This study aims to determine the effects of 8-10 weeks of repeated hot water immersion ("heat therapy") vs. thermoneutral water immersion on vascular function in late middle-aged to older (55-79 years) adults.

Advancing age is the primary risk factor for cardiovascular diseases (CVD), which remain the leading cause of death in the developed world. The key process that links aging to increased risk of CVD is the development of arterial dysfunction, including impaired endothelial function and arterial stiffening. In addition, age-associated impairments in arterial function have been linked to other diseases/disorders of aging, including cognitive impairments, Alzheimer's disease and dementia, and motor disorders/disability.

Passive heat therapy, in the form of repeated use of hot baths and saunas, has been used by several cultures for centuries; however, the physiological benefits are only now being elucidated. For example, recent studies have found that lifelong habitual sauna use is associated with considerably reduced risk of all-cause mortality, sudden cardiac death, CVD-related death, and Alzheimer's disease and dementia. In addition, 8-10 weeks of hot water immersion in young adults has been shown to improve endothelial function and reduces arterial stiffness and blood pressure. Therefore, the investigators expect heat therapy to similarly improve vascular function, as well as cognitive and motor function, in middle-aged to older adults who are at greater risk of CVD and related diseases/disorders.

Inclusion Criteria:

• Ability to provide informed consent.
• Aged 55-79 years. Women must be postmenopausal and not taking hormone replacement therapy within the past year.
• Sedentary to recreationally active: participating in vigorous aerobic exercise <4x/wk for ≥ 3 yrs.
• Able to refrain from the use of dietary supplements, anti-inflammatory medications, and prescription medications for 24-48h prior to experimental testing.
• Generally healthy (no overt clinical disease), as determined by medical history, physical examination, blood and urine chemistries.
• Ankle-brachial blood pressure index >0.7
• Total cholesterol <240 mg/dl (<6.2 mmol/L)
• Fasting plasma glucose <126 mg/dl (<7.0 mmol/L)
• Systolic blood pressure < 160 mmHg and >100 mmHg
• Glomerular filtration rate > 60 ml/min/1.73 m2
• Mini mental state exam score > 20 points
• Body mass index (BMI) <30 kg/m2
• Weight stable in the prior 3 months (≤ 2 kg weight change) and willing to remain weight stable over the course of the study
• Free from alcohol dependence or abuse, as defined by the American Psychiatry Association, Diagnostic and Statistical Manual of Mental Disorders (DSM-IV).

Exclusion Criteria:

• Current use of certain prescription medications that may interfere with thermoregulation and/or blood pressure control during heat stress. These include anti-hypertensives (in particular beta-blockers), nitrates and nitrites (e.g. nitroglycerin), PDE5 inhibitors (e.g. Viagra), anticholinergics (e.g. amitriptyline), and alpha-blockers (e.g. Flomax).
• Orthostatic hypotension
• Current or past smoking within the last 5 years
• Recent major change in health status within previous 6 months (i.e. surgery, significant infection or illness).
• Chronic clinical diseases (e.g., inflammatory bowel diseases/irritable bowel syndrome, coronary artery/peripheral artery/cerebrovascular diseases, diabetes, chronic kidney disease requiring dialysis, neurological disorders or diseases that may affect motor/cognitive functions [multiple sclerosis, Parkinson's disease, polio, Alzheimer's disease, dementia or other brain diseases of aging]).
• History of heat-related illness
• Open wounds or skin lesions. History of skin-related conditions or sensitivities to prolonged water immersion or exposure to pool chemicals.
• Blood donation within the past 2 months
• Inability to tolerate blood draws, intravenous catheters, and/or fingertip blood sampling, including past fainting in response to blood sampling.
• Rectal, anal, vaginal, or prostate surgery within the last 6 months, current or recent (within the last 6 months) hemorrhoids, or related concerns regarding use of rectal thermistor

Investigators collecting and analyzing data will be blinded to subject treatment arms. It is not possible to blind subjects.