Adding Protein and Fiber Before Breakfast and Lunch Doubled Weight Loss
One of the challenges of reduced-calorie diets is the inability to control appetite. Reductions in food intake can lead to activation of neurological pathways that increase hunger and food cravings. Developing dietary interventions that take into consideration these signaling pathways may be important in helping reduce daily food intake to sustain caloric restriction.
Higher protein and fiber diets promote weight management and metabolic health. But most studies use very high amounts—25-35% of daily energy from protein, 30+ grams of fiber per day. These quantities reduce generalizability and introduce difficulty in making recommendations that can be followed by the general population. Recent focus has shifted towards examining whether a combination of more moderate protein and fiber intake may elicit similar beneficial effects.
A study published in The Journal of Nutrition examined whether greater weight loss and positive changes in metabolic outcomes could be achieved with twice-daily consumption of a high-protein and fiber-based supplement shake consumed as a preload before breakfast and lunch compared with an isocaloric low-protein, lower-fiber placebo. Two hundred and six overweight and obese adults were randomly assigned to consume either a shake with 17 g protein and 6 g fiber or an isocaloric control with 1 g protein and 3 g fiber, 30 minutes before breakfast and lunch for 12 weeks. Both groups were prescribed an energy-restricted diet (500 kcal per day deficit).
The high-protein, high-fiber group lost significantly more weight and demonstrated greater improvements in cholesterol, insulin sensitivity, and adiponectin compared with the control group—despite consuming identical calories from the supplement shakes.
The Protocol: Two Preloads Daily, 12 Weeks, 500-Calorie Deficit
Participants were randomized to high-protein and fiber (HPF, n=103) or low-protein and lower fiber (LPF, n=103) groups. The HPF shake contained 17 g protein and 6 g fiber. The LPF control contained 1 g protein and 3 g fiber. Both shakes provided 160 calories each and were matched for color, flavor palatability, and vitamin and mineral fortification.
Both groups were instructed to consume their respective shake preloads 30 minutes prior to both breakfast and lunch (total daily intake: 320 calories from shakes, consumed as two separate doses). Both groups were assigned a 500-calorie per day deficit from calculated energy needs via an exchange-based diet plan and were advised on guidelines for physical activity (2.5 hours per week of moderate to vigorous-intensity exercise).
Participants were overweight and obese adults (BMI 27-35 kg per square meter, 70% female) between ages 25 and 50 who had maintained stable weight for the past 6 months. The study duration was 12 weeks (84 days), with measurements taken at days 0, 28, 56, and 84. Body composition via DXA was assessed at day 0 and day 84.
Of 206 participants initially enrolled, 152 completed all study visits. The per-protocol analysis included 133 participants who met compliance criteria (consumed at least 80% and less than 120% of product provided) and had complete primary outcome data, with 68 in the LPF group and 65 in the HPF group.
Weight Loss: HPF Group Lost 83% More Weight Than Control
In both intent-to-treat and per-protocol analyses, both groups exhibited a decrease in body weight over time, with participants in the HPF group exhibiting greater weight loss compared with baseline than those in the LPF group. At day 84, the HPF group lost 3.3 kg compared with 1.8 kg in the LPF group—a statistically significant difference despite both groups consuming identical calories from the supplement shakes.
Both groups exhibited a decrease in percentage body fat over time, and the HPF group did not differ from the LPF group at any time. Both groups had decreased fat tissue percentage and increased lean tissue percentage from baseline. Both groups exhibited a reduction from baseline in waist and hip circumference starting on day 28.
The increase in weight loss from the HPF preload was comparable to previous studies showing that the addition of 25-30 g protein per day appears sufficient to elicit long-term improvements in weight management. The HPF group was assigned an additional 32 g protein per day compared with the LPF group. Most Americans consume equal to or greater than 25-30 g protein at lunch and dinner but fail to meet this recommendation at breakfast. The habitual addition of a high-protein preload before both breakfast and lunch increases daily protein intake and supports the goal of consuming 25-30 g protein at each meal.
Similarly, higher intake of fiber, particularly soluble fiber, has been associated with significant reductions in body weight and BMI. When taken before a meal, soluble fiber has been associated with slowed rates of consumption, delayed gastric emptying, and increased feelings of satiety. While both groups reported consuming similar energy across the intervention, it is possible there was less absorption of energy in the HPF group, leading to a greater degree of weight loss. Higher fiber intake has been shown to reduce digestible energy, resulting in higher fecal excretion of dietary energy.
Differences in total fiber intake at baseline were observed, in that the HPF group consumed less fiber than the LPF group (18.4 g per day versus 25.0 g per day). Not only did the HPF group consume significantly more fiber across the intervention due to the additional 6 g fiber per day from the preload, but due to this initial imbalance, they saw a larger relative within-group increase in fiber consumption.
The Lean Mass Paradox: Unexpected Body Composition Finding
It was hypothesized that with higher consumption of protein, a greater degree of lean mass retention within the HPF group would be observed. Instead, there was a small but significant decrease in absolute lean mass in the HPF group that did not occur in the LPF group. However, as a percentage of total tissue, lean mass slightly increased in both groups.
Many studies point towards a role of higher protein intake for lean mass preservation during weight loss, with findings strongest among participants with obesity when protein intake reaches 25-35% of daily energy. In this study, the HPF group consumed 21% of their daily energy from protein compared with 14% in the LPF group. Both groups fell within the Acceptable Macronutrient Distribution Range of 10-35% of total energy intake from protein, and both groups consumed protein in quantities greater than the RDA.
It is unclear why the HPF group saw a slight decrease in absolute lean mass, but better preservation of lean mass might have occurred if protein intake was closer to the upper limit of the AMDR (35% of energy). Regardless, the small loss observed is not expected to be clinically relevant, particularly given that lean mass as a percentage of total tissue increased in both groups.
Metabolic Improvements: Cholesterol, Insulin, and Adiponectin
The HPF group demonstrated greater improvements in cardiometabolic outcomes of total cholesterol, LDL cholesterol, and oxidized LDL. While weight loss itself has proven to have an independently powerful impact on obesity-related comorbidities, dietary intake also plays a role. Total cholesterol and LDL cholesterol changed over time and were lower in the HPF group, reaching statistical significance from baseline by day 84 for the HPF group. Oxidized LDL decreased with time but was only significantly lower from baseline in the HPF group.
A meta-analysis demonstrated that an increase of 1 g soluble fiber per day may produce a change in total cholesterol and LDL cholesterol of –0.045 and –0.057 mmol per liter, respectively. Mechanistically, these benefits are derived from the binding of soluble fibers with bile acids in the small intestine, resulting in greater excretion of bile, less absorption of bile acids, and ultimately reduced synthesis of serum cholesterol and LDL cholesterol.
Similar to bile acids, fiber binds and blocks the absorption of glucose in the small intestine, resulting in lower serum glucose concentrations and less glycation of hemoglobin over time, potentially explaining the observed reduction in HbA1c in the HPF group (the only group to demonstrate a decrease from baseline). Given that the fiber in the HPF preload was approximately 40% soluble (2.4 g of 6 g total fiber), it is plausible soluble fiber is a primary mechanism for the beneficial effects observed on cholesterol and HbA1c.
Fasting insulin decreased over time for both groups and was generally lower in the HPF group, reaching statistical significance at day 28. Triglycerides did not change with time but were lower in the HPF group overall.
Another potential mechanism for the metabolic benefits observed lies with the hormone adiponectin. Both groups exhibited increased adiponectin concentrations at day 84, with a greater increase observed in the HPF group (+20% versus +11%). Adiponectin is an endocrine factor that has insulin-sensitizing, antiatherogenic, and anti-inflammatory properties.
Previous weight-loss trials have demonstrated associations between weight loss and increases in circulating adiponectin concentrations. Dietary fiber intake has also been identified as having an independent impact on raising adiponectin concentrations, even in the absence of weight loss. Previous work has observed relations with serum adiponectin concentrations linked to lower concentrations of triglyceride, LDL cholesterol, and oxidized LDL concentrations. Thus, increasing fiber intake alongside weight loss may result in increased circulating adiponectin, which itself may impact the metabolic benefits associated with weight loss.
Study Limitations and the Complex Formula
While this study derives strength from its rigorous design and large sample size, it has limitations. A drop-out rate of approximately 22-30% was observed between randomization and completion. True randomization procedures were used rather than matching, and therefore differences in dietary intake, sex, and weight status occurred. To account for these effects, baseline values and sex were utilized as covariates in the per-protocol analysis.
The HPF formula was more complex than simply protein and fiber and included polyphenol-contributing plant-derived ingredients, prebiotics, probiotics, and vitamins and minerals. One or more of the other components of the preload may have contributed to some of the beneficial effects observed. The LPF group did have higher intake of carbohydrates and sugar, though there was no negative impact on glycemic control as indicated by no significant difference in fasting insulin, glucose, or HbA1c between groups at day 84.
As with all dietary interventions using self-reported food records, limitations of underreporting intake, reduced accuracy over time, and tendency to change dietary behaviors during collection days could have contributed to the lack of differences observed in reported energy intake between groups.
The Practical Takeaway: Nutrient Composition Matters
Both groups consumed identical calories from supplement shakes (320 calories per day total, consumed as two separate 160-calorie doses). Both groups followed a 500-calorie deficit. The primary difference was nutrient composition: 17 g protein and 6 g fiber twice daily versus 1 g protein and 3 g fiber twice daily.
The habitual consumption of a high-protein, high-fiber preload 30 minutes before breakfast and lunch resulted in greater weight loss compared with an isocaloric control. Greater reductions in total cholesterol, LDL cholesterol, and oxidized LDL with greater increases in adiponectin concentrations were observed throughout the 84-day trial. These findings suggest that specific nutrient factors—potentially including protein, fiber, and bioactive content—other than calorie reduction alone influence the success of a weight-loss regimen.
Diet composition, including protein and fiber content, rather than energy reduction alone may be an important consideration for weight management and metabolic health outcomes.
Study: Glynn, E. L., Fleming, S. A., Edwards, C. G., Wilson, M. J., Evans, M., & Leidy, H. J. (2022). Consuming a protein and fiber-based supplement preload promotes weight loss and alters metabolic markers in overweight adults in a 12-week, randomized, double-blind, placebo-controlled trial. The Journal of Nutrition, 152(6), 1415-1425.
