Approximately 30% of patients experience "Long-COVID" syndrome presenting with symptoms such as cognitive difficulties ('brain fog'), fatigue, dyspnea, autonomic dysfunction, depression and anxiety, lasting beyond 12 weeks causing significant disability, and threaten health and wellbeing of millions around the world. At the current time, there is no effective treatment for long-COVID. Walnuts contain a mixture of nutrients and phytochemicals include monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), and alpha-linolenic acid (ALA). Walnut consumption was directly associated with cognitive function. The investigators have shown pomegranate that share the same phytonutrient ellagitannins as walnut and mixed nuts including walnuts could increase blood microbiome metabolites of tryptophan metabolite indole propionate and serotonin levels via change of gut microbiota and therefore play an essential role in gut-brain axis including cognitive function. The proposed pilot study will include 76 adults (ages >40 years) with diagnosis of SARS-CoV2 infection presenting with long-COVID symptoms lasting longer than 12 weeks. They will be randomized 1:1 to usual care vs. an intervention group that will include 2 oz (57g) of walnuts daily into their habitual diet for 12 weeks. The investigators will compare the improvement in symptoms of cognitive complains of "brain fog", fatigue and depression of the active intervention group vs. usual care. The outcome of the investigation of the benefit of walnut consumption will provide important novel information on using dietary sources of polyunsaturated fatty acids and phytochemicals to mitigate the common symptoms of long-COVID. This application was submitted in response to the California Walnut Commission's commodity board topic.

Inclusion Criteria:

Age 40 years and older

History of confirmed SARS-CoV-2 infection

Presence of long-COVID symptoms lasting longer than 12 weeks, including one or more of the following:

• Cognitive complaints ("brain fog," memory, attention, or processing speed difficulties)
• Fatigue
• Mood symptoms (e.g., depression)

Consumption of a low-polyphenol (Western) diet

Adequate visual acuity and hearing to complete neuropsychological testing

Screening laboratory results without clinically significant abnormalities that would interfere with study participation

Ability and willingness to provide written informed consent

Exclusion Criteria:

Diagnosis of probable Alzheimer's disease or other dementia (e.g., vascular, Lewy body, frontotemporal)

Other neurological or medical conditions that may cause cognitive impairment

Evidence of Parkinson's disease based on motor examination

Uncontrolled hypertension (systolic BP >170 mmHg or diastolic BP >100 mmHg)

Allergy to walnuts

Regular consumption of more than 5 oz of walnuts per week

Current diagnosis of a major psychiatric disorder according to DSM-5 criteria

Alcoholism or substance use disorder

Any condition that, in the investigator's judgment, would make participation unsafe or interfere with study completion