New DIETs: New Dietary Interventions Enhancing the Treatment for Weight Loss

This study involves five groups. Participants will be randomly assigned to one of these groups. All groups will receive information on a dietary approach aimed at losing weight. All groups will be asked to favor foods that are low fat and have a low glycemic index, which means they will be asked to favor foods that don't cause a quick rise in blood sugar (for example, choosing oatmeal over cornflakes for breakfast). All groups will also be asked to take a vitamin B-12 supplement each day of their choosing (can be a multivitamin). Participants who are eligible on the online screener will be contacted by phone to complete a few additional screening questions and invited to an orientation meeting. Below are the steps in the study: * If participants are accepted into the study, participants will come to the study site (USC Public Health Research Center) to learn more about the study, have their weight and height measured, learn how to complete questionnaires online, and complete their consent form. This meeting will last approximately 1.5 hours. * Participants will then complete some questionnaires that measure their diet and physical activity levels on a computer at home or other location where participants have computer/internet access. We will also ask participants to keep all their grocery and restaurant receipts during this time to bring back with participants to the next meeting. * Participants will come back for a meeting to have their weight measured again and participants will find out their diet group assignment. Participants will receive a 1.25 hour overview of the diet participants have been assigned to and be provided with materials on this diet. * If participants are assigned to the vegan, vegetarians, pesco-, or semi-vegetarian diets, they will complete the following over the 8-week study: 1. Participants will attend weekly meetings over 8 weeks. 2. We will obtain their weight and collect online questionnaires at weeks 4 and 8. 3. During the 8-week study, we will ask participants to complete an online weekly questionnaire which will assess if participants are following the diet and their weight. This will take approximately 5 minutes to complete. * If participants are assigned to the omnivorous diet, they will only come in for meetings at baseline, 4 weeks, and 8 weeks. 1. We will obtain their weight and collect online questionnaires at weeks 4 and 8. 2. Participants will receive e-mailed lessons of their diet plan each week. 3. During the 8-week study, we will ask participants to complete an online weekly questionnaire which will assess if participants are following the diet, and their weight. This will take approximately 5 minutes to complete. 4. At week 8, participants will also be provided with information about the other 4 diets in case participants would like to try these dietary patterns after the study. * All diet groups will be asked to collect and turn in all grocery store and restaurant receipts for 2 weeks prior to the start of the intervention and for the 8 weeks during the study. * At study completion (8 weeks), participants will receive an incentive for participating ($20). * Participants in all groups will be asked to take a daily multivitamin (or other reliable source of vitamin B-12) over the course of the study. You are responsible for choosing and purchasing this. * During the study, we will ask you to keep your physical activity levels constant (keep your levels the same as what you were doing at study entry).