Characteristics of San Francisco Bay Cold-Water Swimmers

ABSTRACT

To further quantify the characteristics of cold-water swimmers, and to examine the potential exercise benefits, weight changes, perceptions of cold tolerance, and safety concerns related to swimming recreationally in cold water.

After 3 months (December 21 – March 21) of swimming regularly in cold water in the San Francisco Bay (water temperature range: 9.6° C (49.3° F) to 12.6° C (54.7° F)), 103 subjects (76 men, 27 women) underwent a detailed biophysical analysis (weight, body mass index, and % body fat (circumference method)). Swimmers also completed a questionnaire that included questions related to background, exercise patterns, safety, and cold tolerance. To allow for before-and-after comparisons, a sub-group of 50 subjects had biophysical measurements both before, and again after the 3-month winter swim period.

Swimmers were typically in a middle-aged category (mean: 54.3±10.8; range: 24-79, years), employed full-time (69.9%), had a recreational swim background (62.1%), and had body mass index (BMI) (25.9±3.6 kg/m2) and % body fat (23.8±7.5 %) values that were consistent with average American values. The exercise time per week of the subjects (mean: 131.3± 50.8 minutes/week; median: 120 minutes/week) was significantly greater than an intermediate-intensity value of 100 minutes/week based on American College of Sports Medicine /American Heart Association (ACSM/AHA) recommendations (P<0.001). Although 27 of 50 before-and-after subjects lost weight (mean weight loss: 2.1 ±2.6 kg; range of weight loss: 0.1-10.8 kg), subjects overall did not significantly gain or lose weight, or change BMI or % fat (P>0.1 for all before-and-after comparisons). The most frequently reported safety concerns were marine life (47.4%) and cold/hypothermia (46.4%), followed by boats (30.9%), currents (19.6%), cramps/fatigue (15.5%), and other (14.4%). A report of feeling more adversely affected by cold was associated with a lower BMI (Coef. = -2.81; P= 0.002), lower % fat (Coef. = -6.66; P<0.001), and less time in the water per swim (Coef. = -4.23; P= 0.029), while a report of feeling less affected by cold was associated with a higher BMI (Coef. = 2.55; P= 0.001), higher % fat (Coef. = 3.83; P= 0.010), and more time in the water per swim (Coef. = 3.51; P= 0.039).

Individuals with a wide variety of ages, body types, and backgrounds are able to swim recreationally in cold water. Such swimmers typically are not obese and exceed the ACSM/AHA recommendations for weekly exercise. Marine life and cold/hypothermia are frequent safety concerns; our results suggest that swimmers who report worse cold tolerance are more likely to be leaner and limit time in the water, while those who report better cold tolerance are more likely to have a higher % body fat and spend more time in the water per swim.

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