Weekly 2-Day Fast (5:2 Diet) as Effective for Losing Weight and Waist as Continuous Dieting (-500kcal Every Day)

I suspect changing what the subjects ate would have had additional benefits on top of the reduced energy intake.

In the medical literature, there are two different interpretations of "intermittent fasting". While one of them refers more or less to what people from the fitness community will think of if you talk with them about "intermittent fasting" (i.e. eating only within a feeding window of 2-6h in 24h), most researchers use "intermittent fasting" as an umbrella-term for all sorts of time-restricted fasting regimens, including alternate-day fasting regimen and their clones, where you fast say 2, 3, or 4 days per week. Regimen such as the one scientists from the University of Melbourne Department of Medicine have recently compared to "classic dieting" (constant daily energy deficit) in 24 males 55–75 year-old war veterans with stable weight problems (body mass index (BMI) greater than or equal to 30 kg/m²).
The subjects were randomized to consume either a 5:2 diet or a standard energy-restricted diet (SERD | 2050 KJ (500 kcal) deficit per day) for 6 months. Weight, waist circumference (WC), fasting blood glucose, blood lipids, blood pressure and dietary intake were measured at baseline, 3 and 6 months by a blinded investigator. Here are the details from the study:

• The study was a single-centre, parallel group randomized controlled trial.
• SERD: Participants were required to follow a continuous daily energy-restricted diet (500- calorie daily reduction from average requirement) based on Australian guide to Healthy Eating (AGHE) dietary principles (low saturated fat, high fiber, moderate protein and carbohydrate). 
• IER 5:2 Diet (5:2 DIET): Participants were required to ‘fast’ for two non-consecutive days per week (restrict their daily calorie intake to 600 calories for the entire ‘fast day’) and eat ad libitum on the remaining 5 days (i.e. no specific dietary recommendations were made). Participants were encouraged to consume calorie-free beverages on fasting days. 
• Educational material and sample meal plans were provided to participants in each group, and all participants received five individual counseling sessions specific to their dietary intervention. 
• At the conclusion of the 3- month review, participants were asked to continue with their dietary regimen for a further 3 months. During this time, no formal review or further dietetic support was pro vided. 
• To monitor and maximize dietary compliance, participants were asked to keep diet diaries. Adherence to each dietary intervention was assessed by the dietitian by using patients’ self-recorded dietary diaries and self-reported diet histories taken during their dietetic appointments at 2, 4 and 8 weeks and at 3 and 6 months. 
• Participants were advised to maintain their current physical activity levels throughout the study and did not receive specific exercise counseling.

As you can see in Figure 2, both diets effectively reduced body weight (P = <0.001), waist circumference (P = <0.001) and systolic blood pressure (P = 0.001).

Figure 1: Total energy intake at baseline in the weight loss (<3 m) and follow-up (>3 m, but <6 m) phase (Conley 2017).

Without statistically significant differences in weight loss, i.e. 5.3 +/- 3.0 kg (5.5 +/-  3.2%) and 5.5 +/-  4.3 kg (5.4 +/-  4.2%) for the SERD group. A greater, but likewise non-significant difference was observed for the metabolically more relevant reduction in midsection, where the 5:2 group lost 8.0 +/-  4.5 and the SERD group lost 6.4 +/-  5.8 cm off their waists.

Figure 2: Changes (%) in body weight, waist circumference and systolic blood pressure after 3 and 6 months (Conley 2017).

Changes in diastolic blood pressure, fasting blood glucose or blood lipids were not observed in either dietary group. It should be said, however, that the subjects may have been overweight, but not sick - to expect changes in blood glucose and lipids that would achieve statistical significance with only N=24 subjects was thus not necessarily to be expected.

Figure 3: The subjects did not change their diet fundamentally, just like their protein, fat, and carb intake, the subjects' sugar, fiber, calcium, alcohol, and sodium intake was equally reduced (Conley 2017).

That's also because the dietary habits of the subjects obviously didn't change much: With macronutrient ratios of 22 and 21% of dietary energy from protein, 36 and 38% from fat, 35 and 36% from CHO and 7 and 5% from alcohol in the SERD and 5:2 diet groups, respectively, the veterans in both groups probably kept eating the same crap they did before the intervention. With a protein-modified fast and the lion's share of the 600kcal on the fasting day coming from protein, things may have looked fundamentally different, I suspect (more about PSMF).
References:

• Conley, Marguerite, et al. "Is two days of intermittent energy restriction per week a feasible weight loss approach in obese males? A randomised pilot study." Nutrition & Dietetics (2017).