The purpose of this research is to test if eating walnuts will cause any changes in blood lipids (like good and bad cholesterol, for example) and levels of inflammation in the blood, as well as changes in the bacteria living in the gut.

The purpose of this research is to test if eating walnuts will cause any changes in blood lipids (like good and bad cholesterol, for example) and levels of inflammation in the blood, as well as changes in the bacteria living in the gut. Overweight/obese adults between the ages of 45 and 70 yo with elevated LDL-C levels are eligible to participate. The study involves 5 visits (1 initial screening and 4 study visits) and will last about 10 weeks. There is a 1-week run-in period before starting the study, followed by two diet intervention periods each lasting 3 weeks, separated by a 3-week washout period.

Participation in this study will involve an initial 1-week run in period to become accustomed to the study diet. After this 1 week period, participants will be randomly selected to start either a walnut or a no-nut diet. During each phase of the study, participants will be asked to come to the University of California, Davis campus after a 10-12 hour overnight fast. At each visit, body measurements will be taken; blood, urine and fecal samples will be collected.

During the 10 weeks of the study, all nuts and nut-derived products will need to be avoided. The study will provide walnuts to be eaten at home for each intervention phase. Additionally, the investigators will ask that ellagitannin-rich foods, high-fiber and fermented foods be avoided in the diet. Detailed food diaries will be filled out and brought to study visits.

Inclusion Criteria:

• Overweight and obese men and postmenopausal women between 45-70 years of age, BMI range of 25.0-39.9 kg/m2 and elevated LDL-C levels (130-189 mg/dL).

Exclusion Criteria:

• Adults unable to consent, individuals who are not yet adults (infants, children, teenagers), pregnant or lactating women, women planning to become pregnant in the next four months and prisoners will be specifically excluded.

Additional exclusion criteria are listed below:

• Allergy to walnuts or other nuts
• Eating more than 25 grams of fiber per day, assessed by the Block Dietary-Fruit-Vegetable-Fiber Screener©
• Taking medications or supplements known to affect metabolism or gut microbiota composition (i.e. Metformin, statins, antibiotics within the past 3 months, fiber supplements, probiotics, and others)
• Documented chronic diseases including diabetes, renal or liver disease, metabolic syndrome, active cancer, MI or stroke, history of gastric bypass or GI disease (e.g. Crohn's Disease, Irritable Bowel Disease, diverticulosis, diverticulitis, etc.)
• Smoker or living with a smoker, including vaporizer and/or electronic cigarettes.
• Illicit drug use, cannabis usage, or consuming >1 alcoholic drink/day
• Extreme dietary or exercise patterns
• Recent weight fluctuations (>10% in the last 6 months)
• Taking prescription lipid medications or supplements that may affect lipoprotein metabolism (i.e. >1 g of fish oil/day, antioxidant supplements)
• Taking exogenous hormones
• Poor venous access
• Unwillingness to comply with study protocols