Exercise Training and Time-restricted Feeding in Overweight and Obese Adults

Overweight and obesity prevalence in adolescents and adults continues to remain significantly high in the United States in all socioeconomic categories, regardless of racial and ethnic backgrounds. This is important because aging is strongly linked with increases in adiposity and alterations to the distribution of fat in the body, including visceral, hepatic, and intermuscular fat stores. These areas of fat storage are independently associated with increased risk of cardiovascular disease (CVD) and physical dysfunction. These conditions represent a major health problem in the US and are often triggered by multilayered dietary imbalances and lack of physical activity.

CVD is the leading cause of death in the United States, with 30% of adults older than 19 years of age having hypertension, and 16.5% of all deaths being attributed to high blood pressure. Physical disfunction with aging, also referred to as sarcopenia and dynapenia, is the gradual and progressive loss of muscle mass, strength, and endurance. Sarcopenia is characterized by a 3-8% loss of muscle mass per decade after the age of 30 years, affecting 30% of individuals over 60 years and 50% of individuals over 80 years. This age-related decline in muscle mass negatively affects strength, balance, and stability; leading to an increased risk of falls and impaired ability to perform activities of daily living such as walking, personal care, cooking, and chores. The most alarming consequence of decreased muscle strength is its ability to predict future mortality in middle-aged and older adults. While diet and exercise improve many health consequences of obesity and attenuate declines in muscle mass and strength, dietary strategies are not always nutrient sufficient and manageable for long-term use. Thus, highly compliant dietary strategies that facilitate fat loss while maintaining fat-free mass are needed.

Continuous energy restriction (CER), a reduction in daily caloric intake up to 40%, is a primary dietary strategy to help individuals decrease fat mass and lower the risk of CVD. While CER can be effective, it is associated with poor compliance and appears to accelerate the return of pre-deprivation body mass levels once the restraints over feeding are removed. More importantly, CER is known for weight loss consisting of up to 10%-60% fat-free mass, which suggests a large proportion of metabolically active skeletal muscle tissue is lost instead of adipose tissue. TRF, a variant of intermittent fasting, is an increasingly popular dietary approach because it does not require a restrictive energy intake as with CER. TRF allows individuals to consume ad libitum energy intake within a set window of time (example = 8 hours), inducing a fasting window of 16 hours per day. Literature from animal studies have demonstrated reductions in body weight, total cholesterol, and concentrations of triglycerides, glucose, insulin, as well as improvements in insulin sensitivity following TRF. Unfortunately, human studies on TRF are limited and few exist that measure their effects in combination with aerobic or resistance training.

One recent study recruited 34 healthy, resistance-trained males and randomized them into either a TRF (16-hour fast) or NF group. The groups were tested before and after eight weeks of their diet assignment and standardized resistance training for body composition, maximal strength, and multiple health-related biomarkers. These biomarkers included total and free testosterone, IGF-1, blood glucose, insulin, adiponectin, leptin, triiodothyronine (T3), thyroid stimulating hormone, interleukin-6, interleukin-1B (IL-1B), tumor necrosis factor a (TNF-a), total cholesterol, high-density lipoprotein (HDL), low-density lipoprotein (LDL), and triglycerides. After the eight weeks of training, the TRF group significantly decreased fat mass compared to a NF group (-16.4% vs -2.8%). Hormonal responses included significantly reduced levels of total testosterone and IGF-1 with TRF, typically seen during CER. Though these anabolic hormones were reduced, no reductions in fat-free mass and strength were observed. In fact, fat-free mass, as well as arm and thigh cross-sectional area, was maintained in both groups. Leg press one-repetition maximum increased significantly in both groups, and, while not significant, bench press one-repetition maximum increased in both groups. These increase in strength are important to note, considering the subjects were highly resistance trained. Another interesting effect of TRF was reduced blood glucose and insulin levels, which contributed to a significant improvement in HOMA-IR (insulin resistance). Adiponectin increased, while leptin decreased with TRF. These responses were said to be linked to an enhanced regulation of insulin sensitivity and an improved anti-inflammatory effect in the TRF group. Lastly, T3 and Triglycerides decreased significantly and TNF-a and IL-1B were lower in TRF compared to NF. Overall, the study established TRF as a beneficial dietary strategy to improve health-related biomarkers, decrease fat mass, and maintain fat-free mass. Therefore, the feasibility of TRF as a dietary approach, for improving body composition and attenuating the risk factors of CVD and physical dysfunction that occur with obesity and aging, should be investigated further.The study will recruit 40, overweight (determined by body mass index between 25.0-29.9 kg/m2) male and female participants (ages of 45-60 years old) who are not currently following a structured aerobic or resistance training program or dietary plan. This will be a randomized, controlled trial with assessments made pre- and post-intervention. All subjects will be scheduled for an 8-week, standardized aerobic and resistance training program. Participants in TRF group will be required to consume all their energy intake in an 8-hour feeding window (12:00pm to 8:00pm), and will perform their exercise training within that feeding window. Participants in the NF group will maintain their typical dietary habits. Once training is finished, participants will complete post-training assessments that include all pre-training assessment variables.