How Long Will It Take to Lose Weight Once I Start an Exercise Routine?

Waiting to see results after starting an exercise routine can be agonizing. "Weight" loss — the number on the scale — typically happens quite quickly in the early days of working out. But weight loss is not the same as fat loss. Losing fat may take a little longer, depending on how much you are exercising and the quality of your diet.

Weight loss and fat loss are often used interchangeably, but they are distinct concepts relating to body composition. Your body weight is the combined weight of your muscles, bones, organs, water and fat. Fat-free mass is everything in your body besides fat. So when you are talking about fat loss, you're talking about a relatively small proportion of your total body weight.

When you gain weight — the number on the scale — you could be gaining fat, muscle mass or water weight. If you're losing one of these components and gaining another, the number on the scale might not change at all. If you start an exercise program including strength training, you may be building muscle mass and losing fat at similar rates, so you might not see the scale reading budge too much.

That's why it's not a good idea to use the scale as your only judge of progress. A better way is to take account of how you look and feel and how your clothes fit. You can also take measurements of your arms, thighs and midsection, which can tell you if you're losing fat.

If you want a more accurate assessment, you can have your body fat tested at the beginning of your exercise program and then periodically after that. One of the most accurate means of body fat testing is hydrostatic, or underwater weighing, which is typically done in a laboratory setting. Skin-fold testing is more accessible, and you can often ask a personal trainer at the gym to do it for you.

In a skin-fold test, calipers are used to measure the thickness of subcutaneous fat — the adipose tissue just below the skin's surface. This isn't as accurate as other forms of testing because it is subject to the experience and accuracy of the person administering the test as well as others conditions, such as the time of day, pre- or post-exercise, foods and beverages consumed and more.

Read more: 11 Simple Ways to Add Variety to Your Strength-Training Routine

According to an article published in June 2014 in the Journal of the Academy of Nutrition and Dietetics, weight loss occurs in two stages. Although the first stage, lasting several days or weeks, produces more rapid weight loss, it is primarily water, protein and a small pool of stored carbohydrate. Strength training can help combat the muscle loss, so people following a fat-loss program including strength training exercises may not see as rapid a change in weight.

Over time, the body adjusts to increased activity and decreased calorie intake. These changes are both hormonal and neural regulatory mechanisms that cause decreases in energy expenditure, protein breakdown and other metabolic processes. These metabolic processes exhaust carbohydrate stores and reduce the body's usage of protein for energy, thus increasing the reliance on fat oxidation for energy.

This leads to the second phase of weight loss. This stage, lasting months to years, is characterized by a slower rate of weight loss but an increased rate of fat loss.

However, the total energy output during this period slows, due to changes in resting energy expenditure as well as activity and non-activity thermogenesis (heat production). In addition, decreased body weight leads to a reduction in energy expenditure during exercise due to a lower energy cost of activity, as noted in the 2014 Journal of the Academy of Nutrition and Dietetics article.

Weight loss is highly individual, so the rate of weight loss can only be roughly estimated. A myriad of factors, including genetics, age, medical conditions, medications, lack of sleep and stress affect how much weight you will lose and how quickly.

Genetics play a role in fat storage and how easily you lose weight. Body type is also a product of genetics. People with certain body types find it harder to shed fat, or to shed fat specifically from certain areas of their bodies. Age slows metabolism, so if you're older, it may take you longer to lose weight after exercise than a younger person.

Some medical conditions, such as polycystic ovary syndrome and thyroid disease can make it harder for people to lose weight. Even if you exercise a lot and control your diet, your rate of weight loss may be slower. Certain medications may cause weight gain and make it harder to lose weight.

Lack of sleep or poor sleep quality may cause increased calorie intake and slower — or lack of — results. Stress increases levels of the "fight or flight" hormone cortisol, which can cause the body to hold on to and increase fat stores.

Read more: The Fat-Burning Stride-and-Strength Workout

Finally, how soon you lose weight after you start exercising depends on your particular workout program, and it can vary significantly. Are you brisk walking for 30 minutes a few days a week, or are you doing high-intensity interval training (HIIT) three days a week and weightlifting five days a week? This makes a huge difference in energy expenditure — both through physical and activity and through resting metabolic rate.

Lower-intensity forms of aerobic activity, such as brisk walking, don't burn as many calories as more vigorous activities like running. According to Harvard Health Publishing, a person weighing 155 pounds burns 149 calories walking at a pace of 3.5 miles per hour, and 372 calories running at a pace of 6 miles per hour. That's a big difference in calorie expenditure over a week or month and will make a significant impact on how much weight you lose and how quickly.

Secondly, if you aren't strength training, you can expect your weight on the scale to be lower, but you can also expect your rate of fat loss to be slower. This is because muscle mass is more metabolically active than fat mass. Having more muscle raises your resting metabolic rate, so you'll burn more calories throughout the day, even when you're not exercising.

While vigorous exercise isn't for everyone, if you want to see the quickest results, try to increase the intensity of your workout, even if that means jogging instead of walking. Also, you may not want to strength train, but it's crucial to losing weight and keeping weight off. According to the Mayo Clinic, it also provides a host of other benefits, including stronger bones, better balance, reduction in symptoms of chronic illnesses and even improved cognitive function.