruit, vegetables, and grains did not significantly reduce the risk of CHD, stroke, or CVD, in postmenopausal women and achieved only modest effects on CVD risk factors.  Conversely, in an observational study published in 2009 in the International Journal of Environmental Research and Public Health which followed 1,752 rural Swedish men found that the daily intake of fruit and vegetables was associated with a lower risk of coronary heart disease only when combined with a high dairy fat intake, and not when combined with low dairy fat intake. Choosing wholemeal bread, or eating fish at least twice a week showed no association with the outcome. 
A large Cochrane literature review published in 2009 titled “Interventions for preventing obesity in children” concluded that “the current evidence suggests that many diet and exercise interventions to prevent obesity in children are not effective in preventing weight gain, but can be effective in promoting a healthy diet and increased physical activity levels.” Assuming that the goal of exercise and diet interventions is to reduce the amount of fat and total body weight of the children participating, and not just behavioral modifications, which was one of the main outcomes measured, it would seem that most of the interventions (mostly low fat, low calorie diets) were in themselves not efficacious even though the children were successfully able to change their habits. Childhood obesity is a major risk factor for obesity in adulthood and many other co-morbidities including Type II diabetes, heart disease, and psychosocial impairment. 
Therefore, in light of the successes seen clinically and in observational studies of low carbohydrate diet strategies in treating obesity and its sequelae, it would seem that the literature supports a low carbohydrate diet for the loss of excess fat, the long-term maintenance of a healthy body weight and by reducing your risks of some major diseases. Check out Mark Sisson’s post on what he calls the Primal Blueprint Carbohydrate Curve. For most people adopting a “paleo” style of eating consisting of whole vegetables and fruits, nuts and seeds, well-sourced meats, and healthy sources of fats like olive oil, grass-fed butter, and coconut oil will bring them into a roughly “low-carb” diet that is well-supported by the existing science.
1. Hite AH, Berkowitz VG, Berkowitz K. Low-carbohydrate diet review: shifting the paradigm. Nutr Clin Pract. 2011 Jun;26(3):300-8.
2. Thomas D, Elliott EJ, Baur L. Low glycaemic index or low glycaemic load diets for overweight and obesity. Cochrane Database of Systematic Reviews 2007, Issue 3. Art. No.: CD005105. DOI: 10.1002/14651858.CD005105.pub2
3. Ebbeling CB, Leidig MM, Feldman HA, Lovesky MM, Ludwig DS. Effects of a low-glycemic load vs low-fat diet in obese young adults: a randomized trial.JAMA. 2007 May 16;297(19):2092-102.
4. Parks EJ, Hellerstein MK. Carbohydrate-induced hypertriacylglycerolemia: historical perspective and review of biological mechanisms. Am J Clin Nutr. 2000 Feb;71(2):412-33.
5. Parks EJ. Effect of dietary carbohydrate on triglyceride metabolism in humans. Journal of Nutrition. 2001;131:2772S-2774S.
6. Howard BV, et al. Low-fat dietary pattern and risk of cardiovascular disease: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006 Feb 8;295(6):655-66.
7. Holmberg S, Thelin A, Stiernström EL. Food choices and coronary heart disease: a population based cohort study of rural Swedish men with 12 years of follow-up. Int J Environ Res Public Health. 2009 Oct;6(10):2626-38. Epub 2009 Oct 12.
8. Summerbell CD, Waters E, Edmunds L, Kelly SAM, Brown T, Campbell KJ. Interventions for preventing obesity in children. Cochrane Database of Systematic Reviews 2005, Issue 3. Art. No.: CD001871. DOI: 10.1002/14651858.CD001871.pub2