• BMR /basal metabolic rate/ is a rate of oxygen uptake at rest in the fasting and thermo-neutral state.
• TDEE /total daily energy expenditure/ represents the total number of calories a person burns in a day. TDEE is calculated by factoring in your Basal Metabolic Rate (BMR) and the calories burned through physical activity.
• FFM /fat-free mass/ Body mass is divided into two components: fat mass and FFM. The constituents of FFM are skeletal muscle mass, body cell mass, total body water, and bone mineral mass. In men, peak FFM is reached in the mid-30s and progressively declines thereafter; in women, it is stable in young adulthood until about age 50 when it begins to progressively decline with advancing age. Clearly, the homeostatic system regulating FFM is deranged at advanced age. An important component in the age-associated decrease in FFM is the loss of skeletal muscle mass, an important factor in the decrease in muscle strength with age. Although physical exercise with an emphasis on weight-training can decrease the loss of muscle mass in the elderly, even individuals who maintain their fitness have some age-associated loss of muscle.
• Fat mass. Body fat mass increases with age in both men and women through middle age; a slow decrease occurs after age 70. Even in those people whose body weight does not increase with age, body fat increases as lean body mass decreases. The homeostatic regulation of fat mass becomes faulty with advancing age. Sedentary lifestyle plays a role in the age-associated increase in fat mass since exercise is associated with a decrease in fat mass in the elderly. However, exercise attenuates but does not totally prevent the age-associated increase in body fat. There is also a redistribution of fat to the abdominal region with increasing age. Exercise not only decreases the age-associated increase in body fat but, most importantly, it also preferentially attenuates the disproportionate increase in abdominal fat. The great concern about abdominal fat is due to the extensive evidence indicating that it is a risk factor for several age-associated pathologic problems such as coronary heart disease and type 2 diabetes. Indeed, abdominal fat mass is positively associated with mortality in the elderly. Thus interventions aimed at preventing the abdominal accumulation of fat with advancing age are most important to develop.
• LBM /Lean body weight/ Lean body weight is the difference between TBW /total body weight/ and fat mass. LBW represents the highly vascular portion of the body and is significantly correlated to cardiac output (CO), which is an important determinant in the early distribution kinetics of drugs.
• PAR /physical activity ratio/ is the total energy cost when active divided by the measured or predicted BMR.
• PAL /physical activity level/ is a ratio of the total energy expenditure on a 24-hours basis divided by the BMR expressed over the same time period.
• Aging: Body Composition. There is a progressive decrease in fat-free mass and a progressive increase in fat mass as a person ages. Furthermore, bone density decreases, joint stiffness increases, and there is a small reduction in stature (kyphosis). These changes in body composition have probable implications for several conditions, including type 2 diabetes, obesity, heart disease, and osteoporosis.
• Aging: Muscle Tissue. The primary alteration is a reduction in total skeletal muscle mass. After age 30, the average person will lose approximately 3–8% of their muscle mass with each passing decade of life. Specific cellular alterations include reductions in muscle cell number, muscle twitch time and twitch force, sarcoplasmic reticulum volume, and calcium pumping capacity. Sarcomere spacing becomes disorganized, muscle nuclei become centralized along the muscle fiber, the plasma membrane of muscle becomes less excitable, and there is a significant increase in fat accumulation within and around the muscle cells. Neuromuscular alterations include a decrease in the nervous firing rate to muscle, the number of motor neurons, and the regenerative abilities of the nervous tissue. Motor unit size also increases.

BMR is often estimated using equations based on an individual's age, sex, weight, and height, alongside other factors like body composition and physical activity level 'Best-fitting prediction equations for basal metabolic rate: informing obesity interventions in diverse populations' by 2013 Jan 15

One of the well-known methods for BMR calculation, this equation takes into account an individual's weight, height, age, and sex.

This is considered more accurate by many health and fitness professionals, especially for estimating the resting metabolic rate.

Focuses on lean body mass to calculate BMR.

Another established method for estimating BMR based on weight, height, and age.

BMR has a significant relationship with FFM, and equations can be derived to estimate BMR based on FFM.

There's a concern about underestimation or overestimation of BMR using standard equations, especially in individuals with different body compositions, health statuses, or ethnic backgrounds. More accurate BMR estimation may require adjustments or the use of alternative equations that consider these factors. Basal Metabolic Rate

• An equation for BMR calculation was provided in another article, defined as BMR (Kcal/day) = K * weight (kg) + w * height (cm) - K1 * age (years) + e(Kcal/day) ×age (year)+ε(Kcal/day). article