Fennel seed is commonly used in cooking and as an alternative medicine remedy. It has a distinct flavor and, nutritionally, is a good source of potassium, magnesium, calcium and vitamins B and C. Therapeutically, one of its most common applications is to relieve gastrointestinal distress. Fennel seed has powerful antioxidant properties and fights free radicals and irritation.  Fennel also has several noted traits that have lead to its distinction as a beneficial addition to weight loss efforts.
Weight Loss Benefits
Fennel seed essential oil may have a positive role in supporting weight loss as both a metabolic enhancer and an appetite suppressant. Increased metabolic output burns more energy and can accelerate fat loss. Fennel has the added benefit with regard to fat reduction in that it helps “break up” fat deposits in the bloodstream to be used as energy. Using energy sources the body already has stored can reduce the cravings for food. A study conducted at the Thuringian State Institute of Agriculture in Germany, found that dieters who supplemented with fennel seed oil ate less food.
Fennel also contains melatonin. In 2011, Researchers at the University of Granada performed a study that showed melatonin, which is a natural hormone that regulates healthy sleep patterns, also helps helps control weight gain. 
Fennel is an anti-spasmodic and chewing the seeds can help to relieve hunger pangs.
A research group at the School of Nursing, Eulji University, in Korea, found that inhaling fennel essential oil promoted food digestion and lowered caloric intake in rats. 
Fennel can also naturally increase frequency of urination and reduce water weight. In this capacity, it does not have an effect on fat stores and is not regarded as an effective weight loss measure alone. However, its role in a comprehensive weight loss plan may be beneficial specifically when retained water causes bloating. The increased urination also encourages the removal of toxins.
Fennel Seed Essential Oil Within A Weight Loss Strategy
Fennel works best when used as part of an overall weight loss plan that includes a balanced diet of healthy organic foods and other supportive herbs such as green coffee bean, uva-ursi, gymnema, damiana, and hoodia gordonii. Evidence suggests that when taken as a supplement as part of a comprehensive weight loss strategy, fennel can be beneficial in stimulating your metabolism and promoting weight loss.