No treatments have been shown to slow prostate cancer progression after radical prostatectomy. We hypothesize that a carbohydrate restricted diet will slow prostate cancer growth. A total of 60 men with a rising prostate-specific antigen (PSA) after failed primary treatment will be recruited and randomized to either a low-carbohydrate diet (<20 grams carbohydrates/day) or a no-diet control (standard of care) for 6 months. The primary outcome is PSA doubling time.

Inclusion Criteria:

• Received prior radical prostatectomy or definitive local radiation for prostate cancer (either external beam radiation, brachytherapy, or combination)

• PSA within the past 3 months is between 0.4 and 20 if prior radical prostatectomy, or between 3 and 20 ng/ml if prior radiation therapy.

• PSA doubling time (PSADT) >3 months and <36 months

  1. Calculated based at least 2 values (at least 0.2) in the prior 2 years with the first and last PSA separated by at least 3 months
  2. Use all values in the last 2 years to calculate PSADT
  3. PSADT calculated while NOT on androgen deprivation therapy (ADT).
  4. If prior ADT use, then documented either A) normal testosterone or B) a testosterone within 50 points of normal and stable (defined as a second testosterone at least 6 weeks later that is equal or lower than the first testosterone) is required before starting to calculate PSADT.

• BMI >=24 kg/m2

• Willing to be randomized to a no-diet control or a low-carbohydrate diet

• Reads, writes, and understands English

Exclusion Criteria:

• Anticipate needing secondary prostate cancer therapy within the next 6 months (i.e. radiation, or hormonal therapy)
• Current use of weight loss medications including herbal weight loss supplements or enrolled in a diet/weight loss program
• Currently on therapy aimed at lowering testosterone levels (includes gonadotropin-releasing hormone (GnRH) agonist/antagonist, prior bilateral orchiectomy, oral anti-androgens, or 5-alpha reductase inhibitors). Testosterone replacement is allowed but treatment should be stable during the entire study.
• Known distant metastatic disease
• Already consuming a carbohydrate-restricted or vegetarian diet
• Unable or unwilling to adhere to a carbohydrate-restricted dietary intervention
• Weight loss >5% of body weight in the last 6 months
• Medical comorbidities that in the opinion of the investigator limits the patient's ability to complete this study