Background: In a previous study, participants were lived at the NIH and randomly received either a low-fat or a low-carbohydrate diet for 2 weeks and then switched to the other diet for 2 more weeks. Participants who received the low-carbohydrate diet first lost more body fat at the end of the study than those who received the low-fat diet first. Researchers want to see if they can repeat that result in a longer weight loss study when participants live at home. Objective: To test the effects of diet order in people receiving either a low-fat or low-carbohydrate diet first for 4 weeks and then immediately switched to the other diet for another 4 weeks. Eligibility: Adults aged 19 to 50 years with a body mass index of 25 or more. Design: Participants will complete the study at their homes, but there will be 3 required visits to the NIH. Participants will drink a special type of water 2 weeks before the baseline NIH visit and collect urine samples at home to measure how many calories they burn. Before the diets begin, participants will visit the NIH for baseline testing when they will have a metabolism test while relaxing in a bed with a plastic hood over their head to collect the air they breathe out. They will have scans to measure their bone density and how much muscle and body fat they have. They will give stool, blood, and urine samples. Participants will be asked to eat a specific diet for 4 weeks followed by a different diet for 4 weeks. All meals will be delivered to the participants' homes. They will eat only the foods delivered. Participants will weigh themselves daily. They will wear a monitor to track their physical activity and a sensor to measure their glucose levels. They will prick their finger each morning to test a drop of blood for ketones. Participants will meet virtually as a group with the study team weekly. Participants will have two more NIH visits towards the end of each diet period....
* INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: 1. Stated willingness to comply with all study procedures and availability for the duration of the study 2. Adults aged 18-50 years 3. Weight stable (\< +/- 5 percent over past 6 months) as determined by volunteer report 4. Body mass index (BMI) \>= 25 kg/m\^2 5. Agreement to adhere to Lifestyle Considerations throughout the study duration EXCLUSION CRITERIA: 1. Subjects with cardiac pacemakers or other implants that may be affected by or affect the DXA measurements 2. Inadequate access to equipment required for the study (e.g., too little refrigerator storage space or a microwave oven that is too small) as determined by discretion of PI and/or study team 3. Evidence of diseases or conditions that may influence appetite (e.g., cancer, diabetes, alterations of smell or taste post COVID-19), diseases or conditions that affect the immune system, or other conditions at the discretion of the PI and/or study team 4. Taking prescription medications or other drugs that may influence metabolism (including but not limited to diet/weight-loss medication, some psychiatric medications, or other medications at the discretion of the PI and/or study team) 5. People unlikely to adhere to a relatively consistent daily and weekly routine and avoid travel during their participation in the study. For example, people with occupations such as pilots, flight attendants, or frequently travel for work or pleasure. 6. People unlikely to abstain from off study food or supplements that may impact metabolism or appetite at the discretion of the study team 7. Positive pregnancy test or lactating as determined by volunteer report (women only) 8. Inability or unwillingness to use birth control between screening and completion of the study (women only) 9. Symptoms or signs of perimenopause or menopause by volunteer report (women only) 10. Regular vigorous exercise \>150 min per week as determined by volunteer report 11. Alcohol consumption \> 3 drinks per day as determined by volunteer report 12. Regular use of tobacco (smoking, chewing, or vaping), nicotine replacement therapy, or illicit drugs over past 6 months as determined by volunteer report. Subjects may also be excluded based on a urine drug test. 13. Psychological conditions as determined by DSM-5 Self-rated Level 1 Cross-cutting Symptom Measure such as (but not limited to) eating disorders, depression, bipolar disorders, that would be incompatible with safe and successful participation in this study, as determined by investigators 14. Volunteers with strict dietary concerns (e.g. vegetarian or kosher diet, food allergies) or a reported dislike of a significant fraction of the study foods 15. Volunteers unwilling or unable to give informed consent 16. Non-English speakers due to unavailability of required questionnaires in other languages