Reviewed: Morimoto-Kobayashi Y, Ohara K, Ashigai H, et al. Matured hop extract reduces body fat in healthy overweight humans: a randomized, double-blind, placebo-controlled parallel group study. Nutr J. March 9, 2016;15:25. doi: 10.1186/s12937-016-0144-2.

Editor’s note: This study was sponsored by Kirin Company, Ltd. (Yokohama, Japan), a beverage company with a brewery division. Nine of the study authors are employees of the company.

Obesity is on the rise worldwide and is associated with an elevated risk of developing metabolic diseases such as diabetes and cardiovascular disease. Dietary changes, including the addition of functional foods with certain bioactive properties, and other lifestyle modifications are recommended to address this condition. Hops, the flower heads of the hop (Humulus lupulus, Cannabaceae) plant, are traditionally used in the beer-brewing process to add bitterness and flavor. When stored, hops produce bioactive matured hop bitter acids (MHBAs), which have been shown to decrease body fat in animal models. This randomized, double-blind, placebo-controlled, parallel-group trial investigated the impact of a mature hop extract (MHE) on body fat in healthy, overweight subjects.

The trial was conducted by TTC Co., Ltd. (Tokyo, Japan), a contract research organization, from May 2014 to December 2014. Included subjects had a body mass index (BMI) of at least 25 but less than 30 kg/m2. Subjects were excluded from the study if they were on a diet, used pharmaceuticals or dietary supplements to treat body fat or lipid metabolism, regularly consumed hop-containing foods or excessive alcohol, had one or more systemic diseases or abnormal blood work, or were pregnant or lactating, among other criteria.

To create the MHE, the authors aged 300 g of hop pellets (Hopsteiner GmbH; Mainburg, Germany) for 120 hours and then extracted with warm water for one hour. After several filtration steps, the resultant MHE was standardized to 18.3% MHBAs. The material in the trial consisted of 350 mL of MHE test beverage (containing 35 mg of MHBAs) or placebo beverage. The study authors do not describe the contents of the placebo, but mention that it matched the MHE preparation in taste and appearance. The energy (kcal), carbohydrate, and fiber contents of the MHE beverage and placebo also were matched.

From a total of 511 subjects screened, 200 were randomly assigned (100 men and 100 women) to either the treatment group or the placebo group. Subjects consumed either the MHE drink or placebo beverage once daily for 12 weeks and returned for a follow-up visit four weeks after the test period ended. Study visits occurred at baseline, four, eight, and 12 weeks of treatment, and at 16 weeks (the follow-up visit). At study visits, the MHE or placebo drink was consumed after physical parameters were measured. Subjects completed a lifestyle questionnaire during the screening period, and they were encouraged to maintain their lifestyle throughout the study. They were also instructed to avoid alcohol the day before study visits and to fast beginning at 10 p.m. the night before clinical visits.

Body weight, body fat ratio, waist and hip circumference, systolic and diastolic blood pressure, and pulse rate were measured at each visit. Abdominal fat area was measured with a computerized tomography (CT) scan at baseline, eight, 12, and 16 weeks. Blood and urine parameters were assessed, and subjects kept a daily diary recording their food intake, steps taken (measured using a pedometer), and other physical activity. Baseline parameters were not significantly different between groups. The primary endpoint was any decrease of abdominal fat, with secondary endpoints being changes in other physical parameters. Subjects reported any adverse side effects to the investigators.

The treatment group had a significant decrease in energy intake at the 16-week time point compared to baseline (1,728.5 ± 46.3 vs. 1,810.1 ± 50.9 kcal/day, respectively; P < 0.05). However, the treatment group’s decrease in energy intake was not significantly different than that of the placebo group at week 16 (1,769.7 ± 45.7 kcal/day). The treatment group’s fiber intake also decreased after four weeks compared to baseline (9.98 ± 0.34 g/day vs. 10.79 ± 0.47 g/day, respectively; P < 0.05).

In both groups, visceral fat area (VFA) and total fat area (TFA) significantly decreased after 12 weeks compared to baseline (P < 0.01 for both VFA and TFA in both groups). The VFA and TFA of the MHE group were significantly less than those of the placebo group after 12 weeks (P < 0.05 for both VFA and TFA). Subcutaneous fat area (SFA) also decreased significantly in both groups after 12 weeks compared to baseline (P < 0.01 in both groups), but there was no significant difference in SFA between groups at any time point.

Following four, eight, and 12 weeks of treatment, the body fat ratios of those in the treatment group were significantly less than those of the placebo group (P < 0.05 at each time point). Body weight and BMI were significantly lower in the treatment group compared with the placebo group after four weeks (P < 0.01 for both). In addition, body weight and BMI in the treatment group were significantly decreased after 16 weeks compared to baseline (P < 0.01 for both), with no significant changes in the placebo group. Waist and hip circumference also were significantly smaller in the treatment group after four weeks compared to placebo (P < 0.05 for both).

In the placebo group, systolic blood pressure was significantly higher after 16 weeks compared to baseline (P < 0.05). In the treatment group, pulse rate was significantly slower after 16 weeks compared to the placebo (P < 0.05).

Adverse side effects were reported by 47 subjects in the treatment group and 43 subjects in the placebo group, with the most common one being cold-like symptoms. None of the effects were determined to be associated with the test material.

This study demonstrated that the consumption of MHE decreased fat accumulation in overweight subjects. Also, energy intake was significantly decreased in those taking MHE, which suggests that appetite suppression may be involved. The appetite-suppressing effects may be linked to the MHE's bitter compounds, which have been shown to impact appetite and satiety by acting on the gastrointestinal tract. It is suggested that future studies should have a longer duration. This study had significant placebo effects, the reasons for which were unclear to the authors. Also, the average energy intake of the study subjects in Japan was low compared to average energy intake in Western societies. Despite this, MHE may be useful for weight loss.

—Amy C. Keller, PhD