This study is designed to investigate the health impact of a vegan diet compared to a usual, omnivorous diet. The investigators plan to study these diets in twins, where one twin follows a vegan diet and the other twin follows an omnivorous diet, thus the investigators control for genetic differences that might impact the effect of the diet.

During this study, the investigators will evaluate the nutrient intake in both the vegan and the omnivorous diet. The investigators will also measure physiologic markers of health such as lipid levels, HbA1C, heart rate, and weight, and they will also look at the effect of the diets on the microbiota. In addition to measuring the effect of the diet, the investigators will monitor adherence to the diet, and survey participants on the ease/difficulty in following a vegan diet as well as their energy levels and sense of wellbeing. Thus, this study will help us better understand the health impact and feasibility of following a vegan diet. These results will be of much interest to the general public and the health care professionals.

Inclusion Criteria:

• Age 18+
• 1/2 of a pair of twins that will both be participating
• Willing to consume a plant-based diet (vegetables, fruit, whole grains, legumes, etc.)
• Willing to consume meat/eggs (beef, pork/sausage, chicken, eggs) >= 1 time a day
• Willing to consume dairy (milk, yogurt, cheese) >= 1 time a day

Exclusion Criteria:

• Weight < 110 lb

• BMI >= 40

• LDL-C > 190 mg/dL

• Systolic Blood Pressure > 160 mmHg OR Diastolic blood pressure > 90 mmHg

• Pregnant, lactating or planning to become pregnant during the course of the study.

• Use of any of the following drugs/supplements within the last 2 months:

  • systemic antibiotics, antifungals, antivirals or antiparasitics (intravenous, intramuscular, or oral);
  • corticosteroids (intravenous, intramuscular, oral, nasal or inhaled);
  • cytokines;
  • methotrexate or immunosuppressive cytotoxic agents.

• Chronic, clinically significant, or unstable (unresolved, requiring on-going changes to medical management or medication) pulmonary, cardiovascular, gastrointestinal, hepatic or renal functional abnormality, as determined by medical history, Type 1 diabetes, dialysis

• History of active cancer in the past 3 years except for squamous or basal cell carcinomas of the skin that have been medically managed by local excision

• Unstable dietary history as defined by major changes in diet during the previous month, where the subject has eliminated or significantly increased a major food group in the diet.

• Recent history of chronic excessive alcohol consumption defined as more than five 1.5-ounce servings of 80 proof distilled spirits, five 12-ounce servings of beer or five 5-ounce servings of wine per day; or > 14 drinks/week.

• Any confirmed or suspected condition/state of immunosuppression or immunodeficiency (primary or acquired) including HIV infection, multiple sclerosis and Graves' disease.

• Surgery of the GI tract, with the exception of cholecystectomy and appendectomy, in the past five years. Any major bowel resection at any time.

• Regular/frequent use of smoking or chewing tobacco, e-cigarettes, cigars or other nicotine-containing products

• Regular use of prescription opiate pain medication