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Nutrients that enhance fat burning typically affect the fat-burning hormones or their cell receptors. For example, green tea ingestion results in enhanced fat burning by blocking enzymes that degrade norepinephrine. Norepinephrine and epinephrine are the major fat-burning hormones and are responsible for releasing fat from fat cells and inducing fat burning in tissues such as the liver and skeletal muscle. A catechin present in green tea called epigallocatechin gallate (EGCG) appears to be responsible for this effect. An
American Journal of Clinical Nutrition
article showed that drinking 4 cups of green tea a day resulted in a loss of more than 6 pounds (2.7kg) in 8 weeks. However, other clinical trials have not shown a fat-loss effect after green tea consumption. Caffeine also induces fat burning, but does so by elevating sympathetic nervous activity, which elevates levels of epinephrine in the blood, and by blocking the adenosine receptors on fat cells that suppress fat release. Research has shown that drinking 500ml of cold water can increase metabolic rate by a third, mainly due to an increase in fat-releasing hormones such as norepinephrine. Drinking 2 cups of warm rather than cold water resulted in a much smaller increase in metabolic rate.

Consuming omega-3 fatty acids may also contribute to fat loss by triggering fat-burning enzymes inside cells. Omega-3s also appear to improve leptin signalling in the brain, which reduces appetite. Cold-water fish such as tuna and salmon are good sources of omega-3, as are nuts, seeds and flaxseed, which contain fats that are changed to omega-3s when ingested.

Capsaicin, the compound that makes chillis hot, has also been shown to enhance fat burning. Most of the research examining the fat-burning effect of chillis has administered the capsaicin in capsule form, which has been found to be more effective than when it’s contained in food. The mechanisms underlying the capsaicin effect are unclear but are likely to include an increase in energy expenditure, impeded fat-cell growth and a reduction in appetite. Capsaicin can be obtained naturally by consuming raw, cooked, dried or powdered chillis and capsicums. Cayenne pepper or hot sauce can also be added to home-made soups and meals.

We think other nutrients and minerals may enhance or impede fat burning indirectly. For example, having low levels of iron is associated with a slower metabolism, resulting in less fat burning. So correcting low iron levels – anaemia – can improve the body’s ability to burn fat; 1 cup of lentils provides about 35% of daily iron needs.

The thyroid gland affects every cell in the body and has a significant impact on metabolism and fat loss or gain, immunity, hormones, heart rate and blood pressure. Thyroid dysfunction often has a genetic influence; it isn’t unusual for grandparents, parents and children to all possess thyroid dysfunction. However, nutrients in food can also increase or decrease thyroid function. For example, iodised salt contains iodine, which is essential for thyroid function. Avoiding table salt and consuming little iodine is likely to result in a depressed thyroid in most people. Processed foods typically contain lots of salt but not the iodised kind. Meat and chicken typically contain antibiotics and growth stimulants that have been shown to reduce thyroid function. Surprisingly, some vegetables impede iodine absorption, resulting in reduced thyroid function. Such vegetables usually contain a chemical called goitrogen however cooking typically negates this goitrogen effect. Thyroid-reducing vegetables include green leafy vegetables such as raw cabbage, broccoli and spinach. Soy also contains goitrogens, and studies have shown that although it is thought of as a healthy food, soy is detrimental for thyroid function; it follows that cooking oils and margarine that contain soybean should be avoided. Replace oils containing soy-bean with coconut oil and extra-virgin olive oil. Kelp contains high levels of iodine and, in contrast to soy, has been shown to improve thyroid function.

The status of your thyroid needs to be medically evaluated, which typically involves an assessment of the thyroid hormones in your blood by a doctor know-ledgeable in endocrinology. Hypothyroidism refers to a thyroid gland that is underperforming, whereas hyperthyroidism reflects an overactive thyroid.

Most processed foods suppress fat burning. These include anything that has been deep-fried, soft drinks, milk chocolate, margarine, cakes and biscuits, pies, donuts, take-away burgers, hot dogs, sausages and so forth. The mechanism underlying the fat-burning suppression of these foods is most likely that processed foods take fewer calories to digest than unprocessed food. One study indicated that people burned fewer calories after eating a processed meal rather than a meal comprised of whole foods. Researchers first gave volunteers a cheddar cheese sandwich on whole-grain bread, then, on a second day, a processed cheese sandwich on white bread. Both sandwiches had the same amount of calories. The metabolic rate of the subjects was measured and revealed that 137 calories were burned after eating the sandwich made with cheddar cheese, compared to 73 calories after eating the processed cheese sandwich. In this case, unprocessed food resulted in the burning of 43% more calories.

The replacement of processed foods with whole foods results in burning more calories due to what is called the thermic effect of eating whole foods. The cheese sandwiches made with whole-grain bread contained high levels of fibre, too, which lowers the surge in blood insulin levels that occurs with eating; high levels of insulin after a meal decrease fat burning. Do what you can to avoid these foods and replace them with healthy eating, such as the Mediterranean eating plan.

Nutrients that reduce fat absorption

A number of nutrients have an effect on the amount of fat we absorb from what we eat. For example, ingestion of oolong tea – a form of green tea – with a high-fat meal significantly lowered fat absorption.
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One study showed that the amount of total lipids and cholesterol remaining in the faecal mass of subjects increased by 52% after drinking 750ml of oolong tea each meal per day. The healthy components of green tea are called catechins, which are powerful antioxidants. Green tea contains many catechins, although epigallocatechin gallate (EGCG) seems to be responsible for the greatest lipid absorption effect. EGCG suppresses the uptake of lipids in the small intestine and is also effective at lowering the absorption of cholesterol.

It has also been found that green tea lowers the ab-sorption of organic pollutants, which humans can easily absorb. This leads to high levels of tissue toxicity and the prevention of fat loss. For example, eating trans fats and oxidised and polyunsaturated vegetable oils results in increased accumulation of free radicals in our bodies. Free radicals are atoms with an odd number of electrons and are typically created when oxygen interacts with other molecules. Free radicals cause damage by reacting with the DNA inside our cells, resulting in cell death and disease. Antioxidants can prevent the negative effects of free radicals by terminating their ability to damage cells. To help the body produce enough antioxidants, a diet containing vitamins such as beta-carotene, vitamin C and vitamin E is necessary.

Eating fruit and plants containing insect pesticides can also result in a build-up of toxins in adipose cells. The accumulated toxins inside the adipose cell can be released when stimulated, which has been shown to slow fat loss and therefore may make people fat. Green tea prevents absorption of organic pollutants, so people who regularly drink green tea may experience more fat loss by reducing fat absorption in the meals they eat and by decreasing the toxicity effect of organic pollutants.

Fat and sugar in the blood after eating a meal

Eating 3 meals per day containing 20–40 grams of saturated fat or 50 grams of fructose typically causes elevated triglyceride blood levels lasting up to 18 hours. This phenomenon is called postprandial lipemia and is a major risk factor for atherosclerosis and cardio-vascular disease, because higher levels of fat in the blood increase inflammation and disrupt the lining of the arteries.

Postprandial lipemia levels vary, but they are decreased or increased by food intake, fat type, dietary intake of protein, nutrients and exercise. For example, consuming 30 grams or more of saturated fat typically results in postprandial lipemia, whereas eating monounsaturated and polyunsaturated fats does not. Consuming protein, fibre and green tea with saturated fat results in lowered postprandial lipemia, whereas drinking alcohol and smoking enhances it.
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One session of aerobic exercise lasting 45 minutes resulted in significantly lower postprandial lipemia. People regularly performing aerobic exercise every week also exhibit lower postprandial lipemia. Recently, our studies have shown that 20 minutes of interval sprinting at night also reduced fat in the blood of women who ate a high-fat meal the next morning.
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Postprandial lipemic individuals can be inflamed for a significant proportion of the day, and this inflamed state is likely to impede fat loss. Inflammation raises insulin and cortisol levels; these are hormones that enhance fat storing. Therefore, eating meals high in saturated fat may increase belly fat stores to a greater extent than eating less saturated fat or consuming healthier fats. Those individuals who ingest high-saturated-fat meals but consume protein, fibre or green tea simultaneously may experience a suppressed postprandial lipemia and inflammatory response.

Eating lots of simple sugar can also cause postprandial elevations in glucose, which in turn can increase inflam-mation, endothelial dysfunction and hyperinsulinaemia. Eating healthy foods such as vegetables, fruits, seeds, grains and nuts results in a far lower rate of postprandial glucose elevation.
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Drinking 1–2 glasses of alcohol before eating a high-sugar meal also results in significantly lower postprandial glucose levels, and drinking a moderate amount of alcohol decreases insulin resistance for 12–24 hours. Consuming olive oil and fish oil significantly lowers postprandial plasma glucose levels, and such reduction in postprandial glucose levels leads to decreased blood insulin levels. As previously discussed, insulin enhances fat storage, and high blood insulin levels are an impediment to fat loss. Therefore, as dis-cussed with postprandial lipemia, ingestion of different types of nutrients with a meal that affect postprandial glucose and insulin could influence belly fat loss response.

What to eat before, during and after exercising

Digesting nutrients before, during and after exercising influences fat burning. Eating 30 grams of fructose or glucose 1 hour before a 60-minute aerobic exercise session significantly suppressed whole body fat burning by 32% (fructose) and 50% (glucose). Unfortunately, consuming a high-protein meal also results in a significant increase in blood insulin levels. It is important not to eat sugar or protein before or immediately after exercise, as these nutrients elevate blood insulin levels, which impair fat burning. Digesting low-glycemic meals before exercise also suppresses fat oxidation, but the effect is much greater after high-glycemic meals. Low-glycemic meals contain fewer simple sugars, such as glucose, whereas high-glycemic meals contain lots of glucose; consuming sugary drinks and protein snacks before or after exercise will tend to suppress fat burning and reduce long-term fat loss.

In contrast, other nutrients can enhance fat burning during and after exercise. For example, we have shown that ingestion of green tea before interval sprinting increased fat oxidation by over 20% during the hour after exercise.
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Another study demonstrated that green tea ingestion before steady-state cycle aerobic exercise resulted in a 17% increase in fat oxidation during exercise.
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Time of day for optimal belly fat loss with interval sprinting exercise

The time of day you choose to exercise and whether or not you have eaten a meal before your session affects fat burning during exercise. Studies have shown exercising following an overnight fast results in significantly greater fat burning during exercise, compared to exercising soon after a meal.

One such study also investigated the effect of exercise on glucose intolerance and insulin resistance in the fasted and fed states.
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It found that fasted training was more effective than fed training for improving glucose tolerance and insulin resistance: fasted compared to fed subjects displayed the greatest insulin sensitivity levels. The fasted subjects also significantly increased fat burning, which resulted in a much smaller weight increase in the fasted group. Thus, even though they consumed a high-caloric diet, fasted subjects gained much less weight after the 6-week program. This and other studies have shown that the body burns up much more fat during and after exercise if no food is eaten before exercise. This means the best time of day to exercise in order to lose belly fat is early in the morning, before breakfast.

Exercise and appetite

Overall, the evidence indicates that engaging in exercise does not automatically increase appetite; with most people who exercise, food intake remains unchanged, and in certain circumstances exercise may suppress appetite. It is possible that exercising may change the quality of nutrients eaten; for example, after hot, sweaty exercise there may be a switch away from high-fat foods to fruit and water. However, this has not been examined thoroughly and is an important future research area.

Hard exercise, such as running a marathon, has been shown to induce a moderate decrease in appetite, lasting up to several hours after the session, but moderate exercise does not seem to have any effect on appetite. When people change from a sedentary lifestyle to an active one, food intake is not increased accordingly, which seems to suggest there is no biological mechanism that matches energy output to energy intake; rather, social and psychological factors may determine how much and what we eat.

The mechanisms underlying the appetite-suppressant effect incurred by hard, vigorous exercise are unknown, but increased body temperature may inhibit food intake; people in cold climates normally eat more than those in temperate or warm regions. Recent research has also shown that a number of gut peptides change after exercise and may inhibit food intake by suppressing central nervous system appetite circuits.
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Other studies have shown that satiety hormones released from the brain during hard exercise also suppress appetite, and that elevated blood lactate levels blunt appetite by affecting appetite centres in the brain.
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In contrast, however, other studies have shown that cold-water workouts increase appetite – individuals who exercise in cool water eat more afterwards.
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The effect of interval sprinting on appetite has not been examined and is an important area for future research, but we already know lactate levels are increased in the blood during interval sprinting.