This will be the shortest chapter, by far. Although short, as the title clearly states, it is the most important, which is why it is near the beginning and why we decided to make it a separate chapter. Please don’t let the brevity of it diminish its significance.

We hope the introduction helped to clarify the fact there are many factors that influence our behaviors and ultimately our weight. Furthermore, there is no doubt that many people struggle to reach and maintain a healthy weight (more about this in Chapter 2). There are many real and challenging reasons for this, which the rest of this book will be covering in detail. These many aspects are very important. However, unless one basic fact about weight management is fully embraced these real and modifiable reasons will not be the focus of a person’s time and effort, which will negate or significantly reduce their chance for success. The basic fact that we are alluding to is the calorie balance equation, also referred to as calories in versus calories out or CICO. CICO means that bodyweight will change based on amount of calories ingested (more specifically absorbed) versus the amount of calories the body uses (burns). If weight goes up, intake is greater than expenditure. If weight stays the same, then intake and expenditure are matching up. Finally, if weight loss is happening, then intake is less than expenditure. There are a few caveats to CICO, see footnote (1). This should NOT be controversial as there is overwhelming evidence this is how weight regulation works (2).  Conversely, there is no quality evidence to dispute this fact. Therefore, if weight is not moving down, then a calorie deficit is not being consistently created. We wish everyone would accept this. Many people do. However, because of some frequent misleading or just plain false information regarding this concept, a fair amount of people will not embrace this fact. Regrettably, not accepting this fundamental fact leads to a variety of “ideas”(3) being put forth to try to support the idea a person is creating a calorie deficit but it is just not working. This often manifests as the following comments;

  • “I really don’t eat much”
  • “I am probably eating too little”
  • “I have been gaining weight but I am not eating more”
  • “I think I am eating less but I am not losing weight”
  • “I eat healthy”
  • “I am eating less sugar/carbs”
  • “I am not drinking enough water”

The fact is CICO always works, which means creating a calorie deficit will always lead to weight loss (1). Therefore, when weight is not moving down, the first thought a person should have (if it has been more than a week or two of no change) is something like this;

“I have not lost weight in about 2 weeks. I must not be consistently creating a calorie deficit. What can I do to do this consistently?”

Losing weight and keeping it off is often challenging, so don’t make it harder by not accepting the fundamental CICO concept. By accepting this, you can then focus on the “how to” of consistently creating a calorie deficit. The rest of the book will explore the many what’s and how’s of losing weight and maintaining a healthy weight.



(1)-A few caveats. First, short–term water balance can mask weight loss, however, this can only last for so long, as 3-5lbs of EXTRA fluid will likely go away and/or weight (fat) loss past this amount will show up on the scale. Second, some muscle gain, for most people, 2-5 lbs could happen over a number of months, which could mask some of the fat loss that is happening. However, it is very unlikely, again for most people, for it to mask all the progress with weight loss. For example, if a person is gaining one lb of lean tissue a month (this is very good for most people) and losing 4lbs of fat tissue a month they would still have a net loss of 3 lbs a month. Third, there is a duration aspect to consider. In the short-term, days  or maybe a week or two, even if a person was consistently creating a calorie deficit, at an amount that would likely lead to about 1 lb of fat loss per week, but they happen to also, during that time, be holding onto a bit of extra fluid, then it will not show up. However, when looking at it over weeks and certainly months, a consistent calorie deficit will always result in weight loss. To be clear, none of these factors negate the CICO concept, rather they are things that can temporarily mask it.

(2)-There is some much quality evidence for this concept (CICO) we don’t understand how it cannot be accepted. This is covered in greater detail in Chapter 3, a number of sections, but specifically the section “If You are Not Losing Weight it is Not Because You are in “Starvation Mode”

Also see the following, which are some of the MANY quality articles and papers on the topic;

Aragon, A. et al (2017). International society of sports nutrition position stand: diets and body composition. Journal of the International Society of Sports Nutrition; 14:16

Hall, K. & Guo, J. (2017). Obesity energetics: Body weight regulation and the effects of diet composition. Gastroenterology; 152(7): 1718-1727.

Hill, J. et al (2012). Energy balance and obesity. Circulation; 126: 126-132

Westerterp, K. (2010). Physical activity, food intake, and body weight regulation: insights from doubly labeled water studies. Nutrition Reviews; 68(3): 148–154.

Schoeller, D. (2009). The energy balance equation: looking back and looking forward are two very different views. Nutrition Reviews; 67(5): 249-254.

Buchholz, A. & Schoeller, D. (2004) Is a calorie a calorie? AJCN; 79(suppl): 899s-906s.

Body Weight Regulation: What Controls the Scale?

11 Reasons People Think Calories Don’t Count — And Why They’re Wrong,

Countering Anti-Energy Balance, CI=CO Arguments ~ Part I The 3500 Cal = 1 Pound Fat

Countering Anti-Energy Balance, CI=CO Arguments ~ Part II: We Can’t Calculate It

The Energy Balance Equation

Also the textbook, Metabolic Regulation: A human perspective by Frayn

(3)-The “ideas” that CICO is not true is regrettably being put forth by a handful of people, some that have significant influence/reach, such as Gary Taubes and Jonathan Bailor. These authors and others that are health professionals should know better. The evidence does NOT support their position. We understand that this idea can be enticing; however, it is just not true.