How much weight can I safely lose in 1 week?
The simple answer: about one pound.
The scientific answer:
It depends if you’re talking about fat loss or water loss.
Tolerance to water loss depends on your cardiovascular physiologic reserve. Our bodies consist mainly of water, and blood volume is dependent on your water composition.
Cardiac Output = Heart Rate x Stroke Volume
Your body’s ability to compensate for dehydration depends on it’s tolerance to function at a higher heart rate. So if your heart can pump at a higher rate despite a lower stroke volume, you can maintain your cardiac output.
To put things in perspective a heart rate of 110-120 beats per minute (BPM) is comparable to a light jog at 4.5 mph for an average person. A heart rate between 120-150 is comparable to a brisk run at 6 mph for an average person. A normal resting heart rate is between 50-100 BPM at rest with a regular rhythm.
Normal resting heart rate: 50-100 BPM
Light jog: 110-120 BPM
Brisk run: 120-150 BPM
So the next time your heart rate is in this range, ask yourself could I sustain a light jog or brisk run at that heart rate considering a marathon (26.2 miles jogged at 4.5 mph) would take close to 6 hours.
One can easily lose about one pound in one day due to dehydration from sweating, not drinking, and evaporative losses. Drugs such as diuretics also can rid excess body water as well but also at the expense of depleting intravascular volume.
So the next time you see a billboard with a smiling skinny person proclaiming that they lost 100 lbs in a month, take it with a grain of salt. Advertisements can be misleading. In reality, it takes several weeks once your caloric intake drops close to 1,300 calories and fat loss occurs close to one pound per week.
For more info, read “How fighters cut 10-15 lbs days before a weigh-in” by Dr. Lee.