During the first few days at a higher altitude with thinner air that contains less atmospheric oxygen, one’s body will respond as follows

• breathing rate (ventilation) increases

• blood pH decreases, becoming more acidic

• muscle pH also decreases and becomes more acidic

• use of carbohydrates as an energy source increases

• use of fat as an energy source reduces

These changes lead to a slight increase in basal metabolic rate (BMR), the amount of energy needed to keep your body working correctly at rest for 24 hours.

Research studying this phenomena have found between a 6 to 28 percent increase in overall BMR in women and men at high altitudes.

With time, the increase in BMR lowers, but does not return completely to baseline, so metabolism is slightly elevated at higher altitudes.

With this said, it seems that someone can expend many more calories while exercising at high altitudes; however, this is not necessarily the case.

There isn’t a immense difference of calories utilized at a higher altitude during exercise even though it could feel as though a individuals is working much harder.

The few extra calories someone will use will be a result of increased breathing to get more oxygen into the body, which means increased work by the rib cage and other muscles involved in ventilation.

So even though ventilation increases and oxygen delivery is enhanced, it doesn’t necessarily mean a individuals is expending more energy while exercising.

Here are some factors that do influence the amount of calories a individuals expends in general

Genetics

Some people are born with high metabolisms (the rate at which one’s body uses calories) and others are born with low metabolisms. the primary culprit of these phenomena is the thyroid gland.

A few individuals produce more thyroxin (the hormone that is secreted from the thyroid) than others.

Thyroxin is responsible for metabolism, so when a person’s body produces a high-normal amount of thyroxin, the person will utilize calories more quickly.

When a person’s body produces a low-normal amount of thyroxin, the person will utilize calories more gradually.

NOTE – if thyroxin is out of normal range in either direction, it is hazardous and needs medical attention.

Gender

Usually, men have greater muscle mass than women. Since muscle requires more calories to maintain, men tend to have 10 to 15 percent faster metabolisms than women. Similarly, men have a lower body fat percentage than women.

Age

Metabolic rate is higher in childhood than in adulthood. Children are growing and need more calories to fulfill their bodies’ requirements. After the age of 20 years, metabolism drops 2 percent per decade.

Brain Power

The brain is only 2 percent of the body’s weight, but accounts for additional than 20 percent of total calories used.

Additionally, the length of time per day spent awake affects the amount of calories utilized. We expend more calories when we’re awake than when we’re asleep.

Fever

For every increase of 0.5.C (32.9.F) in body temperature, BMR increases by approximately 7%. For example, if someone has a fever of 42.C (107.6.F), the person would have an increase in metabolic rate of 50%.

The reason for this is that chemical reactions in the body occur more rapidly at higher temperatures.

Medications

A few medications, like anti-depressants, can slow down metabolic processes and lead to weight gain.

During exercise, the following factors influence calorie expenditure

Cardiovascular Exercise Intensity

The intensity of aerobic activity has the greatest impact on calorie usage during exercise. As exercise intensity increases, the greater the caloric expenditure during and after exercise.

Intensity refers to the rate of exertion during exercise, which may be measured by VO2 max (oxygen consumption), heart rate, or perceived exertion.

Here is an example of cardiovascular exercise intensity – running on a treadmill at 6.5 mph is more intense than running at 5.5 mph, so increasing speed can impact intensity.

Notwithstanding, two people could be running at 5.5 mph, but if one runs on an incline, that activity would be more intense.

Cardiovascular Exercise Duration

The length of time per fitness session not only impacts the volume of calories utilized during exercise, but also the volume of calories to be utilized after exercise.

The longer the bout of exercise, the more calories will be expended right after it is over, a.k.a. the ‘after-burn.’

Intermittent vs. Single Bouts of Exercise

A few studies have concluded that intermittent aerobic exercise expends more calories overall than continuous exercise.

People used more calories during two, 25-minute sessions when compared to a continuous 50-minute bout of the same exercise.

This occurs for two reasons – first, the body has to work harder at rest to move from an anaerobic state, using glucose and other simple and complex carbohydrates during the first few minutes of exercise, to an aerobic state, which relies on fat as its main energy source during more sustained activity.

This will happen twice in intermittent exercise bouts versus once in a continuous bout. Additionally, there’s more ‘after-burn’ of calories in intermittent workout sessions versus one continuous session.

Strength Training

The intensity of strength training also influences calories utilized. Heavier lifting (3 sets, 8 exercises, 3 to 8 reps at 80 to 90 percent of 1RM) will use more calories during and right after than lighter resistance training (4 sets, 8 exercises, 15 reps at 50 percent of 1RM).

1RM refers to the weight of one repetition at maximal strength. For  instance, when a person’s 1RM of a squat is 100 pounds, then heavy lifting would be doing 3 sets of 3 to 8 reps of squats at 80 to 90 lbs. Lighter lifting weights would be squatting 50 lbs for 4 sets of 15 reps.

Fitness Level

Individuals who are more fit expend fewer calories during and right after exercise than individuals who are less fit.

This occurs because individuals who have been exercising more consistently have faster recovery time in breathing and heart rate, and repair muscle more quickly.

Overall, metabolism increases slightly at higher altitudes with some greater caloric expenditure, but this difference is insignificant, since metabolism is influenced to a bigger degree by the above eleven factors.