Weight loss occurs when you consistently consume fewer calories than you burn each day.

Conversely, weight gain happens when you consistently eat more calories than you burn.

Any food or beverage you consume that has calories counts towards your overall calorie intake.

That said, the number of calories you burn each day, which is known as energy or calorie expenditure, is a bit more complicated.

Calorie expenditure is composed of the following three major components (1):

• Resting metabolic rate (RMR). This is the number of calories your body needs to maintain normal bodily functions, such as breathing and pumping blood.
• Thermic effect of food (TEF). This refers to the calories used to digest, absorb, and metabolize food.
• Thermic effect of activity (TEA). These are the calories you use during exercise. TEA can also include non-exercise activity thermogenesis (NEAT), which accounts for the calories used for activities like yard work and fidgeting.

If the number of calories you consume equals the number of calories you burn, you maintain your body weight.

If you want to lose weight, you must create a negative calorie balance by consuming fewer calories than you burn or burning more calories through increased activity.

Summary Weight loss occurs when you consistently consume fewer calories than you burn each day.

Several factors affect the rate at which you lose weight. Many of them are out of your control.

Gender

Your fat-to-muscle ratio greatly affects your ability to lose weight.

Because women typically have a greater fat-to-muscle ratio than men, they have a 5–10% lower resting metabolic rate than that of men of the same height (2).

This means that women generally burn 5–10% fewer calories than men at rest. Thus, men tend to lose weight quicker than women following a diet equal in calories.

For example, an 8-week study including over 2,000 participants on an 800-calorie diet found that men lost 16% more weight than women (3).

Yet, while men tended to lose weight quicker than women, the study did not analyze gender-based differences in the ability to maintain weight loss.

Age

One of the many bodily changes that occur with aging is alterations in body composition — fat mass increases and muscle mass decreases.

This change, along with other factors like the declining calorie needs of your major organs, contributes to a lower RMR (4, 5).

In fact, adults over 70 years old can have RMRs that are 20–25% lower than those of younger adults (2, 6).

This decrease in RMR can make weight loss increasingly difficult with age.

Starting point

Your initial body weight also affects how quickly you can expect to lose weight.

The amount of weight you lose, especially within the first few weeks, tends to be proportional to your body weight.

People who are heavier will lose more pounds than people who are lighter. However, the rate of weight loss tends to be similar percentage wise (7).

For example, a person weighing 300 pounds (136 kg) may lose 10 pounds (4.5 kg) after reducing their daily intake by 500 calories for 2 weeks.

Conversely, someone of the same age and gender weighing 150 pounds (68 kg), may lose only 5 pounds (2.3 kg) following the same method.

Although the heavier person lost double the amount of weight, both individuals lost an equal percentage of their body weight (290/300 = 9.7% versus 145/150 = 9.7%).

Calorie deficit

You must create a negative calorie balance to lose weight. The extent of this calorie deficit affects how quickly you lose weight.

For example, consuming 500 fewer calories per day for 8 weeks will likely result in greater weight loss than eating 200 fewer calories per day.

However, be sure not to make your calorie deficit too large. Doing so would not only be unsustainable but also put you at risk of nutrient deficiencies. What’s more, it might make you more likely to lose weight in the form of muscle mass rather than fat mass.

Sleep

Sleep tends to be an overlooked yet crucial component of weight loss.

Chronic sleep loss can significantly hinder weight loss and the speed at which you shed pounds.

Just one night of sleep deprivation has been shown to increase your desire for high-calorie, nutrient-poor foods, such as cookies, cakes, sugary beverages, and chips (8, 9).

One 2-week study randomized participants on a calorie-restricted diet to sleep either 5.5 or 8.5 hours each night.

Those who slept 5.5 hours lost 55% less body fat and 60% more lean body mass than those who slept 8.5 hours per night (10).

Consequently, chronic sleep deprivation is strongly linked to type 2 diabetes, obesity, heart disease, and certain cancers (11, 12, 13).

Other factors

Several other factors can affect your weight loss rate, including:

• Medications. Many medications, such as antidepressants and other antipsychotics, can promote weight gain or hinder weight loss (14).
• Medical conditions. Illnesses, including depression and hypothyroidism, a condition in which your thyroid gland produces too few metabolism-regulating hormones, can slow weight loss and encourage weight gain (7, 15).
• Family history and genes. There is a well-established genetic component associated with people who are overweight or obese, and it may affect weight loss (16. 17).
• Yo-yo dieting. This pattern of losing and regaining weight can make weight loss increasingly difficult with each attempt, due to a decrease in RMR (18).

Summary Age, gender, and sleep are just a few of the many factors that affect weight loss. Others include some medical conditions, your genetics, and the use of certain medications.

With innumerable weight loss diets available — all promising impressive and quick results — it can be confusing to know which one is best.

Yet, though creators and proponents deem their programs superior to the rest, there is no single best weight loss diet (19, 20).

For example, low-carb diets like keto may help you lose more weight initially, but studies find no significant differences in weight loss in the long term (21, 22, 23).

What matters most is your ability to stick to a healthy, reduced-calorie eating pattern (24, 25).

However, following a very-low-calorie diet for long periods is difficult for many people and the reason why most diets fail (26).

To increase your chances of success, only moderately reduce your calorie intake, individualize your diet according to your preferences and health, or work with a registered dietitian.

Combine diet with exercise, including both aerobic and resistance training, to maximize fat loss and prevent or minimize muscle loss (27).

By eliminating highly-processed foods and incorporating more healthy, whole foods, such as vegetables, fruits, whole grains, healthy fats, and proteins, you can further promote weight loss and your overall health.

Summary Adhering to a weight loss diet is difficult for most people. Regardless of your goals, choose a dietary pattern based on your individual preferences and health status.

While most people hope for fast, rapid weight loss, it’s important that you don’t lose too much weight too quickly.

Rapid weight loss can increase your risk of gallstones, dehydration, and malnutrition (28).

Other side effects of rapid weight loss include (29, 30):

• headaches
• irritability
• fatigue
• constipation
• hair loss
• menstrual irregularities
• muscle loss

Though weight loss may occur faster at the start of a program, experts recommend a weight loss of 2.2–6.6 pounds (1–3 kg) per week, or about 1% of your body weight (31).

Also, keep in mind that weight loss is not a linear process. Some weeks you may lose more, while other weeks you may lose less or none at all (32, 33).

So don’t be discouraged if your weight loss slows or plateaus for a few days.

Using a food diary, as well as weighing yourself regularly, may help you stay on track.

Research shows that people who employ self-monitoring techniques, such as recording your dietary intake and weight, are more successful at losing weight and keeping it off than those who don’t (34).

Summary Losing weight too quickly can lead to problems like gallstones, muscle loss, and extreme fatigue. Experts recommend a moderate weight loss of 1–3 pounds (2.2–6.6 kg) per week, or about 1% of your body weight.

Weight loss occurs when you eat fewer calories than you burn.

Many factors affect your weight loss rate, including your gender, age, starting weight, sleep, and the extent of your calorie deficit.

Aiming to lose 1–3 pounds (2.2–6.6 kg) per week is a safe and sustainable way to reach your goals.