Advanced Functional Medicine's mission is to utilize the latest scientific research to diagnose and formulate a customized treatment plan for each individual patient to help reverse chronic disease, eliminating the need for dangerous or unnecessary drug or surgical interventions.  This is healing in its healthiest, truest form- from the inside out.

The way it should be.


We have created a practice that we believe in and choose for our own family members. 



Based on cutting-edge science, functional medicine offers a whole new way to understand and look at chronic illnesses from diabetes to heart disease, cognitive declineautoimmune disease and skin issues, to fibromyalgia, thyroid, anxiety, digestive issues, and more. The core concept is that chronic illness is an outcome of imbalances in the key underlying physiological systems in your body. When inflammation, blood sugar imbalances, insulin resistance, oxidative stress, microbiome changes, hormonal problems, and systemic upsets occur, the body spirals out of balance.

Functional Medicine identifies and restores these imbalances, leaving the patient feeling younger, healthier, energetic, and more hopeful about the future.


Dr. Matthew Willis, DC, DABCI - Founder

(Board Certified Diplomate Internist)

Dr. Willis is a nationally board certified diplomate internist (DABCI), and has completed over 9 years of post-doctorate education and holds advanced degrees and certifications in functional medicine, functional nutrition, internal medicine, clinical laboratory diagnosis, nutrigenomics, specialty laboratory testing, & natural treatment of internal disorders. 


Dr. Willis believes that we all deserve a life of great health—and that we can create it for ourselves. That’s why he is dedicated to treating the root causes of chronic disease by using the dynamic model of Functional Medicine, which has been transforming the way healthcare is practiced in today's world as patients demand healthier and more effective alternatives to traditional drug therapy.


He is a practicing doctor, a recognized leader, speaker, author, educator, and advocate in his field.  In 2020 and 2021, Dr. Willis was selected by the prestigious Marquis Who's Who in America, and also awarded one of America's Top Doctors for professional integrity, innumerable contributions to society, and outstanding achievement in his respective field.


Dr. Willis is the founder and director of Advanced Functional Medicine of San Diego (awarded best medical clinic in San Diego 4 years consecutive), Advanced Functional Medicine of Orange County and Los Angeles, a speaker for the nonprofit group Doctors for Health and Wellness, and has been a regular medical contributor on many functional medicine and integrative health websites around the country.  He has been featured on the Discovery Health Channel, KUSI news, iHeart radio, and is also the author of the best selling online book "The Truth About Type 2 Diabetes".  In 2019, Dr. Willis founded Advanced Functional Formulations, a nutraceutical brand of professional grade supplements created in an FDA-approved facility to help patients overcome a variety of chronic illnesses.


He is a member of the nationally recognized Institute for Functional Medicine, the Council on Diagnosis of Internal Disorders, an active member of the Alzheimer's Family Organization, the Cancer Control Society, as well as a professional member of the American Diabetes Association.

Dr. Sonia Mohan, MD

Dr. Sonia Mohan has years of research experience in pathology at the renowned Cedars Sinai Medical Institute in Los Angeles, CA.


This was followed by post-graduate residency training in the field of Pathology and Laboratory medicine at Loma Linda University Medical Center.  She is now an apprentice of Dr. Willis.


Most recently, seeking a transition from the confines of traditional medicine and learning about the many advantages of a whole-body approach, Dr. Mohan has partnered with the mission of Dr. Willis and now dedicates her career to healing using the latest research in functional medicine.


It is Dr. Mohan’s full intention to target the root causes of illness and embrace the patient’s true path to a long-term well being. Serving as co-owner and medical director of Advanced Functional Medicine has enabled Dr. Mohan to fulfill her patient’s needs in a more comprehensive and fulfilling manner than typical conventional medicine allows.


We have a 96% success rate in patient outcomes

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We have created a practice that we believe in and choose for our own family members.



Functional medicine offers a whole new way to understand and look at chronic illnesses from diabetes to heart disease; autoimmune disease and digestive issues, to chronic pain, cognitive issuesthyroid, anxiety, depression, weight gain, and more. The core concept is that chronic illness is an outcome of imbalances in the key underlying physiological systems in your body. When inflammation, blood sugar imbalances, insulin resistance, oxidative stress, hormonal problems, toxicity, and systemic upsets occur, the body spirals out of balance. 

We use a unique clinical approach with this.  We run diagnostics typically not used conventionally that find the root cause of the issues creating symptoms. 

We then take a more natural approach to restore balance within the body through nutrition, dietary changes, and supplements.  Patients feel remarkably better, unlike the way they feel when they are merely given chemical drugs by their traditional doctors.



  • Patients are feeling years younger.

  • Medications are rapidly being eliminated.

  • Blood chemistry tests are normalizing.

  • Hormones are being balanced.

  • Stubborn weight is coming off.

  • Energy levels are being dramatically improved.

  • Pain and inflammation are reduced and resolved.

  • Brain fog is lifted.

  • Mental clarity returns.

  • Sleep difficulties are improved.

Dr. Susanne Macsay, DDS, NMD, ND, aka Dr. Susy

Dr. Susy is a board-certified and licensed Naturopathic Physician in California, Arizona, Oregon, as well as Puerto Rico.

As a former Dentist – still licensed in her home country of Denmark - she was transitioning over the years to a more comprehensive and holistic system of healthcare and was very happy to discover Naturopathic and Functional Medicine early in her career.

Dr. Susy’s years of practicing have been shared between her helping patients with chronic disease with a focus on research-based natural cancer care, including Gerson Therapy and advanced Integrative Cancer care uniting Traditional & Modern Naturopathic Medicine with Acupuncture & Oriental Medicine.

Dr. Susy was also a professor at the Naturopathic Doctoral program in Puerto Rico, teaching clinical/medical sciences as well as supervising general care, acupuncture and physical medicine clinic, educating medical students at the doctorate level.

Dr. Susy’s passion is to help people feel their best, reduce pain, improve and maintain their health & well-being, and her guidance is focused on self-care as key which incorporates Mind-Body Medicine.

Dr. Susy is also a certified Upledger CranioSacral Therapist since 1995, as well as a certified Yoga teacher, and she has practiced, taught and lived in Denmark, Greece, Turkey and United States, incl. Puerto Rico.

Furthermore, due to her background as a Dentist Dr. Susy pays specific attention to the dental health of her patients.  Science is revealing on an almost daily continuum that dental health is paramount to overall health, and that mouth and body are inseparable.

Dr. Susy believes that prevention and daily personal practice is the best cure, and that nature and laughter are some of our greatest healers. She loves nature in all its glory, the beach, the mountains, and is very excited to be exploring San Diego and SoCal, the salsa dance scene, culture, and the performing/arts while enjoying being close to her grown children, one of which is a Naturopathic Doctor as well.

Dr. Grace Victor, ND

Dr. Grace is a CA Licensed Naturopathic Doctor.  She earned her Doctorate of Naturopathic Medicine from Bastyr University in San Diego, CA. Her journey to Naturopathic Medicine began after completing her undergraduate degree with a desire to find a model of medicine that focuses on chronic disease management, prevention, and a patient-centered health care model that allows patients to heal from the inside- not just mask symptoms with pharmaceuticals. Dr. Victor was drawn to the success of the Functional Medicine healthcare model at Advanced Functional Medicine. Her passion is to support patients on their healthcare journey through utilizing nutrition, botanical medicine, homeopathy, lifestyle, and nutrient supplementation. Outside of the office, Dr. Victor enjoys spending time with her family, and enjoying the sunny SoCal weather!

Sara Leckman, BSN, RN, HHC


Sara is a Registered Nurse and Board-Certified Health and Wellness Coach originally from Washington State where she completed her Bachelor’s degree in the Science of Nursing at Seattle Pacific University. Before joining the team at Advance Functional Medicine, Sara worked as a Nurse Clinician at a Community Health Clinic in Seattle, where she practiced chronic disease management and led diabetes education programs. Early on in her career, she recognized that the conventional-medicine model fails to identify and treat the root-cause of disease, and often leaves patients desperate for more help. Sara decided that she could no longer be a part of the ineffective and costly medical system that simply “manages chronic disease”. In search of something greater, Sara discovered the power of Functional Medicine. Since then, Sara has completed additional training in Nutrition, Holistic Nursing and Functional Medicine.

Sara believes the human body is innately intelligent and fully capable of healing given the right environment. As an RN Health Coach, Sara sees her patients as active-participants in their care, as they are supported in creating lasting-lifestyle change. Sara’s mission is to educate and empower patients to take back control of their health. What Sara loves most about her role here at Advanced Functional Medicine, is seeing patients get off numerous prescription medications while they continue to reverse disease and live more fulfilling lives.

Dr. Kris Larson, MSOM, DACM


Dr. Kris has a masters in Oriental Medicine, and is a Doctor of Acupuncture and Chinese Medicine. Her journey towards becoming a medical professional was an uncommon one. She started having her own personal health struggles in her early twenties- nagging, unexplained symptoms that would not go away.  After several years, she finally turned to alternative and natural forms of medicine for healing, and like many, her life was forever changed!  This started her journey of becoming a doctor that could fully embrace the patient, healing from the inside-out- breaking the confines of the traditional drug-therapy model.


She is passionate about Functional Medicine by helping patients heal through nutrition, supplements, modalities like acupuncture and a positive mind-set. She is a believer that educating and empowering her patients to live a more clean, non-toxic lifestyle combined with healthy eating can be the key foundations to optimal health and aging. 

Leslie Katousky - Clinical Nutritionist

Leslie started her health journey when she was just in high school. After seeing incredible results with changes to her diet she decided to pursue a career in nutrition. Leslie attended San Diego State University and received her Bachelors of Science in Foods and Nutrition. After graduation she began working at Palomar Hospital then a Skilled Nursing Facility as a Dietary Supervisor. She loved working with patients but wanted to help prevent these illnesses. She went on to work for weight loss company then found her way to Advanced Functional Medicine where her passion for preventative and natural medicine was able to expand and be used. With over 10 years of patient experience, Leslie has been apart of the our team for many years and has enjoyed learning from Dr. Willis and getting to work with patients to transform their health. She enjoys continuing her education in her spare time, cooking, and spending time with her husband and daughter enjoying the beautiful city of San Diego. 


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Phone: (858) 412-5744
Hours: Mon: 9am - 4pm
Tues - Thurs: 9am - 6pm
Fri.  9:00am to 2:00pm 

Dr. Willis made Top Doctors list in Who's Who in America:

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