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Calories, Carbs & Obesity: Physics for the Physician (and Everyone Else)

Dr. David Ludwig
6 min readOct 21, 2021


This essay is adapted from an article in MedPage Today entitled: Let’s Focus More on What We Eat, Not How Much

The First Law of Thermodynamics, formulated in the 1800s, says that energy can neither be created nor destroyed. For the human body, this principle of physics means that if you consume more calories than you burn (or excrete), the excess is stored in the body (mainly as fat).

Variation on this “energy balance” principle form the foundations for how we think about, and treat, obesity. For example, the Dietary Guidelines for Americans 2020–2025 advises that “Losing weight …requires adults to reduce the number of calories they get from foods and beverages and increase the amount expended through physical activity.” The American Diabetes Association, in a Standards of Care report this year, concluded, “Behavioral changes that create an energy deficit, regardless of macronutrient composition, will result in weight loss.”

But there’s an obvious problem. The obesity pandemic shows no signs of abating, despite incessant focus on calorie balance in government dietary recommendations, on packaged food labels, and in patient care sessions with doctors and dietitians. Quite the opposite, according to CDC data from just before and during the Covid-19 pandemic. Currently, more than 70% of US adults have high BMI, putting them at risk for diabetes, heart disease, and numerous other chronic conditions — including severe illness from COVID-19.

Maybe we need to focus on energy balance even harder. Or maybe the problem is how we think about energy balance in the first place. Let’s consider three ways of translating this principle of physics to obesity.

The Uninformative View

One interpretation is with an equal sign:

Energy intake Energy expenditure = Energy stored (Δ body fat)

That is, weight gain can only occur with a positive energy balance and weight loss can only occur with a negative energy balance, as exemplified in Fig 1.



Dr. David Ludwig

Physician, Nutrition Researcher, and Public Health Advocate. #1 NY Times bestselling author ofALWAYS HUNGRY? and ALWAYS DELICIOUS