Losing weight quickly is unhealthy for a number of reasons; the quickly weight loss is unlikely to stick for a long period of time, you're much less likely to take in 100% of the nutrients your body needs each day and your energy levels will plummet, impacting many areas of your life. When you think of losing weight as a temporary diet, you will lose it in an unhealthy manner. Weight loss should be thought of as a long term lifestyle change rather than a quick way of losing a few pounds for a party, beach trip or wedding.
1. Too Much Change. Losing weight at a healthy rate (1-2 pounds per week) already requires a lot of change. You have to cut calories, increase you physical activity and eat less of some of the foods you love. If you're starting a healthy weight loss plan, you typically spread this change out over a few weeks so you can build a solid foundation. When you try and lose weight at a pace quicker than two pounds per week (crash dieting), you won't have a lot of time to adjust. This will place a heavy strain on your body and will eventually force you to abandon your diet all together. Slow, steady change over time will lead to healthy habits, while quick changes will lead to temporary, unsustainable behaviors.
2. Not Enough Food. If you're trying to lose weight faster than the healthy pace, you'll find yourself eating close to nothing. Losing two pounds per week (healthy weight loss rate) requires a 1,000 calorie per day deficit. If you want to lose four pounds per week, you'll need to double your deficit to 2,000 calories per day. If your body burns 3,000 calories each day, you'll only be able to eat 1,000 calories to reach your weekly weight loss goal. Cutting too many calories out of your diet can have a detrimental effect on your metabolism. Your metabolism can be described as how much energy your body burns in a day to stay alive. It isn't set in stone and can be adjusted based on the environment you create (diet and exercise). Drastically cutting your calorie intake (required by quick weight loss) will slow your metabolism to a point where your calorie deficit is significantly smaller than what you planned it to be. You can cut your calories to the point of almost starving yourself but your body has the means to thwart your plans.
3. Yo-Yo Dieting. Yo-yo dieting is described as quick weight loss immediately followed by quick weight gain. With each bounce, your starting weight gets higher which means that over time, your weight will trend upward. Quick weight loss leads to this yo-yo pattern because the changes that you make to facilitate the weight loss is only sustainable for a very short period of time. Once your body says no more, you will go back to your old eating and exercise habits causing a quick return to your starting weight plus a little extra. Each time you yo-yo, you lose less weight and gain back more.
4. Nutrients. Each day, your body needs a certain amount of nutrients (vitamins, minerals, antioxidants, fiber). If you're on a normal and balanced diet, this isn't a big issue. When you start cutting calories, you're also giving your body less nutrients to work with. Eating less than the required amount of nutrients will negatively affect the way your body functions. Supplements are not as effective as food.
5. Energy and Exercise. To lose weight at a fast pace, you need to do a lot of exercising. Unfortunately, you won't get very far exercising while drastically cutting your calorie intake. Food (calories) fuels exercise. Exercise (and the recovery phase) is very energy intensive. Without enough nutrients to power the recovery phase, your muscles will get smaller and weaker making it even more difficult to workout. This will eventually make it difficult if not impossible to workout leading to the up swing of the yo-yo phase.
The Bottom Line
Rather than looking at your weight loss as purely a cosmetic problem, try and see it as it deals with your overall health. Making changes to your lifestyle that will improve your health in the long run will also help you lose weight. Weight loss that is associated with healthy, long term habits such as an improved exercise routine and a balanced diet is much more likely to stick when compared to weight loss that is achieved through crash dieting.