oods provide fuel to your body. Your body burns fuel to produce energy. The complex that breaks down food to produce energy is known as metabolism. The speed of metabolism is linked to the body weight. One of the causes of excess weight gain is slow metabolism. Your body needs energy all the time, even when you are resting. While foods are the source of energy, certain foods are believed to slow down the metabolism. Therefore, to maintain the healthy metabolism process you should eat the right type of foods.
The rate at which your body burns fuel is called the metabolic rate of the body. It is regulated by the hormone secreted by the thyroid gland. When the metabolic rate is slow, fewer calories are burned and more is stored as fats. Sluggish metabolism is a possible cause of excess weight gain. However, serious metabolic disorder is present in a small number of overweight or obese individuals. The most common cause of excess weight gain is excessive calorie consumption and less energy expenditure. When you eat too much and do not participate in physical activities, you gain weight. As the hunger signals in humans tend to be stronger than the satiety signals, we are vulnerable to overeating. Furthermore, the pleasure of eating and excess intake of sugar and fats affect the serotonin level of the brain that encourages us to overeat. Sluggish metabolism is also associated with nutritional deficiency. A diet deficient in iodine, zinc and the tyrosine (amino acid) is known to reduce the metabolic rate. 
The individual metabolic rates are usually predetermined by the genes. Though you cannot change your genes, nevertheless, you can speed up your sluggish metabolism by avoiding foods that are known to slow down metabolism. At the same time add foods that are known to boost metabolism to your diet. To sustain the healthy metabolic rate, it is advisable to eat five times a day. Your blood sugar level falls when you miss a meal, which stimulates excess production of hormone that breaks down muscles instead of fats to produce energy. 
1. Refined Grain
Grains are the stable food in the traditional diets. They are traditionally consumed either in the whole form or are grounded with stone grinders and consumed as coarse flour. The whole grains, apart from being an important source of carbohydrate, are known for their multiple health benefits. However, removal of the bran and the germs from the grain through grinding and milling with the help of modern technology depletes the nutritional value of the grains. Refined grains contain more starch than whole grains. But, they are deficient in dietary fibers, minerals, vitamins and phytochemicals that are concentrated in the outer bran layer. Since refined grains are rapidly digested, they quickly increase the blood glucose level after meals.  Compared to consumption of whole grains, refined grain consumption is associated with lower resting metabolic rate and lower daily energy loss.  Furthermore, excess refined grain intake is known to cause insulin resistance and metabolic syndrome, and higher systolic and diastolic blood pressure, waist circumference and serum triglyceride levels and lower high-density lipoprotein level. 
What to Eat
White rice, white bread, degermed cornmeal and products made with white flour are common examples of refined grain products that we consume frequently. Foods made with refined grains include corn tortillas, flour tortillas, corn bread, crackers, noodles, pasta, macaroni, spaghetti, corn flakes, white bread, pitas, pretzels and white sandwich buns and rolls. Replace these refined grain products with whole grains such as brown rice, buckwheat, bulgur, oatmeal, popcorn, whole wheat cereal flakes, muesli, whole grain barley, whole rye, whole grain cornmeal, whole wheat bread, whole wheat sandwich buns and rolls, whole wheat crackers, whole wheat tortillas and wild rice. Amaranth, quinoa, millet, triticale and sorghum are some of the less common whole grains that you can include in your diet. 
2. Saturated Fats
Excess intake of saturated fats can slow down your metabolism. Diets that contain higher amount of saturated fats and fewer polyunsaturated fatty acids are known to reduce the resting metabolic rate. The polyunsaturated fatty acids are rapidly oxidized, whereas saturated fats have a slower oxidation rate. High ratio of polyunsaturated fatty acids to saturated fatty acids in a diet is known to boost the resting metabolic rate and stimulates diet-induced thermogenesis.  In a study conducted in Spain, healthy male participants between 18 and 30 years of age were divided into three groups. One group ate walnuts, another group took olive oil and the third group consumed dairy products. Walnuts contain polyunsaturated and monounsaturated fatty acids. Olive oil comprises primarily of monounsaturated fatty acids. Dairy products are an important source of saturated fats. All the three portions of fatty foods had similar calorie content. After measuring the thermogenesis rate of the participants, researchers found that the thermogenesis rate was lowest among participants who ate dairy and was highest in the groups that ate walnuts. The thermogenesis rates of the walnut and olive oil groups were respectively 28 percent and 23 percent higher than the thermogenesis rate of the dairy group. 
What to do?
Animal fats are the primary source of saturated fats. Fatty beef, lamb, pork, tallow, poultry, lard, and butter, cream, cheese and other dairy products made from whole or 2% milk are important dietary sources of saturated fats. Saturated fats are also found in certain plant-based oil such as palm oil and coconut oil. According to the American Heart Association’s dietary guidelines not more than 5 to 6 percent of the daily calories should be obtained from saturated fats. If the daily energy need of your body is 2000 calories, your daily saturated fat intake should be limited to 13 grams. 
3. Vegetable Oils
Vegetable oils such as canola oil, corn oil, soybean oil, sunflower oil and safflower oil that are widely used in modern diets are believed to be harmful for the human body. The modern oil extraction methods involving exposure of the oil seeds to enormous temperature and pressure and presence of toxic compounds in the extraction chemicals are blamed for the poor quality of the vegetable oils. The vegetable oils are high in free radicals that damage the body cells and are known to slow down metabolism. Furthermore, some of these vegetable oils undergo hydrogenation during the extraction process, resulting in production of harmful trans fats.  Vegetable oils such as corn and soybean oil are believed to inhibit the functions of the thyroid gland that could slow down metabolism. 
What to do?
To derive the benefits of fat intake without damaging your health and slowing down metabolism, replace the vegetable oils with extra virgin olive oil, flaxseed oil and coconut oil.  Peanut oil and canola oil can be used for frying while olive oil can be used for non-frying cooking. 
4. Fruit Juices and Soft Drinks
Because of their high simple sugar content, fruit juices are believed to be harmful for the metabolism. Simple sugar is quickly digested and absorbed into your blood when you drink fruit juice. The spike in the blood sugar level stimulates excess insulin secretion. Too much insulin circulating in the blood lowers your blood sugar level and depletes energy, which encourages you to overeat to meet your body’s energy need. Eliminating fruit juice from the daily diet is believed to help in reducing excess body weight.  Soft drinks are another major source of simple sugar. One can of fruit juice or sweetened soda is known to contain nine teaspoons of sugar or fructose. 
What to Eat and Drink?
Eat whole fruits instead of drinking fruit juices. While the juices require very little energy for digestion, your body burns more calories while breaking down the whole fruits, thereby boosting your metabolism. Moreover, whole fruits contain fewer calories than equal amount of fruit juice. The glycemic index of a whole fruit is lower than the glycemic index of the juice extracted from the fruit. Consuming foods that reduce the glycemic load helps in regulating metabolism and assists in weight loss.  To keep your body hydrated, instead of drinking fruit juice, drink water. Increasing water intake is believed to boost the metabolic rate of the body. Research suggests that by drinking 34 ounces of extra water daily you can lose up to five pounds of body weight in one year. Drinking cold water is believed to be more effective in increasing the body’s metabolic rate. 
5. Sugar Substitutes and Artificial Sweeteners
Sugar substitutes and artificial sweeteners are more harmful for the metabolism than regular sugar or fructose. High-fructose corn syrup (HFCS) widely used as a sweetening agent is more rapidly absorbed than regular sugar. Unlike sugar, HFCS does not require insulin to enter the body cells. It is known to increase the cholesterol and triglyceride levels and is believed to be one of the major causes of fatty liver and liver abnormalities. Liver problems slow down metabolism. Artificial sweeteners such as aspartame, saccharin, acesulfame, neotame, dihydrochalcones and sucralose are associated with health risks. They are found in artificially sweetened foods, candy, gum, sodas, mints and various packaged foods.  Given the harmful side effect of excess sugar intake, use of sugar substitutes has increased worldwide. However, studies suggest that frequent use of sugar substitutes impairs the normal metabolism. Research suggests that consumption of foods sweetened with non-caloric or reduced-calorie artificial sweeteners interferes with the body’s learned responses associated with intake of sweet-tasting foods and glucose and energy homeostasis. Excess intake of artificially sweetened foods is known to increase risks of metabolic syndrome, obesity, type-2 diabetes and cardiovascular disease. 
How to Sweeten Foods?
Instead of using sugar, sugar substitutes and artificial sweeteners, consider sweetening your foods and beverages with honey. Honey being a natural sweetener is free from chemical additives. It is believed that unlike refined sugar consumption, honey is less likely to trigger frequent sugar cravings. Furthermore, honey contains a number of proteins, antioxidants, vitamins and minerals. It can help in improving the body’s ability to process sugar. Since honey is sweeter than refined table sugar, a small amount of this concentrated sweetener is enough to sweeten your foods and beverages.  Another natural sweetener that is believed to be safe is stevia. For hundreds of years steviol glycoside extracted from the stevia leaf is being used as a sweetener in South America. This plant-based natural sweetener is 50 to 350 times sweeter than sugar. A number of food regularity and safety authorities around the world have confirmed the safety of stevia. 
6. Hidden Food Allergens
Hidden food allergies can slow down your body’s metabolism. The allergens impair the normal thyroid function. The sluggish thyroid gland slows down metabolism. It causes hormonal imbalance and increases food cravings. Hidden food allergies are also associated with water retention.  Food allergies associated with wheat or gluten, dairy, egg, shellfish, soy, peanuts, tree nuts and vegetables of the nightshade family are known to increase after the age of 30. Symptoms of allergic reactions include digestion disorders, headache, fatigue, sinus congestion and depression. 
What to do?
As the allergens slow down your metabolism, eliminating them from your diet helps in revving up your metabolism. However, detecting the hidden food allergens is not easy. It is believed that the body tends to crave for the food it is intolerant to. You may crave for cookies if you are intolerant to wheat. Detecting your strong cravings for a particular food that is a source of food allergens can help you in identifying the offending food.  However, elimination diets are usually most effective in identifying foods allergens. 
7. Non-Organic, Processed and Packaged Foods
Processed and packaged foods are known to contain artificial sweeteners, preservative, pesticides and packaging chemicals that tend to disrupt the hormonal balance of your body and slow down your metabolism.  Fast foods are loaded with highly processed sugar, artificial sweeteners, high-fructose corn syrup, trans fat, nitrates and monosodium glutamate. Moreover, the meat and dairy products served by fast-food restaurants are known to contain growth hormones, antibiotics and various drugs. The nonorganic foods that are dominant in modern diets are known to contain pesticides, herbicides, chemical fertilizers and a number of chemicals and antibiotics that are harmful for the body. Increased consumption of non-organic foods is believed to be a possible reason for the rising incidence of obesity. 
What to Eat?
To boost your metabolism, eat the best quality foods that contain the highest nutritive value and minimal amount of harmful additives and byproducts. Foods that have undergone least amount of processing are known to be of the best quality. Eat foods that had been grown organically. Organic fruits and vegetables are produced with the help of natural fertilizers and they are free from chemical contaminants. Opt for grass-fed meat as they are of the highest quality.