Researchers at the University of Adelaide in South Australia have shown that that there is an inverse relationship between PSA levels in the blood and the body mass of the men being tested.
According to members of this research team:
We have shown for the first time that the concentration of PSA in the blood is lower in men with severe obesity (with a body mass index or BMI of 30 or higher) than in lean men, and that this can be attributed to lower concentrations of circulating testosterone.
and
The results of this study have important implications for how we should interpret PSA levels in men who are obese.
The paper by Aref et al. was just published in the journal Endocrine-Related Cancer. Additional information can be found on the ScienceDaily web site.
Aref and his colleagues set out to investigate the nature of the effects of obesity on PSA and to see if they could determine potential contributing mechanisms.
To do this, they looked at data from a cohort of 1,195 men aged 35 years and over at recruitment who were participants in the Florey Adelaide Male Ageing Study. Men who had been diagnosed with prostate cancer or for whom there were no PSA data collected were excluded.
Based on all the available data from the eligible group of 970 men remaining, Aref et al. were able to show the following:
- PSA levels were significantly lower in men with greater waist circumference (p = 0.001).
- In a multivariable model including waist circumference, age, estradiol/testosterone levels (E2/T), and plasma volume as predictors,
- There was no statistically significant association between PSA level and waist circumference (p = 0.36).
- There was no statistically significant association between PSA level and plasma volume (p = 0.49).
- There was a strong association between PSA level and E2/T (p < 0.001).
- There was a strong between PSA level and age (p < 0.001).
- In what is known as “mediation analysis”
- When plasma volume was used as the mediator, the average causal mediation effect explained about 20 percent of the total effect of waist circumference on levels of PSA (p = 0.31).
- When E2/T was used as the mediator, the average causal mediation effect explained about half of the effect of waist circumference on levels of PSA (p < 0.001).
What this all means, according to Aref et al., is that
… lower PSA levels in obese men, as compared to normal weight men, can be explained both by hormonal changes (elevated E2/T ratio) and hemodilution. Hormonal factors therefore represent a substantial but underappreciated mediating pathway.
Exactly what the “important implications” are going to be for the interpretation of PSA data in assessment of risk for prostate cancer and monitoring of men with prostate cancer when those men are overweight or obese will require further research. However a possible implication is that an otherwise healthy man with a PSA level of 2.5 ng/ml may be at greater risk for prostate cancer if he is obese (i.e., if he has a body mass index or BMI of ≥ 30) than if he is lean (with a BMI of ≤ 25). This could be one more factor in helping to determine which patients really need a biopsy based on their PSA level and other test data.
THE "NEW" PROSTATE CANCER INFOLINK 
So does this mean that at 6’1″ and 175 pounds my PSA is actually lower than an “average” 6’1 man at 215 pounds (obese) and therefore that the whole PSA grading system is skewed towards the overweight and I’m doing better than them?
Previous studies have already shown this inverse BMI and PSA relationship, e.g.: this study and also this one.
Dear Tom:
No. You can’t take data like these and start applying them to individuals without a great deal more research. Also no, it would be highly misguided to think that “the whole PSA grading system is skewed towards the overweight”.