In 1958, a now infamous study concluded one pound of body fat was equal to 3,500 calories1. It's been conventional diet wisdom for over half a century: a 3,500 calorie increase or decrease, above or below maintenance levels results in a one pound change in weight. Most diets base their weight loss targets on this old rule: spread 3,500 calories over a week to see a gain or loss of one pound. Given the advances in our understanding of how the body works, does this 3,500 rule still stand today?
Calorie Deficit Basics
Energy balance governs body weight. The body stores energy in many forms including: body fat in adipose tissue, carbohydrates in the blood, liver and muscles and protein in the form of muscle and other tissue. During a positive energy balance (consuming more than you expend) the body increases energy stores. During a negative energy balance (expending more than you consume) the body draws upon previously stored energy to make up the difference between calories burned and calories eaten, this is also known as a calorie deficit. A calorie deficit causes weight loss; it's the difference between energy consumed and energy burned. For example:
- Calories consumed = 2,000 calories
- Calories burned = 2,500 calories
- Difference = 2,000 - 2,500 = -500 calories
In the above example, the body burns 500 more calories than it's taking in. This means the body MUST find 500 calories from non-food sources. The body has the ability to use fat, carbohydrate or protein stores to make up the shortfall or deficit of energy. The 3,5000 rule says a 500 calorie deficit over the period of seven days (500 X 7 = 3,500 calories) results in a one pound loss.
The question isn't whether a calorie deficit works; we know it does. The question is how many calories do we need to cut in order to lose a certain amount of weight. Traditional diet advice has maintained 3,500 calories spread over a certain period of time results in a one pound change.
Old Research - History of the 3,500 calories per pound claim
Max Wishnofsky’s authored the first high profile study looking into how many calories it takes to lose a certain amount of weight. He concluded a pound of body fat was equivalent to 3,500 calories. He used prior studies showing 87% of adipose tissue in humans was fat (87% of 454 grams [one pound] is about 395 grams of fat). This means one pound of adipose tissue contains 395 grams of fat. Fat has a caloric density of nine calories per gram; 395 grams X 9 calories/gram = 3,555 calories. This is where the 3,500 calories per pound of body weight comes from1.
The 3,500 calorie rule made sense for a long time, however, as our understanding of the body increased, the rule started to unravel.
Weight loss happens in three distinct phases. The first phase is characterized by rapid weight loss lasting days or weeks. Weight loss in the first phase is made up mostly of fat-free mass (carbohydrates, protein, water, electrolytes). The second phase is characterized by slow weight loss lasting months. Weight loss in the second phase is made up mostly of fat mass. The third phase is not typical; it happens once fat stores are depleted. Weight loss in this phase is mainly in the form of protein catabolism (the breakdown of muscle and other body tissue). This phase is not seen in normal weight loss2.
Phase I. This phase starts as soon as energy intake is reduced and lasts between 5 and 25 days. Obese individuals stay in this phase longer than lean individuals. Weight loss in this phase is rapid and is a result of the body's initial response to a lower energy intake. In the initial stages of calorie deficiency, the body begins to burn its glycogen and protein stores. Glycogen and protein are both bound to water; the loss of these macronutrients results in the shedding of water weight. In addition to glycogen, protein and water, the body also loses electrolytes which further increases the amount of water lost2.
As energy intake remains low, the body beings to go through hormonal and neural regulatory changes. These changes result in a lower resting energy expenditure, slowing the rate of weight loss2.
Phase II. As the body begins to adjust to a negative energy balance, the rate of weight loss slows. Phase two lasts for months and ranges from 100 days in lean individuals to 300 days in obese individuals. Fat loss is seen in greater quantities in this phase compared to the previous one. As glycogen and protein stores are burned up in the initial weeks of weight loss, the body shifts to fat stores2.
Weight loss continues to slow as phase two progresses. Eventually, the rate of weight loss may become unnoticeable until it stops completely. At this point, the body has found a new energy equilibruim3.
Weight Loss is not Linear
The 3,500 calorie rule relies on the assumption that weight loss is linear. It assumes a 3,500 calorie deficit is worth the same one pound of body weight throughout the weight loss journey. We now know this is not the case.
One Pound is Equal to Varying Amounts of Calories
During the early stages of weight loss, a pound of body weight is worth much less than 3,500 calories. As the period of caloric deficiency increases, a pound of body weight increases to more than 3,500 calories; it approaches 7,700 calories. As weight loss continues, the caloric deficiency needed to lose one pound increases; this makes it increasingly difficult for weight loss to continue at a constant pace. Over time, weight loss slows, and eventually stops3.
Weight Loss is Dynamic
Wishnofsky’s 3,500 calorie rule was devised when we had a much smaller understanding of how variable the body was. It was a good start to understanding how the body reacted to decreased energy intakes. Today, we understand the body's response to weight loss as dynamic.
Through hormonal and neurological pathways, the body slows metabolic processes in response to decreased energy intakes. Whereas at one point, you may have been able to squeeze a pound out of 3,500 calories, the dynamic nature of the body means a pound of bodyweight increases in "value" to almost double that figure in the later stages of weight loss.
The Bottom Line
The 3,500 calorie rule is a dated, yet popular concept. We know the body is dynamic and responds to significant changes in energy intake. Still the basic idea of losing weight remains the same: calories expended must remain greater than calories consumed. The problem is that energy expenditure decreases with decreased calorie intakes. While the exact difference between expenditure and intake needed for a certain amount of weight loss is dynamic and difficult to predict, there is no way around it.
- Wishnofsky, M. (1958). Caloric Equivalents of Gained or Lost Weight. American Journal of Clinical Nutrition, 6(5), 542-546. Retrieved from http://ajcn.nutrition.org/content/6/5/542.extract
- Heymsfield, S. B. (2010). Voluntary weight loss: Systematic review of early phase body composition changes. Obesity Reviews, 12(5). Retrieved from http://onlinelibrary.wiley.com/doi/10.1111/j.1467-789X.2010.00767.x/full
- Thomas, D. M. (2014). Time to Correctly Predict the Amount of Weight Loss with Dieting. Journal of the Academy of Nutrition and Dietetics, 114(6), 857-861. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4035446/