As in the previous post, BMR accounts for 70% of total energy expenditure. Thus, changes in BMR can significantly affect the amount of weight we will get or lose during weight management programs.
Many factors influence the basal metabolic rate (BMR). BMR increases with the increase of weight (mainly due to more fat free mass in heavy people), decrease with age (2% drop per decade), and higher in male, lower in female. Body temperature, activity level, and drugs can also change BMR. But most important, the genetic makeup probably accounts for the most difference of BMR between persons.
From the weight change point of view, the most important factors are fat free mass, the dietary factors, and physical activity. They are modifiable and often the weapons of weight loss programs.
For a person with 1800 kcal BMR, a small change in the BMR, say 2%, may be equivalent to 36 kcal. This is very important for weight gain. For example, given the same amount of energy intake and physical activity, a person with a BMR 2% lower than the population average BMR may gradually gain 4 lb fat (and some fat free mass too) in one year. This is not a trivial amount of fat.
On the other hand, during the weight loss process, The decrease of BMR due to the loss of fat free mass is small, and this small difference may have little effect on overall weight loss, since most weight loss programs target at losing 2 lb per week. People have to reduce at least 500 kcal a day. Compared with this, the 36 kcal is quite negligible.
However, BMR does have a big change during weight loss, particular among those who practice only dieting. During a low calorie diet session, the BMR will gradually slow down too—sometimes up to 30% less than that of pre-dieting. This is the body defense mechanism against food shortage developed before the existance of human beings.
The 30% change of BMR is very big. Think about it, for a person with an 1800 kcal BMR at baseline, after a month of dieting, the BMR now becomes 1260 kcal, close to the daily energy intake for a low calorie diet (800-1200 kcal per day). Without other ways to create further energy deficiency, that person will experience a diminished weight loss progress, and his/her weight loss may reach the plateau.
Furthermore, even within a day, during a prolong starvation (e.g., by skipping a meal), the BMR slows down too. In addition the long fasting also increases the food intake in the next meal, resulting a binge eating pattern. This is the main reason that dieters should not skip any meal.
To overcome the decrease of BMR during the course of weight loss, one solution is to increase daily physical activity. It is known that physical activity has a extended effect on BMR after the activity is finished. The BMR still remains higher than normal 30 minutes after a moderate exercise. This extended effect may be longer with longer and more vigorous exercise. My personal experience is that my body becomes warmer— an indicator of increased BMR—during current weight loss session.
Since the basal metabolic rate reflects the underline intensity of the biochemical reactions in the body, what is the effect on longevity of human life? This is a very interesting question. In fact, this hypothesis has been proposed 60 years ago. People think that a lower BMR may be related to a longer life because the body works slower, thus is worn out slower. There are some animal studies suggesting this possibility. For example, C. elegant and genetically modified mouse can live longer by moving slower. In reality, slow moving animals such as turtle live much longer than similar species. Monks and nuns also seem to live longer than common people because their vegetarian diet may cause their lower BMR. However, this hypothesis has not been confirmed, nor been refuted. Further research is needed.