Effect of Oral Contraceptives on Weight and Body Composition in Young Female Runners

Hormonal Contraceptive Use and Physical Performance, Body Composition, and Musculoskeletal Injuries during Military Training

PURPOSE

To investigate associations between hormonal contraceptive use and physical performance, body composition, and musculoskeletal injuries in basic military training.

METHODS

Female British Army recruits ( n = 450) were grouped as nonusers ( n = 182), combined oral contraceptive users (COCP; n = 184), or progestin-only users (POC; n = 144). Physical performance (2.4-km run, lift strength, leg power), body composition, iron and vitamin D status, and bone metabolism were measured at the start (week 1) and end (week 13) of training. Lower body musculoskeletal injuries were recorded from medical records.

RESULTS

Training decreased 2.4-km run time (-3.7%) and fat mass (-9.6%), and increased lift strength (4.5%), leg power (1.5%), lean mass (5.4%), and whole-body (0.9%), arms (1.8%), and legs (1.4%) areal bone mineral density ( P ≤ 0.015); the training response was not different between groups ( P ≥ 0.173). Lift strength was lower in COCP users than nonusers ( P = 0.044). Whole-body, trunk, and leg areal bone mineral densities were lower in POC users than nonusers and/or COCP users ( P ≤ 0.041). There were no associations between hormonal contraceptive use and musculoskeletal or bone stress injury ( P ≥ 0.429). Training did not change ferritin ( P = 0.968), but decreased hemoglobin and total 25-hydroxyvitamin-D, and increased parathyroid hormone, c-telopeptide cross-links of type 1 collagen (βCTX), and procollagen type 1 N-terminal propeptide (PINP; P ≤ 0.005); the training response was not different between groups ( P ≥ 0.368). Total 25-hydroxyvitamin-D was higher, and βCTX and PINP were lower, in COCP users than nonusers and POC users; parathyroid hormone was lower in COCP users than nonusers; and βCTX and PINP were higher in POC users than nonusers ( P ≤ 0.017).

CONCLUSIONS

Hormonal contraceptive use was not associated with performance or injury outcomes in military training.

The Influence of Menstruation and Hormonal Birth Control on the Performance of Female Collegiate Lacrosse Players

This study compared the mechanical and physiological load placed on Division I female collegiate lacrosse athletes (1) with and without hormone contraceptive (HC) use and (2) with and without menstruation during training and games. Athletes' (20.6 ± 1.5 years, HC users = 9, naturally cycling (NC) athletes = 9) workloads-total distance traveled (TD, m), max speed (km∙h-1), accelerations (repetitions), decelerations (repetitions), and high-intensity distance (HID, m)-were measured through VX Sport wearable microtechnology in training sessions (n = 87/athlete) and games (n = 17/athlete). Analyses showed no statistical group differences based on HC use or not, and no differences during menstruation versus non-menstruation for training or games. However, while not statistically different, athletes taking HCs performed worse during menstruation, with a 5.1% decline in decelerations, 3.4% decline in TD and HID, 1.2% decline in max speed, and 1% decline in accelerations. NC athletes did not show this same decline with menses. Given that withdrawal bleeding exacerbates performance reduction of HC users, it may be beneficial for these athletes to consider skipping their withdrawal bleed if it is likely to coincide with a game. Further research needs to be carried out to see if these trends are consistent across other female athletes in other sports.

Effect of combined oral contraceptive on cardiorespiratory function and immune activation in premenopausal women involved in exercise: A systematic review protocol

BACKGROUND

The use of combined oral contraceptive (COC) is common among women of reproductive age despite the potential risk of them developing thrombotic events. There is a need to understand how COC affects cardiorespiratory function and markers of immune activation in premenopausal women involved in exercise. This highlights a need for a systematic review to enhance our understanding of how the use of COC affects cardiovascular health in premenopausal women subjected to exercise.

METHOD

This systematic review protocol was prepared following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols (PRISMA-P) 2015 statement. An extensive search of relevant literature by two independent reviewers will be conducted through the EBSCOhost interface to access databases such as MEDLINE, EMBASE, and CINAHL. Other health sources, including Cochrane CENTRAL, unpublished studies and grey literature, will also be searched. The search will include all studies that report the effect of COC on essential parameters of cardiorespiratory function and markers of immune activation in premenopausal women involved in exercise. All included studies will be appraised using appraisal tools, while appropriate extraction tools will be used for data extraction. Where possible, eligible studies will be pooled for meta-analysis. If statistical pooling is not feasible, our findings will be presented in a narrative format. The certainty of evidence will be assessed using the Grading of Recommendations, Assessment, Development and Evaluation Assessment (GRADE) tool.

TRIAL REGISTRATION

PROSPERO registration number: CRD42021265257.

Combined hormonal contraceptive use is not protective against musculoskeletal conditions or injuries: a systematic review with data from 5 million females

OBJECTIVE

Assess the association between combined hormonal contraceptives (CHC) use and musculoskeletal tissue pathophysiology, injuries or conditions.

DESIGN

Systematic review with semiquantitative analyses and certainty of evidence assessment, guided by the Grading of Recommendations Assessment, Development and Evaluation approach.

DATA SOURCES

MEDLINE, EMBASE, CENTRAL, SPORTDiscus, CINAHL searched from inception to April 2022.

ELIGIBILITY

Intervention and cohort studies that assessed the association between new or ongoing use of CHC and an outcome of musculoskeletal tissue pathophysiology, injury or condition in postpubertal premenopausal females.

RESULTS

Across 50 included studies, we assessed the effect of CHC use on 30 unique musculoskeletal outcomes (75% bone related). Serious risk of bias was judged present in 82% of studies, with 52% adequately adjusting for confounding. Meta-analyses were not possible due to poor outcome reporting, and heterogeneity in estimate statistics and comparison conditions. Based on semiquantitative synthesis, there is low certainty evidence that CHC use was associated with elevated future fracture risk (risk ratio 1.02-1.20) and total knee arthroplasty (risk ratio 1.00-1.36). There is very low certainty evidence of unclear relationships between CHC use and a wide range of bone turnover and bone health outcomes. Evidence about the effect of CHC use on musculoskeletal tissues beyond bone, and the influence of CHC use in adolescence versus adulthood, is limited.

CONCLUSION

Given a paucity of high certainty evidence that CHC use is protective against musculoskeletal pathophysiology, injury or conditions, it is premature and inappropriate to advocate, or prescribe CHC for these purposes.

PROSPERO REGISTRATION NUMBER

This review was registered on PROSPERO CRD42021224582 on 8 January 2021.

The importance of estradiol for body weight regulation in women

Obesity in women of reproductive age has a number of adverse metabolic effects, including Type II Diabetes (T2D), dyslipidemia, and cardiovascular disease. It is associated with increased menstrual irregularity, ovulatory dysfunction, development of insulin resistance and infertility. In women, estradiol is not only critical for reproductive function, but they also control food intake and energy expenditure. Food intake is known to change during the menstrual cycle in humans. This change in food intake is largely mediated by estradiol, which acts directly upon anorexigenic and orexigenic neurons, largely in the hypothalamus. Estradiol also acts indirectly with peripheral mediators such as glucagon like peptide-1 (GLP-1). Like estradiol, GLP-1 acts on receptors at the hypothalamus. This review describes the physiological and pathophysiological mechanisms governing the actions of estradiol during the menstrual cycle on food intake and energy expenditure and how estradiol acts with other weight-controlling molecules such as GLP-1. GLP-1 analogs have proven to be effective both to manage obesity and T2D in women. This review also highlights the relationship between steroid hormones and women's mental health. It explains how a decline or imbalance in estradiol levels affects insulin sensitivity in the brain. This can cause cerebral insulin resistance, which contributes to the development of conditions such as Parkinson's or Alzheimer's disease. The proper use of both estradiol and GLP-1 analogs can help to manage obesity and preserve an optimal mental health in women by reducing the mechanisms that trigger neurodegenerative disorders.

Effect of oral contraceptives on energy balance in women: A review of current knowledge and potential cellular mechanisms

Body weight management is currently of major concern as the obesity epidemic is still a worldwide challenge. As women face more difficulties to lose weight than men, there is an urgent need to better understand the underlying reasons and mechanisms. Recent data have suggested that the use of oral contraceptive (OC) could be involved. The necessity of utilization and development of contraceptive strategies for birth regulation is undeniable and contraceptive pills appear as a quite easy approach. Moreover, OC also represent a strategy for the management of premenstrual symptoms, acne or bulimia nervosa. The exact impact of OC on body weight remains not clearly established. Thus, after exploring the potential underlying mechanisms by which OC could influence the two side of energy balance, we then provide an overview of the available evidence regarding the effects of OC on energy balance (i.e. energy expenditure and energy intake). Finally, we highlight the necessity for future research to clarify the cellular effects of OC and how the individualization of OC prescriptions can improve long-term weight loss management.

Investigation of taste function and eating behavior in women with polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder in women of reproductive age that is associated with eating disorders and disordered eating. No data is available regarding taste function in women with PCOS. The aim of this study was to assess taste function and eating behavior in patients with PCOS compared to healthy women and investigate potential impact of oral contraceptive (OC) use on those. Forty-four patients with PCOS and 36 age and body-mass-index matched healthy controls were enrolled. Gustatory function was assessed by taste strips (sweet, sour, salty, bitter) and Food Cravings Questionnaire-Trait (FCQ-T), Night Eating Questionnaire (NEQ) and Three Factor Eating Questionnaire-R18 (TFEQ-R18) were applied. All measurements were repeated in patients after receiving an OC along with general lifestyle advice for 3 months. At baseline, PCOS group had lower total taste strip test (TST) scores compared to controls (11.7 ± 2.2 vs. 13.1 ± 1.4; p = 0.001). Subgroup analysis showed lower sour and salty taste scores in PCOS group (2.4 ± 0.9 vs. 2.9 ± 0.7; p = 0.004; and 2.6 ± 1 vs. 3.1 ± 0.7; p = 0.01 respectively). Sweet and bitter taste scores were similar. No difference was determined in eating behavior. Linear regression analysis revealed that hyperandrogenism was significant predictor for total TST score (R² = 0.22, p < 0.001). Higher free androgen index (FAI) was associated with lower total TST score (p = 0.01). Total TST score, TFEQ-R18 and NEQ scores remained unaltered after treatment in the PCOS group whereas FCQ-T scores showed significant reduction (p = 0.02), mainly due to a decrease in lack of control subscale (p = 0.01). Our results suggest that taste perception is reduced in PCOS, and short-term OC use does not alter taste functions in the syndrome.

Influence of Menstrual Cycle or Hormonal Contraceptive Phase on Energy Intake and Metabolic Hormones-A Pilot Study

Sex hormones are suggested to influence energy intake (EI) and metabolic hormones. This study investigated the influence of menstrual cycle (MC) and hormonal contraceptive (HC) cycle phases on EI, energy availability (EA), and metabolic hormones in recreational athletes (eumenorrheic, NHC = 15 and monophasic HC-users, CHC = 9). In addition, 72-h dietary and training logs were collected in addition to blood samples, which were analyzed for 17β-estradiol (E2), progesterone (P4), leptin, total ghrelin, insulin, and tri-iodothyronine (T3). Measurements were completed at four time-points (phases): Bleeding, mid-follicular (FP)/active 1, ovulation (OVU)/active 2, mid-luteal (LP)/inactive in NHC/CHC, respectively. As expected, E2 and P4 fluctuated significantly in NHC (p < 0.05) and remained stable in CHC. In NHC, leptin increased significantly between bleeding and ovulation (p = 0.030) as well as between FP and OVU (p = 0.022). No group differences in other measured hormones were observed across the MC and HC cycle. The mean EI and EA were similar between phases, with no significant differences observed in macronutrient intake over either the MC or HC. While the MC phase might have a small, but statistically significant effect on leptin, the findings of the present study suggest that the MC or HC phase does not significantly alter ad libitum EI or EA in recreational athletes.

Evaluating the effects of oral contraceptive use on biomarkers and body composition during a competitive season in collegiate female soccer players

High training demands throughout the competitive season in female collegiate soccer players have been shown to induce changes in biomarkers indicative of stress, inflammation, and reproduction, which may be exacerbated in athletes using oral contraceptives (OCs). This study aimed to compare biomarkers and body composition between OC-using and nonusing (CON) female soccer players throughout a competitive season. Female collegiate soccer players were stratified into two groups based on their reported OC use at the start of preseason (OC: n = 6; CON: n = 17). Before the start of preseason and immediately postseason, athletes underwent a battery of performance tests. Blood draws and body composition assessments were performed before preseason, on wks 2, 4, 8, and 12 of the season, and postseason. Area-under-the-curve ratios (OC_AUC:CON_AUC) indicated the OC group were exposed to substantially higher levels of sex hormone-binding globulin (AUC_ratio = 1.4, probability = P > 0.999), total cortisol (1.7; P > 0.999), C-reactive protein (5.2; P > 0.999), leptin (1.4; P = 0.990), growth hormone (1.5; P = 0.97), but substantively lower amounts of estradiol (0.36; P < 0.001), progesterone (0.48; P = 0.008), free testosterone (0.58; P < 0.001), follicle-stimulating hormone (0.67; P < 0.001), and creatine kinase (0.33, P < 0.001) compared with the CON across the season. Both groups increased fat free mass over the season, but CON experienced a greater magnitude of increase along with decreased body fat percentage. Although similar training loads were observed between groups over the season, the elevated exposure to stress, inflammatory, and metabolic biomarkers over the competitive season in OC users may have implications on body composition, training adaptations, and recovery in female athletes.NEW & NOTEWORTHY This study highlights the influence of OC use on physiological changes that occur over a 4-mo intense, competitive season and the differential systemic exposure to biomarkers, specifically those of inflammation, stress, anabolism, and energy balance, between OC-using and nonusing soccer players. Additionally, this study provides insight into changes in body composition with prolonged training between female athletes with and without OC use.

Effect of administration of monophasic oral contraceptive on the body composition and aerobic and anaerobic capacities of female athletes

AIM

To examine the effects of administration of monophasic oral contraceptive (OC) on the body composition and aerobic and anaerobic capacities of female athletes.

METHODS

Ten female athletes (23.0 ± 4.1 years old) participated in this study. All the subjects were examined during the menstrual cycle (early follicular [EF], middle luteal [ML]) and OC cycle (inactive OC, and active OC phases). After the menstrual cycle measurements, all the subjects began taking OC. The athletes tested using a body composition, circumferences, the lactate curve test, the maximal oxygen consumption test, and the Wingate test during all phases.

RESULTS

Waist circumference was lower during the OC cycle than menstrual cycle (P < 0.05), and the other circumferences of parts were not different during the menstrual or OC cycles. However, there were no differences in body composition before and after taking OC. The phase of the menstrual or OC cycle did not affect maximal oxygen consumption, heart rate max or power output at a blood lactate concentration of 2 and 4 mmol/L between the menstrual and OC cycle phases. Although, the peak blood lactate level after the Wingate test was higher during the OC cycle (inactive OC phase, 13.0 ± 2.4 mmol/L; active OC phase, 12.4 ± 3.0 mmol/L) than during the menstrual cycle (EF phase, 11.9 ± 2.1 mmol/L; ML phase, 11.4 ± 2.1 mmol/L; P < 0.05), the peak and average power in the Wingate test did not change during the menstrual and OC cycle phases.

CONCLUSION

Administration of monophasic OC did not affect the body composition or aerobic and anaerobic capacities of female athletes.

Molecular markers of skeletal muscle hypertrophy following 10 wk of resistance training in oral contraceptive users and nonusers

The objective was to determine whether skeletal muscle molecular markers and SC number were influenced differently in users and nonusers of oral contraceptives (OCs) following 10 wk of resistance training. Thirty-eight young healthy untrained users (n = 20) and nonusers of OC (n = 18) completed a 10-wk supervised progressive resistance training program. Before and after the intervention, a muscle tissue sample was obtained from the vastus lateralis muscle for analysis of muscle fiber cross-sectional area (fCSA) and satellite cell (SC) and myonuclei number using immunohistochemistry, gene expression using PCR, protein expression, and myosin heavy chain composition. Following the training period, quadriceps fCSA (P < 0.05), SCs/type I fiber (P = 0.05), and MURF-1 mRNA (P < 0.01) were significantly increased with no difference between the groups. However, SCs/total fiber and SCs/type II fiber increased in OC users only, and SCs/type II fCSA tended (P = 0.055) to be greater in the OC users. Furthermore, in OC users there were a fiber type shift from myosin heavy chain (MHC) IIx to MHC IIa (P < 0.01), and expression of muscle regulatory factor 4 (MRF4) mRNA (P < 0.001) was significantly greater than in non-OC users. Use of second-generation OCs in young untrained women increased skeletal muscle MRF4 expression and SC number following 10 wk of resistance training compared with nonusers.NEW & NOTEWORTHY The effect of oral contraceptive use on the skeletal muscle regulatory pathways in response to resistance training has not been investigated previously. Here we present novel data, demonstrating that use of second-generation oral contraceptives in young untrained women increased skeletal muscle regulatory factor 4 expression and satellite cell number following 10 wk of resistance training compared with nonusers.

Influence of Second Generation Oral Contraceptive Use on Adaptations to Resistance Training in Young Untrained Women

Dalgaard, LB, Jørgensen, EB, Oxfeldt, M, Dalgaard, EB, Johansen, FT, Karlsson, M, Ringgaard, S, and Hansen, M. Influence of second generation oral contraceptive use on adaptations to resistance training in young untrained women. J Strength Cond Res XX(X): 000-000, 2020-The study purpose was to determine effects of using second generation oral contraceptives (OC) on muscle adaptations to resistance training in young untrained women. Twenty users and 18 nonusers of OC completed a 10-week supervised progressive resistance training program. Before and after the intervention, muscle cross-sectional area (mCSA) of the quadriceps was measured using magnetic resonance imaging and muscle fiber CSA (fCSA) was determined by immunohistochemistry. In addition, body composition (DXA, fat mass/fat-free mass), maximal isometric muscle strength (dynamometry), 5 repetition maximum (5RM) leg press strength, counter movement jump (CMJ) height, and average power using a modified Wingate test were determined. Serum hormone analysis ensured OC compliance and 4-day food records documented dietary intake. After the training period, quadriceps mCSA (OC: 11.0 ± 6.0% vs. non-OC: 9.2 ± 5.0%, p = 0.001), type II fCSA (OC: 19.9 ± 7.9% vs. non-OC: 16.6 ± 7.2%, p = 0.05), muscle strength (knee extension, knee flexion and 5RM, p < 0.001), and functional power (CMJ, AP, p < 0.001) were significantly increased with no significant difference between the groups. However, a tendency toward a greater increase in fat-free mass (FFM) in the OC group was observed (OC: 3.7 ± 3.8% vs. non-OC: 2.7 ± 3.5%, p = 0.08). Collectively, use of second generation OCs in young untrained women did not significantly improve adaptations to 10 weeks of resistance training compared with nonusers. The trend toward greater gains in FFM in the OC group warrant future studies.

Impact of Combined Hormonal Contraceptive Use on Weight Loss: A Secondary Analysis of a Behavioral Weight-Loss Trial

OBJECTIVE

This study aimed to perform a preliminary investigation of the impact of combined hormonal contraceptive (CHC) use on weight loss during an 18-month behavioral weight-loss trial.

METHODS

Adults (n = 170; 18-55 years; BMI 27-42 kg/m2 ) received a weight-loss intervention that included a reduced-calorie diet, a progressive exercise prescription, and group-based behavioral support. Premenopausal women (n = 110) were classified as CHC users (CHC, n = 17) or non-CHC users (non-CHC, n = 93). Changes in weight were examined within groups using a linear mixed model, adjusted for age and randomized group assignment.

RESULTS

At 6 M, weight was reduced from baseline in both CHC (mean, -6.7 kg; 95% CI: -9.8 to -3.7 kg) and non-CHC (-9.1 kg; -9.1 to -6.4 kg). Between 6 and 18 M, CHC regained weight (4.9 kg; 0.9 to 8.9 kg), while weight remained relatively unchanged in non-CHC (-0.1 kg; -1.8 to 1.6 kg). At 18 M, weight was relatively unchanged from baseline in CHC (-1.8 kg; -7.3 to 3.6 kg) and was reduced from baseline in non-CHC (-7.9 kg; -10.2 to -5.5 kg).

CONCLUSIONS

In this secondary data analysis, CHC use was associated with weight regain after initial weight loss. Prospective studies are needed to further understand the extent to which CHC use influences weight loss and maintenance.

Combination contraceptives: effects on weight

BACKGROUND

Weight gain is often considered a side effect of combination hormonal contraceptives, and many women and clinicians believe that an association exists. Concern about weight gain can limit the use of this highly effective method of contraception by deterring the initiation of its use and causing early discontinuation among users. However, a causal relationship between combination contraceptives and weight gain has not been established.

OBJECTIVES

The aim of the review was to evaluate the potential association between combination contraceptive use and changes in weight.

SEARCH METHODS

In November 2013, we searched the computerized databases CENTRAL (The Cochrane Library), MEDLINE, POPLINE, EMBASE, and LILACS for studies of combination contraceptives, as well as ClinicalTrials.gov and International Clinical Trials Registry Platform (ICTRP). For the initial review, we also wrote to known investigators and manufacturers to request information about other published or unpublished trials not discovered in our search.

SELECTION CRITERIA

All English-language, randomized controlled trials were eligible if they had at least three treatment cycles and compared a combination contraceptive to a placebo or to a combination contraceptive that differed in drug, dosage, regimen, or study length.

DATA COLLECTION AND ANALYSIS

All titles and abstracts located in the literature searches were assessed. Data were entered and analyzed with RevMan. A second author verified the data entered. For continuous data, we calculated the mean difference and 95% confidence interval (CI) for the mean change in weight between baseline and post-treatment measurements using a fixed-effect model. For categorical data, such as the proportion of women who gained or lost more than a specified amount of weight, the Peto odds ratio with 95% CI was calculated.

MAIN RESULTS

We found 49 trials that met our inclusion criteria. The trials included 85 weight change comparisons for 52 distinct contraceptive pairs (or placebos). The four trials with a placebo or no intervention group did not find evidence supporting a causal association between combination oral contraceptives or a combination skin patch and weight change. Most comparisons of different combination contraceptives showed no substantial difference in weight. In addition, discontinuation of combination contraceptives because of weight change did not differ between groups where this was studied.

AUTHORS' CONCLUSIONS

Available evidence was insufficient to determine the effect of combination contraceptives on weight, but no large effect was evident. Trials to evaluate the link between combination contraceptives and weight change require a placebo or non-hormonal group to control for other factors, including changes in weight over time.

Combination contraceptives: effects on weight

BACKGROUND

Weight gain is often considered a side effect of combination hormonal contraceptives, and many women and clinicians believe that an association exists. Concern about weight gain can limit the use of this highly effective method of contraception by deterring the initiation of its use and causing early discontinuation among users. However, a causal relationship between combination contraceptives and weight gain has not been established.

OBJECTIVES

The aim of the review was to evaluate the potential association between combination contraceptive use and changes in weight.

SEARCH STRATEGY

We searched the computerized databases CENTRAL (The Cochrane Library), MEDLINE, POPLINE, EMBASE, and LILACS for studies of combination contraceptives, as well as ClinicalTrials.gov and International Clinical Trials Registry Platform (ICTRP). Searches were conducted from January to May 2011. We also wrote to known investigators and manufacturers to request information about other published or unpublished trials not discovered in our search.

SELECTION CRITERIA

All English-language, randomized controlled trials were eligible if they had at least three treatment cycles and compared a combination contraceptive to a placebo or to a combination contraceptive that differed in drug, dosage, regimen, or study length.

DATA COLLECTION AND ANALYSIS

All titles and abstracts located in the literature searches were assessed. Data were entered and analyzed with RevMan. A second author verified the data entered. For continuous data, we calculated the mean difference and 95% confidence interval (CI) for the mean change in weight between baseline and post-treatment measurements using a fixed-effect model. For categorical data, such as the proportion of women who gained or lost more than a specified amount of weight, the Peto odds ratio with 95% CI was calculated.

MAIN RESULTS

We found 49 trials that met our inclusion criteria. The trials included 85 weight change comparisons for 52 distinct contraceptive pairs (or placebos). The four trials with a placebo or no intervention group did not find evidence supporting a causal association between combination oral contraceptives or a combination skin patch and weight change. Most comparisons of different combination contraceptives showed no substantial difference in weight. In addition, discontinuation of combination contraceptives because of weight change did not differ between groups where this was studied.

AUTHORS' CONCLUSIONS

Available evidence was insufficient to determine the effect of combination contraceptives on weight, but no large effect was evident. Trials to evaluate the link between combination contraceptives and weight change require a placebo or non-hormonal group to control for other factors, including changes in weight over time.