1.a Low-Calorie Diets

Rationale: Thirty-four RCT articles examined the impact on weight loss of an LCD consisting of approximately 1,000 to 1,200 kilocalories/day. Many of these LCDs also promoted low fat intake as a practical way to reduce calories. Twenty-five RCT articles covered interventions lasting 6 or more months (346-348, 350, 354, 355, 359, 363-366, 368, 369, 373, 375, 378-380, 399, 400, 406, 413, 431-433). Another nine RCTs lasted 12 to 21 weeks (349, 351, 356, 370, 371, 377, 384, 434, 435). 

All of the studies, regardless of the length of the intervention, showed that LCDs result in weight loss. From the 25 RCT articles with a duration of 6 months, compared to controls, LCDs brought about a mean weight loss of approximately 8 percent of body weight over a period of 6 months and up to 1 year. The nine studies with an intervention lasting from 3 months up to 6 months also averaged approximately 8 percent weight loss compared to controls. Four studies that included a long-term weight loss and maintenance intervention lasting 3 to 4.5 years reported an average weight loss of 4 percent over the long term (348, 359, 366, 379).

Four RCTs testing the effects of LCDs alone on weight loss also had measures of abdominal fat, as measured by waist circumference (365, 369, 375, 399). The studies showed consistently that waist circumference decreases along with weight loss produced by LCDs. After 6 to 12 months, LCDs resulted in a mean weight loss of as little as 0.3 BMI unit to as much as 11 kg (24.3 lb) body weight, and a concomitant reduction in waist circumference of 1.5 to 9.5 cm compared to controls.

Rationale: Ten RCTs examined whether weight loss through diet alone without increased physical activity had an effect on cardiorespiratory fitness as measured by VO2 max when compared to controls with no weight loss. Four RCTs reported no improvement in VO2 max (363, 375, 399, 435), while six RCTs reported an improvement (346, 369, 380, 384, 406, 432). In three of the six studies, improvement in those on the LCD was shown because the control group had a large decrease in VO2 max (346, 406, 432). In all six studies that showed improvement, VO2 max was expressed as ml/kg/min rather than liters/min. Because the ratio of ml O2 /kg/min increases with weight loss alone and not necessarily from an improvement in oxygen uptake, results from studies reporting VO2 max in terms of ml O2 /kg/min are equivocal. The three studies that reported VO2 max as liters/min consistently showed no improvement from diet alone (375, 399, 435).