Reanalysis of the effects of phenylalanine, alanine, and aspartame on food intake in human subjects

The Combined Effects of Aspartame and Acesulfame-K Blends on Appetite: A Systematic Review and Meta-Analysis of Randomized Clinical Trials

Aspartame (Asp) and acesulfame-K (Ace-K) are nonnutritive sweeteners (NNSs) commonly used in combination to replace added sugars in reduced- or low-calorie foods and beverages. Despite Asp/Ace-K blends having negligible calories, their effects on appetite have not been reviewed systematically. We therefore undertook a systematic review and meta-analysis of the metabolic effects of Asp/Ace-K blends on energy intake (EI), subjective appetite scores, blood glucose, and the incretin hormones glucose-dependent insulinotropic peptide and glucagon-like peptide. MEDLINE, Web of Science, and Cochrane CENTRAL databases (Embase, PubMed, and CINAHL) were searched (May 2021) for randomized controlled trials (RCTs). Human RCTs using Asp/Ace-K blends compared with sugar and water controls were included, whereas isolated cell and animal studies were excluded. An overall 4829 publications were identified and 8 studies, including 274 participants, were retrieved for review. The Asp/Ace-K group's EI was significantly reduced compared with sugar [mean difference (MD): -196.56 kcal/meal; 95% CI: -332.01, -61.11 kcal/meal; P = 0.004] and water (MD: -213.42 kcal/meal; 95% CI: -345.4, -81.44 kcal/meal; P = 0.002). Meta-analysis of subjective appetite scores and incretins could not be undertaken due to inconsistencies in data reporting and insufficient data, respectively, but of the 4 studies identified, no differences were observed between Asp/Ace-K blends and controls. The Asp/Ace-K group's blood glucose was nonsignificantly reduced compared with sugar (MD: -1.48 mmol/L; 95% CI: -3.26, 0.3 mmol/L; P = 0.1) and water (MD: -0.08 mmol/L; 95% CI: -0.62, 0.47 mmol/L; P = 0.78). Lower EI in participants who were predominantly healthy and assigned to Asp/Ace-K blends could not be reliably attributed to changes in subjective appetite scores. Blood glucose and incretins were also generally not affected by Asp/Ace-K blends when compared with controls. Additional short- and long-term RCTs using NNSs and sugars at dietarily relevant levels are needed. This trial was registered at the International Prospective Register of Systematic Reviews (PROSPERO: CRD42017061015).

Differential effects of L- and D-phenylalanine on pancreatic and gastrointestinal hormone release in humans: A randomized crossover study

AIM

To investigate the effects of L-phenylalanine on gastroenteropancreatic hormone release, glucose levels, subjective appetite and energy intake in humans, and to determine whether these effects were stereoisomer-specific by comparing them with D-phenylalanine.

MATERIALS AND METHODS

A dose-finding, non-randomized, unblinded, crossover study was conducted during October-December 2017 at the NIHR Imperial Clinical Research Facility in five participants, in which the tolerability of escalating doses of oral L-phenylalanine was assessed (0, 3, 6 and 10 g). Also, an acute, randomized, double-blind, placebo-controlled crossover study was conducted during January-May 2018 at the NIHR Imperial Clinical Research Facility in 11 participants, in which the effects of oral 10 g L-phenylalanine relative to D-phenylalanine and placebo on gastroenteropancreatic hormone (insulin, glucagon, glucose-dependent insulinotropic polypeptide [GIP], peptide tyrosine tyrosine [PYY], glucagon-like peptide-1) and glucose concentrations, visual analogue scales for subjective appetite and energy intake at an ad libitum meal served 70 minutes postingestion, were investigated.

RESULTS

L-phenylalanine was well-tolerated and increased insulin and glucagon concentrations prior to meal ingestion at several time points relative to placebo and D-phenylalanine (P < .05). L-phenylalanine also increased GIP concentrations relative to D-phenylalanine (P = .0420) and placebo (P = .0249) 70 minutes following ingestion. L-phenylalanine reduced postprandial glucose area under the curve (AUC)_70-150mins relative to placebo (P = .0317) but did not affect subjective appetite or energy intake (P > .05). D-phenylalanine increased postprandial PYY AUC_70-150mins concentrations relative to placebo (P = .0002).

CONCLUSIONS

Ingestion of L-phenylalanine, but not D-phenylalanine, increases insulin, glucagon and GIP concentrations without appearing to have a marked effect on appetite.

Non-nutritive sweeteners for diabetes mellitus

BACKGROUND

Products sweetened with non-nutritive sweeteners (NNS) are widely available. Many people with type 1 or type 2 diabetes use NNS as a replacement for nutritive sweeteners to control their carbohydrate and energy intake. Health outcomes associated with NNS use in diabetes are unknown.

OBJECTIVES

To assess the effects of non-nutritive sweeteners in people with diabetes mellitus.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE Ovid, Scopus, the WHO ICTRP, and ClinicalTrials.gov. The date of the last search of all databases (except for Scopus) was May 2019. We last searched Scopus in January 2019. We did not apply any language restrictions.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) with a duration of four weeks or more comparing any type of NNS with usual diet, no intervention, placebo, water, a different NNS, or a nutritive sweetener in individuals with type 1 or type 2 diabetes. Trials with concomitant behaviour-changing interventions, such as diet, exercise, or both, were eligible for inclusion, given that the concomitant interventions were the same in the intervention and comparator groups.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened abstracts, full texts, and records retrieved from trials registries, assessed the certainty of the evidence, and extracted data. We used a random-effects model to perform meta-analysis, and calculated effect estimates as risk ratios (RRs) for dichotomous outcomes and mean differences (MDs) for continuous outcomes, using 95% confidence intervals (CIs). We assessed risk of bias using the Cochrane 'Risk of bias' tool and the certainty of evidence using the GRADE approach.

MAIN RESULTS

We included nine RCTs that randomised a total of 979 people with type 1 or type 2 diabetes. The intervention duration ranged from 4 to 10 months. We judged none of these trials as at low risk of bias for all 'Risk of bias' domains; most of the included trials did not report the method of randomisation. Three trials compared the effects of a dietary supplement containing NNS with sugar: glycosylated haemoglobin A1c (HbA1c) was 0.4% higher in the NNS group (95% CI -0.5 to 1.2; P = 0.44; 3 trials; 72 participants; very low-certainty evidence). The MD in weight change was -0.1 kg (95% CI -2.7 to 2.6; P = 0.96; 3 trials; 72 participants; very low-certainty evidence). None of the trials with sugar as comparator reported on adverse events. Five trials compared NNS with placebo. The MD for HbA1c was 0%, 95% CI -0.1 to 0.1; P = 0.99; 4 trials; 360 participants; very low-certainty evidence. The 95% prediction interval ranged between -0.3% and 0.3%. The comparison of NNS versus placebo showed a MD in body weight of -0.2 kg, 95% CI -1 to 0.6; P = 0.64; 2 trials; 184 participants; very low-certainty evidence. Three trials reported the numbers of participants experiencing at least one non-serious adverse event: 36/113 participants (31.9%) in the NNS group versus 42/118 participants (35.6%) in the placebo group (RR 0.78, 95% CI 0.39 to 1.56; P = 0.48; 3 trials; 231 participants; very low-certainty evidence). One trial compared NNS with a nutritive low-calorie sweetener (tagatose). HbA1c was 0.3% higher in the NNS group (95% CI 0.1 to 0.4; P = 0.01; 1 trial; 354 participants; very low-certainty evidence). This trial did not report body weight data and adverse events. The included trials did not report data on health-related quality of life, diabetes complications, all-cause mortality, or socioeconomic effects.

AUTHORS' CONCLUSIONS

There is inconclusive evidence of very low certainty regarding the effects of NNS consumption compared with either sugar, placebo, or nutritive low-calorie sweetener consumption on clinically relevant benefit or harm for HbA1c, body weight, and adverse events in people with type 1 or type 2 diabetes. Data on health-related quality of life, diabetes complications, all-cause mortality, and socioeconomic effects are lacking.

Does low-energy sweetener consumption affect energy intake and body weight? A systematic review, including meta-analyses, of the evidence from human and animal studies

By reducing energy density, low-energy sweeteners (LES) might be expected to reduce energy intake (EI) and body weight (BW). To assess the totality of the evidence testing the null hypothesis that LES exposure (versus sugars or unsweetened alternatives) has no effect on EI or BW, we conducted a systematic review of relevant studies in animals and humans consuming LES with ad libitum access to food energy. In 62 of 90 animal studies exposure to LES did not affect or decreased BW. Of 28 reporting increased BW, 19 compared LES with glucose exposure using a specific ‘learning' paradigm. Twelve prospective cohort studies in humans reported inconsistent associations between LES use and body mass index (−0.002 kg m⁻² per year, 95% confidence interval (CI) −0.009 to 0.005). Meta-analysis of short-term randomized controlled trials (129 comparisons) showed reduced total EI for LES versus sugar-sweetened food or beverage consumption before an ad libitum meal (−94 kcal, 95% CI −122 to −66), with no difference versus water (−2 kcal, 95% CI −30 to 26). This was consistent with EI results from sustained intervention randomized controlled trials (10 comparisons). Meta-analysis of sustained intervention randomized controlled trials (4 weeks to 40 months) showed that consumption of LES versus sugar led to relatively reduced BW (nine comparisons; −1.35 kg, 95% CI –2.28 to −0.42), and a similar relative reduction in BW versus water (three comparisons; −1.24 kg, 95% CI –2.22 to −0.26). Most animal studies did not mimic LES consumption by humans, and reverse causation may influence the results of prospective cohort studies. The preponderance of evidence from all human randomized controlled trials indicates that LES do not increase EI or BW, whether compared with caloric or non-caloric (for example, water) control conditions. Overall, the balance of evidence indicates that use of LES in place of sugar, in children and adults, leads to reduced EI and BW, and possibly also when compared with water.

Effect of l-phenylalanine supplementation and a high-protein diet on pharmacokinetics of cefdinir in healthy volunteers: an exploratory study

BACKGROUND

Upregulation of oligopeptide transport activity by dietary protein, certain dipeptides and amino acids has been reported in the rat intestine and a human intestinal cell line.

OBJECTIVE

In this study, the pharmacokinetics of cefdinir were investigated after L-phenylalanine supplementation and a high-protein diet (HPD) in humans to explore changes in the activities of intestinal and renal oligopeptide transporters.

METHODS

A normal-protein diet (NPD, 73.2 +/- 2.6 g/day), NPD + l-phenylalanine (7.5 g/day), or HPD (141.3 +/- 3.7 g/day) was given to six male healthy volunteers for 12 days followed by a single dose of cefdinir after an overnight fast in a randomized three-way crossover study with a 22-day washout. Blood and urine were collected over a 12-h period after administration of cefdinir. Concentrations of cefdinir in plasma and/or urine were measured by high-performance liquid chromatography.

RESULTS

Plasma concentrations and urinary excretion of the drug did not change throughout the study. Physiological variables and laboratory values did not reveal any differences between the three periods except for serum and urinary nitrogen levels and serum triglyceride.

DISCUSSION

A reason for the unchanged pharmacokinetics of cefdinir may be due to lower doses of L-phenylalanine and protein in humans than in animals when converting animal effective doses to humans.

CONCLUSION

In humans, L-phenylalanine supplementation and HPD do not seem to upregulate intestinal and renal oligopeptide transport in the ranges of duration and dose examined.

Effect of a dietary herbal supplement containing caffeine and ephedra on weight, metabolic rate, and body composition

OBJECTIVE

To evaluate the effect of a dietary supplement containing herbal caffeine (70 mg/dose) and ephedra (24 mg/dose; C&E) on metabolic rate, weight loss, body composition, and safety parameters.

RESEARCH METHODS AND PROCEDURES

In phase I, 12 healthy subjects with a BMI of 25 to 35 kg/m2 had resting metabolic rate (RMR) measured for 2 hours after ingesting C&E or a placebo on two occasions 1 week apart, followed by a 1-week washout before phase II. In phase II, these 12 and 28 additional subjects were randomized to a 12-week, double-blind trial comparing C&E (3 times/day) to placebo. In phase III, the C&E group was given open-label C&E for 3 months, and the placebo group was given C&E for 6 months.

RESULTS

In phase I, C&E gave an average 8 +/- 0.1% (SE) rise in RMR over 2 hours compared with placebo (p < 0.01). In phase II, weight loss at 12 weeks was 3.5 +/- 0.6 kg with C&E compared with 0.8 +/- 0.5 kg with placebo (p < 0.02). The percentage fat lost, shown by DXA, was 7.9 +/- 2.9% with C&E and 1.9 +/- 1.1% with placebo (p < 0.05). Pulse decreased more in the placebo group that in the C&E group (p < 0.03). There were no differences in lipid levels or blood pressure. In phase III, there was a 6-month loss of 7.3% and 7.8% of initial body weight for the groups on placebo and C&E during phase II, respectively. There were no serious adverse events.

DISCUSSION

C&E increased RMR significantly by 8% compared with placebo, promoted more weight and fat loss than placebo, and was well tolerated.

Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis

Numerous peripheral signals contribute to the regulation of food intake and energy homeostasis. Mechano- and chemoreceptors signaling the presence and energy density of food in the gastrointestinal (GI) tract contribute to satiety in the immediate postprandial period. Changes in circulating glucose concentrations appear to elicit meal initiation and termination by regulating activity of specific hypothalamic neurons that respond to glucose. Other nutrients (e.g., amino acids and fatty acids) and GI peptide hormones, most notably cholecystokinin, are also involved in short-term regulation of food intake. However, the energy density of food and short-term hormonal signals by themselves are insufficient to produce sustained changes in energy balance and body adiposity. Rather, these signals interact with long-term regulators (i.e., insulin, leptin, and possibly the orexigenic gastric peptide, ghrelin) to maintain energy homeostasis. Insulin and leptin are transported into the brain where they modulate expression of hypothalamic neuropeptides known to regulate feeding behavior and body weight. Circulating insulin and leptin concentrations are proportional to body fat content; however, their secretion and circulating levels are also influenced by recent energy intake and dietary macronutrient content. Insulin and leptin concentrations decrease during fasting and energy-restricted diets, independent of body fat changes, ensuring that feeding is triggered before body energy stores become depleted. Dietary fat and fructose do not stimulate insulin secretion and leptin production. Therefore, attenuated production of insulin and leptin could lead to increased energy intake and contribute to weight gain and obesity during long-term consumption of diets high in fat and/or fructose. Transcription of the leptin gene and leptin secretion are regulated by insulin-mediated increases of glucose utilization and appear to require aerobic metabolism of glucose beyond pyruvate. Other adipocyte-derived hormones and proteins that regulate adipocyte metabolism, including acylation stimulating protein, adiponectin, diacylglycerol acyltransferase, and perilipin, are likely to have significant roles in energy homeostasis.

Postingestive inhibition of food intake by aspartame: importance of interval between aspartame administration and subsequent eating

Aspartame administered in capsules (i.e., without tasting) 1 h before a meal significantly reduces the amount eaten in that meal. In the present study 36 young men and women were divided into 3 groups of 12 to receive aspartame (400 mg) or placebo (400 mg starch) on separate occasions either 5 min (Group A), 30 min (Group B) or 60 min (Group C) before beginning an ad lib test meal. Compared with placebo, aspartame reduced food intake in Group C (by 18.5%, p < 0.01), but did not reliably affect intake in Groups A or B. There were, in contrast, no significant effects of aspartame on premeal ratings of hunger, desire to eat or fullness for any of the groups. These results confirm a postingestive inhibitory action of aspartame on appetite, which may involve the amplification of the satiating effects of food. The lack of effect of aspartame administered at the shorter intervals before eating suggests a postgastric or even postabsorptive mechanism of action. This observation is also important in its implications for the possible therapeutic exploitation of the anorexic effect of capsulated aspartame.