The question of how many calories below your Basal Metabolic Rate (BMR) you should eat to lose weight is based on a misunderstanding of how the body uses energy. BMR represents the minimum number of calories your body needs to perform life-sustaining functions (such as breathing and circulating blood) while completely at rest.
This number is not the correct baseline for calculating a calorie deficit because it does not account for any activity performed throughout the day. Attempting to calculate a weight loss plan directly from the BMR will lead to an overly restrictive and potentially harmful target. The appropriate number from which to subtract calories for weight loss is the Total Daily Energy Expenditure (TDEE), which captures all energy your body uses.
Calculating Total Daily Energy Expenditure
Total Daily Energy Expenditure (TDEE) is the complete measure of calories burned in a 24-hour period. It is composed of three main factors: your BMR, the thermic effect of food (TEF), and energy expended through physical activity. TEF refers to the energy required to digest, absorb, and metabolize nutrients, typically accounting for about ten percent of total calories burned.
The most significant variable added to BMR is the energy used for physical activity, which includes structured exercise and non-exercise activity like walking, standing, and fidgeting. To estimate TDEE, the calculated BMR is multiplied by an activity factor corresponding to a person’s average daily movement level. Online calculators often use formulas like the Mifflin-St Jeor equation to estimate BMR.
Activity factors range from 1.2 for a sedentary lifestyle to 1.9 for someone engaged in very hard daily exercise or a physically demanding job. For example, a lightly active person who exercises one to three days per week would use a multiplier of approximately 1.375. Multiplying your BMR by the appropriate activity factor provides the maintenance calorie number (TDEE).
The TDEE is the total energy required to maintain your current weight, making it the proper starting point for any weight loss calculation. It ensures the baseline calorie intake accounts for the energy needed to fuel your daily life.
Setting a Sustainable Calorie Target
The goal for healthy weight loss is to create a negative energy balance by consuming fewer calories than your calculated TDEE. This forces your body to tap into stored energy reserves to meet its energy needs. A safe and sustainable daily calorie deficit is typically between 500 and 1,000 calories below your TDEE.
A daily deficit of 500 calories, sustained over a week, equates to a total energy deficit of 3,500 calories—the approximate energy content of one pound of body fat. This deficit generally results in a healthy weight loss rate of about one pound per week. Doubling the deficit to 1,000 calories per day can result in a weight loss of roughly two pounds per week, considered the upper limit for sustainable loss.
Selecting a deficit within this range promotes consistent fat loss while minimizing muscle mass loss, which is important for maintaining a healthy metabolism. Excessively large deficits are difficult to sustain and can lead to burnout, nutritional deficiencies, and counterproductive metabolic responses. As weight is lost, the TDEE naturally decreases because a smaller body requires less energy, necessitating a recalculation of the deficit every few weeks to continue progress.
Why Eating Below Your BMR Is Harmful
The BMR represents the floor for your body’s energy needs, and consistently eating below this level signals a state of starvation. When calorie intake is severely restricted, the body activates protective mechanisms to conserve energy and increase the efficiency of its remaining fuel stores. This primary response is known as metabolic adaptation.
Metabolic adaptation involves a significant reduction in the rate at which the body burns calories, often called a metabolic slowdown. This is the body’s attempt to survive by lowering the TDEE, making further weight loss progressively more challenging. The body also begins to break down muscle tissue for energy.
Muscle tissue is metabolically active, meaning it burns more calories at rest than fat tissue. Losing muscle further lowers the BMR and compounds the metabolic slowdown. Severe restriction also causes hormonal disruptions, notably affecting thyroid hormones and leptin, which regulate metabolism, hunger, and satiety. These changes increase hunger and cravings while reducing the energy available for daily activities, leading to fatigue and poor performance.
Chronic calorie restriction below BMR increases the risk of nutrient deficiencies because the small volume of food consumed may not provide adequate vitamins and minerals. This can lead to serious health issues, including a weakened immune system, bone density loss, and anemia. For safe and effective weight loss, the calorie target must be set relative to the TDEE, ensuring the body supports its minimal BMR functions while achieving a moderate energy deficit.