Resting Metabolic Rate (RMR)
Long-term rapid weight loss programs can have a profound negative effect on your resting metabolic rate. The resting metabolic rate is the body’s total caloric requirement over a 24-hour period to sustain all its functions at rest, including vital organ functions, maintaining body temperature, thought processes, and respiration. In 1993, researchers examined 328 healthy men (17–80 years old) and 194 women (18–81 years old) volunteers who were characterized for RMR, body composition, physical activity, peak oxygen consumption (peak VO2), anthropometrics, and energy intake. Measured RMR was 23% higher in men, approximately 1750 kcal/day, than in women at about 1350 kcal/day. Your resting metabolic expenditure, another term for RMR, varies with your body size, age, muscle mass, and climate – the rate is lower in warmer climates because you need to expend less energy to maintain your body temperature.
If moving to a colder climate is not your way of altering your RMR, you can increase the resting energy expenditure by being more physically active, increasing muscle mass. With more muscle mass, you burn more per day at rest. How many, you ask? A change of just 100 calories can theoretically equate to 36,500 calories per year or roughly 10 pounds of body fat [1]
Of course, you can also reduce your RMR by opting for a sedentary lifestyle, losing muscle mass, or excessive caloric restriction. That’s right, lowering your calories, say with a diet, can reduce your RMR and reduce the efficiency of your dieting.
The reduction in your resting metabolic rate is a natural protective mechanism by the brain to prevent what it interprets as insufficient calories to maintain vital organ function and all the related physiological functions that occur 24 hours per day, seven days a week. Let me give you an example. While running an exercise physiology lab, I had a chance to measure the RMR in two of our employees. Both had decided to pursue an 800-calorie-per-day liquid weight loss program offered through a local endocrinologist due to their excess weight. Before the start of their 800 calorie dietary quest, both fell into the normal range of 1,200–1,300. One month into their weight loss efforts, their RMRs declined to roughly 950–1,000 calories per day. This 200 – 300 calorie reduction in RMR equates to burning the equivalent of 20 – 30 pounds less body fat over a year at rest.
The body’s reduction in resting metabolic expenditure in the face of “diminishing caloric intake” is a major issue overlooked by most consumers. It is a significant reason why most who pursue rapid weight loss programs find themselves regaining the weight faster the second time around and finding it increasingly more difficult to lose any significant fat mass in the future. Combining a now abnormally low RMR with the combined limited ability to exercise makes the task of losing excessive fat mass extremely difficult.
Many weight loss programs use various equations to predict one’s RMR. However, studies have demonstrated these equations are often inaccurate, misleading participants, and significantly impacting their long-term success. One study reported an overestimation of the RMR of approximately 10% for men,15% for women, and possibly as high as 20–30% for other demographic groups. Another concluded that equations predicting RMR within 10% of an actual measured value were only accurate approximately 40% of the time, regardless of gender and weight classifications. The authors concluded, “in clinical weight management settings, direct measures of RMR should be made wherever possible.” Lastly, in 2017 researchers again validated that compared to direct measured RMR prediction equations, overestimate resting metabolic rates and energy requirements by 10% to 30% depending upon the equation. Therefore, without direct measurement, many obese individuals may be greatly overestimating their caloric needs at rest, as well as during exercise, as pointed out previously, deterring their efforts to successfully lose or manage their weight.
The takeaway points are as follows:
- Long-term success at weight loss and control involves effort, pain, difficulty, as well as time. There are no long-term successful shortcuts as advertised.
- You need a base level of calories per day for normal resting physiological processes to function normally. Caloric levels sustained below these levels will initiate a “starvation mode” physiological adaptation that will hinder both short-term and long-term success at weight control.
- Expecting more than 1-2 pounds per week in actual fat mass loss is unrealistic for most. Anything greater than this is water weight.
- Calories needed at rest as well as during exercise increase with improved physical conditioning.
This article highlighted the dangers associated with underestimating basic resting caloric needs to lose unwanted body fat. In the next part of this series, I will highlight how overestimating your caloric needs for exercise will also hinder your weight control efforts and the considerable variations between fit and unfit individuals.
[1] The usual calculation is that there are 3,500 calories in one pound of body fat
