Many factors can affect your weight, but often weight loss comes down to a matter of “energy balance”. We measure this energy with the unit calories and because energy cannot be created nor destroyed we have 3 possible scenarios when it comes to energy balance.
When we’re maintaining our weight we’re utilizing the same amount of calories we’re consuming. Meaning the energy we’re putting out is the same as the energy we’re taking in. When we’re gaining weight we have a surplus of calories because we’re taking in more than we’re burning and that energy gets stored in the body. When we consume less calories than our body requires we use the calories the body already has stored for energy.
For a deeper dive, I wrote a 3 part series around Nutrition basics and Nutrition Strategy and within those articles I included the math for weight loss. However, for the purposes of this article, we’re looking at creating a negative energy balance and the math of how much you may reasonably be able to expect to lose in 100 days.
The Math
To begin, 1 pound = 3,500 calories. The average basal metabolic rate for adult humans is 1,400 calories for women and between 1,600-1,800 calories for men. This means that if we were to sleep/rest all day and do absolutely nothing, our body will naturally burn around 1,500 calories a day while it is going through all of the biological processes that keep us alive.
Many of us will not be able to burn 2,000 additional calories in one day. This is why it’s better to look at weight loss weekly instead of daily so we can spread out that 3,500 calorie deficit over multiple days.
If you want to lose a pound a week, you simply would divide 3,500 calories by 7 days to get 500 calories. Meaning, if you want to lose 1 pound in 1 week, you would need to reduce your calories by 500 calories a day.
If we were to look at the standard American diet, which is thought to be 2000 calories daily, reducing your calories by 500 calories daily would put you at 1500 calories per day in your daily calorie allowance. While a 1500 calorie diet can work, there can be better ways to see body composition change. Furthermore, for this method to work we are assuming that you are burning more than 1,500 calories in a day. Otherwise you will be maintaining your weight as it is if you are only consuming 1,500 calories a day and burning 1,500 calories a day.
This is where exercise comes in for that extra calorie burn. If the person above has a 300 calorie workout and is only consuming 1,500 calories, the deficit is created which means our body must burn stored calories because it is not consuming enough calories.
The Research Shows that Noticeable Change Requires an Increase/Decrease of at least 250 Calories Daily, 1750 weekly
Traditionally, it’s recommended that if you want to see weight come off you should reduce your calories by 300-500 calories a day. Studies have found that calorie deficits of 250 calories or less are seen as negligible and ineffective.
If we’re looking at calorie deficits over a week, effective calorie deficits are believed to be in the 300-500 calorie for daily increments therefore giving us weekly deficits in increments of 2100-3500 calories across your week.
For both weight gain and weight loss, the general recommendation is to add or reduce by 300 calories per day for a couple of weeks while your body adapts and adjusts. Then, if necessary, repeat this process and reduce your calories for another set of weeks by another 300-500 calories.
The Recommended Maximal Threshold that is Considered “Safe”
Studies have shown that 1% weight loss per week as a maximal daily deficit upper threshold is generally considered safe for most people.
Depending on what your current baseline is, how much you’re consuming, and how much weight you’re looking to lose, guidelines say it is safe to only lose 1-2 lbs/week which can equate to ~500-1000 calorie reduction per day.
How Much Weight Can You Safely Lose in 100 Days?
So if we were to decide we simply wanted to reduce our calories by 500 calories a day for 100 days we’d lose about 14 pounds.
(500 calorie reduction * 100 days=50,000 calories lost)
(50,000 calories lost divided by 3500 calories=14.28 lbs)
If we were to reduce our calories by 1000 calories a day to lose 2 lbs a week over 100 days we’d lose about 29 lbs.
(1000 calorie reduction * 100 days=100,000 calories lost)
(100,000 calories lost divided by 3500 calories=28.57 lbs)
Keep in mind that unless you’re currently eating 3,000 calories a day, this may manifest as a 500 calorie deficit for your intake and an extra 500 calorie burn through exercise.
Do NOT reduce your daily allowance of caloric intake by 1000 calories a day without medical supervision. Medical supervision is required for any extreme calorie restriction.
And, believe me, even reducing your calories by 500 a day isn’t fun.
Regardless, it’s helpful to see the math and how you can realistically lose 15 lbs in 100 days by reducing your calories.
Use Caution and Always Consult a Licensed Medical Professional
There can be a lot of nuance that goes into calorie restriction. For example, as a protection mechanism if we are losing weight too quickly our body will employ automatic defenses to cause your body to stop burning calories so efficiently. This is a defense mechanism our body adapted in the cave man days when we needed to preserve our calories in order to avoid death by starvation. Essentially, our body has safety features in place to protect us from excessive weight loss.
Another nuance is that for women, our weight loss is better tracked biweekly instead of daily because of our hormone fluctuations due to our monthly hormone cycles. (Even though men’s hormones reset every 24 hours, it’s still best to track weight loss weekly.)
Other considerations from imbalances, hormones, inflammation, etc. can all affect our weight and the bottom line is to always consult your licensed medical professional if you’re looking for help with weight loss. However, if you’re only looking to gradually and sustainably lose a pound or two a week, you can begin by reducing your calories daily by 300-500 or weekly by 2100-3500.