By Jessica Chrisman, Certified Family Nurse Practitioner
Updated on March 6, 2023
Published on May 16, 2019
A quick breakdown of what you need to know about this new weight loss/ lifestyle trend.
Will Fasting Cause Weight Loss?
Ok, let’s start with the basics, YES when you eat less, you will lose weight. If you do a fast or juice cleanse you will definitely see the number on the scale decrease, however, you probably aren’t loosing as much as you think.
If you juice for 2-3 days and the scale says you dropped 5lbs than in most cases you probably only lost about 2-3 pounds. It’s safe to say that about half of the weight loss you see on the scale will be water and stool loss. If you fast for longer periods of time, however, like for 10 days or do intermittent fasting for a few weeks, then the number on the scale will be more accurate.
This is just plain math, the fewer calories in, the more weight you lose, but it’s very hard to lose more than 1 pound in a day. To lose 1 pound you need to have a loss 3,500 calories. This can be done by eating fewer calories or with exercise.
To burn off 3,500 calories with exercise would take a 140lb person, running a 10 min mile, 5 hours and 50 mins of running and/or 33 miles. That is a lot of running!
Now, let’s look at diet. Since it takes a 3,500 calorie deprivation to lose one pound and most people consume around 1800- 2500 calories per day to maintain weight, then it is not really possible to lose more than a pound a day unless you are an athlete or heavily over weight.
Lets say you need 2,000 calories a day to maintain your weight and you go on a 500 calorie a day juice fast for 3 days, then on a 4,050 calories deprivation plan for 3 days you will get about 1.5-2lbs of weight loss in 3 days, the rest of the weight loss on the scale will likely be from an empty bowel and loss of water. This is why the scale may show more like 3-5 pounds until you eat again and then likely will jump back to 1-2lbs loss.
How did we get this number?
2,000 per day x 3 days= 6,000 is your normal calorie intake to maintain for 3 days
Subtract the 500 calories per day juice x3 = 1500
And you get a 4,500 calories deprivation in 3 days
So if it takes 3,500 calories to lose 1 pound, then you’re not going to lose more than 1-2 pounds in 3 days.
LOOPHOLE: if you are greatly overweight or an athlete who is burning hundreds to thousands of calories per day you can possibly lose 5 pounds 3 in a day. However, for the average to slightly overweight person this will not be the case.
Fasting vs Low Calorie
There are two famous cases of weight loss that many people refer to in this debate; the Minnesota starvation diet that was conducted in 1944 to understand the effect of calorie restriction on the body and the case of Agnus Barbieri, an overweight Scottish man who in 1965, despite the advice of his doctor, decided to stop eating for a whole year.
The Minnesota study was made up of 36 men, all around 5’10 and 160lbs. For 3 months they ate 3,200 calories and then for 6 months ate between 1,000-1,570 calories per day, with the goal of losing 1lb per week. The diet was high carb and low fat and protein. The men complained of fatigue, low body temp, decreased strength, and were hungry all the time, to the point that some became so obsessed with food they were digging in the trash for scraps.
Angus Barbieri, was an extremely overweight man who decided that eating nothing would be the most effective way to take the weight off. For an entire year, he consumed only water, coffee, tea, and a multivitamin a day. He went from 456 pounds to 180lbs and when they went to check up on his progress, they were shocked to learn that he was able to keep the weight off and stayed in the 190lb range. During the year he didn’t eat he regulated his metabolism hormones and shrunk his stomach, so it was easier for him to keep the weight off than someone like a contestant on the show the biggest loser who is put on a low calorie diet. In addition, unlike the men in the Minnesota study, he didn’t complain of excessive hunger. Then again, he was very overweight and the men in Minnesota study were not so it’s not a perfect comparison.
From these studies and several others, many medical professionals believe that fasting is not only more effective for weight loss than calorie restriction, but is actually better for the body and will result in less muscle loss and hunger. The belief is that calorie restriction will prevent ketosis (fat burning metabolism) from occurring and staying in glucose burning metabolism (with calorie restriction) will cause you to stay far hungrier then someone who is fasting and in ketosis.
To understand this better it is helpful to look at the hormones associated with eating and fasting. Ghrelin is the notorious hunger hormone, it is released in waves and is highest at breakfast, lunch, and dinner times. What this means is the body starts to remember when you should be eating and will release Ghrelin at those times. It is also noted that ghrelin will decrease after 2 hours without food. Have you ever been really hungry, but didn’t have time to eat and noticed that if you didn’t eat your hunger actually went away till the next meal time.
Interestingly if you are doing a fast, you will notice the most hunger occurs at set meal times, but will go away and the longer you do the fast the ghrelin levels will actually start to decrease in general. Thus fasting will help lower the hunger hormone that is associated with weight gain.
Will fasting slow my metabolism?
Well, yes, but let me go on tangent first on what metabolism is. In simple terms, metabolism is the process of converting food into energy for the cells. When there is excess food intake the excess energy will be stored as glucose in the liver for quick use when we have dips in blood sugar OR when we have extra calorie intake, the spillover from the full liver cells will be stored as fat cells. Unlike the cells that store glucose, the fat cells don’t ever get full, they can keep on growing.
When you lose weight you will need to eat fewer calories at your lower weight then you did at your higher weight. Let’s say at 190lbs you could eat 2300 calories a day, then you ate 1400 calories per day for a few weeks to lose weight, now at your goal weight of 150lbs you can eat 1800 calories per day to maintain weight. Your smaller body needs fewer calories. These numbers are all just estimates, to calculate your calories needed for weight loss, weight gain, or maintenance use a calorie intake calculator.
If you still want to eat 2300 calories a day at 150lbs you could, you would need to increase the energy burn rate and to do that would need to workout at the gym for an hour and burn off those extra 500 calories.
While lemon water, spicy food, fidgeting, and increased muscle mass can increase the number of calories you burn and thus “increase metabolism,” its only a small amount, not enough to be able to eat 2300 calories and maintain 150lbs. Everyone is a unique individual so some people will need fewer calories than others even with the same lifestyle.
What does fasting do to metabolism?
Over time fasting will slow it, but again, this referring to how much energy (calories) your body needs and how it utilizes those calories. In men, it takes about 5-6 days of extreme fasting to see a 10-20% dip in calorie burn rate, which will take about 2-3 months of re-feeding (normal eating) to increase back to their baseline. In women, a drip in calories burns rate of 10-20% will be seen after 2-3 days and again, will take about 2-3 months of re-feeding to increase back to their baseline.
Prolonged fasting will lower the calorie burn rate, this is why intermittent fasting is often considered the better and safer option as it helps prevent that more drastic dip in calorie burn rates.
With fasting and weight loss most will also experience a reduced body temperature, ability to keep warm, as well as a dip in adrenal function– so why is this good?
Benefits of Fasting and Weight Loss
The majority of people want to lose weight to look better, but weight loss and fasting will also help your insides also. The benefits of weight loss include: reduced cholesterol levels, lower insulin resistance, and drops in blood pressure. If you are overweight reducing your body weight just 10% will lower your risk of many diseases such as stroke, gallstone, heart attack, sleep apnea, allergies, and chronic inflammation which as we know is linked to early death and cancer.
Where fasting takes normal weight loss a step further is in waste production and healing abilities. Those who fast regularly notice a dip in cortisol (the stress hormone), improved insulin sensitivity, increased growth hormone, improvements in GI, autoimmune, and inflammatory conditions, and less waste is being dumped in the body, which results in more healing time.
Every time you eat you are stressing the body to an extent– this is why longevity doctors love the idea of fasting. Having time where you eat less food, less often, will reduce the amount of toxins your body produces and allow your body time to heal. Having an empty stomach for example, allows the gut lining time to heal. For people who suffer from indigestion and heartburn, fasting can help improve symptoms.
This is when you have you ask yourself what kind of health do I want and does fasting fit into that goal? There are 3 main health categories people can choose from.
FITNESS enthusiasts: those who work out 1-3 hours a day and are able to consume more calories, but also are often overstressing the joints and muscles and many end up in adrenal fatigue. For them, fasting will allow for an increased ability of the body to switch from carb burning to fat burning metabolism during long workouts and boost hGH for muscle growth. The goal? get ripped and increase endurance.
LONGEVITY enthusiasts: those who do moderate workouts with regular fasting. The goal here is to produce less inflammation and toxins in the body to help preserve the body and lower risk for diseases and aging. Just but be okay with eating less food more often. The goal? live longer.
BALANCE enthusiasts: eat 3-4 small healthy meals a day with moderate exercise 4-5x per week. This is the more traditional, I will make smart choices everyday approach. For balance enthusiast fasting once in a while, like 2 days a month or doing intermittent fasting would be a good approach. The goal? everything in moderation.
Regardless of which approach you take, regular and occasional fasting has many health benefits, so just figure out where and when it fits into your life. Or better yet, consult with your medical provider to figure out which approach would be best for you.
