Energy metabolism is the process by which the body transforms the food we eat into usable energy to sustain life. Every cell in the human body requires a continuous supply of energy to carry out basic chemical reactions, maintain structural integrity, and perform specialized functions like muscle contraction and nerve signaling. This energy is primarily stored and transported in the form of adenosine triphosphate (ATP).
The overall energy expenditure of an individual is divided into distinct components:
In pregnancy and lactation, the maternal body undergoes significant metabolic shifts. The total energy requirement increases to support fetal growth, development of the placenta, and the production of breast milk. Understanding these foundational concepts helps midwives appreciate why nutritional counseling must go beyond simple calorie counting to look at how the body actually processes and utilizes fuel.
Basal Metabolic Rate represents the minimum amount of energy required to keep the human body alive while resting in a neutrally warm environment. To measure true BMR accurately, strict conditions must be met: the individual must be completely at rest physically and mentally, in a post-absorptive state (usually 12 hours after the last meal), and breathing room air at a comfortable temperature.
In practical clinical settings, measuring true BMR can be difficult because these strict conditions are rarely achievable outside of a laboratory. Therefore, healthcare providers often measure Resting Energy Expenditure (REE) instead. REE measures the energy spent when a person is at rest, but it does not demand the absolute fasting and strict environmental controls of a true BMR test. In everyday language, clinicians often use BMR and REE interchangeably, though REE is usually slightly higher—about 10% more—due to recent food intake and minor physical movements.
BMR accounts for the lion's share of total daily energy expenditure for most sedentary adults, making up roughly 60 to 75 percent of the calories burned each day. It keeps the heart beating, the lungs expanding, the kidneys filtering waste, and the brain processing information, all without any conscious effort from the individual.
Several biological traits dictate how fast or slow an individual's basal engine runs. Recognizing these factors helps midwives explain why different pregnant women have unique nutritional needs.
Beyond fixed biological traits, external conditions and disease states can temporarily or permanently alter how much energy the body burns at rest.
Environmental temperature plays a noticeable role. When exposed to cold environments, the body increases its metabolic rate to generate extra heat through shivering and non-shivering thermogenesis. Conversely, extremely hot environments can also prompt minor metabolic shifts.
Fever is a powerful pathological driver of energy expenditure. For every one degree Celsius rise in core body temperature, the basal metabolic rate increases by roughly 10 to 13 percent. This is why patients fighting infections rapidly lose weight if their nutritional intake is not adjusted to match the higher internal demand.
Nutritional status also changes metabolic output. During prolonged fasting or semi-starvation, the body enters a conservation mode, intentionally lowering its BMR to preserve remaining energy stores. This adaptive slowdown is a key reason why strict crash diets often fail over time. Hormonal disorders such as hyperthyroidism cause severe metabolic acceleration, while hypothyroidism causes marked sluggishness and weight gain.
A pregnant woman's metabolism is far from static. As gestation progresses, her BMR rises steadily to support the growing fetus, enlarged uterus, placenta, and increased maternal cardiovascular workload. By the third trimester, basal metabolic rate can increase by as much as 15 to 25 percent compared to pre-pregnancy levels.
Midwives must understand these metabolic changes to provide accurate nutritional guidance. If a pregnant woman does not consume enough calories to match her rising BMR and the demands of fetal development, her body may break down its own tissues, leading to poor gestational weight gain, fetal growth restriction, and maternal fatigue.
Similarly, during lactation, the metabolic cost of producing breast milk is substantial, requiring an additional daily energy intake. Recognizing that a nursing mother's BMR and total energy expenditure remain elevated prevents undernutrition and supports successful breastfeeding outcomes. Assessment of weight changes alongside clinical signs of energy depletion allows midwives to intervene early with appropriate dietary advice.