One study, published in the journal Annals of Internal Medicine, looked at the impact obesity has on remaining life expectancy at 40 years of age. The study involved 3,457 individuals who were divided into three groups: normal weight (BMI < 25), overweight (BMI between 25 and 30), and obese (BMI > 30).

During the forty year study period, 1,647 (47.6%) of the participants died. The researchers found that individuals in the obese group died on average 6 to 7 years earlier than the normal weight group regardless of gender or smoking status. In the overweight group, life expectancy was reduced by 3 years for non-smokers, and by 1 year for male non-smokers. Overweight female smokers did not have a significantly reduced life expectancy compared to female smokers of a normal weight.

The full results of the study are presented in the graph below.

A second study, published in the Journal of the American Medical Association in 2003, looked at the impact obesity has on life expectancy at different stages of life. The study found that obesity in younger people, particularly those under the age of 30, appears to have the greatest effect on life expectancy. On the other hand, obesity had little effect on life expectancy in adults aged over 60 years.

Cardiovascular diseases such as heart disease and strokes are the major cause of premature death in obese individuals. Almost 40% of heart attacks in people under the age of 50 are thought to be caused by obesity. Obesity is strongly correlated with several heart disease risk factors including high blood pressure, poor physical fitness, high blood lipid levels, and diabetes.

Almost 65% of Americans are overweight and 30% are obese. In 1980, overweight and obesity rates were just half of what they are today. Life expectancy at birth in the United States is currently 74 and 80 years for men and women respectively. Life expectancies have plateaued in the western world in the last decade and it is thought the huge increase in obesity rates is partly responsible for this. If overweight and obesity rates in the United States had remained constant since 1980, it is estimated that life expectancy would be almost three years greater than what it is today.

One study, published in the International Journal of Obesity in 1997, looked at the effect of high doses of fish oil on total weight, body fat, and the rate of fat oxidization. The research involved six volunteers who were fed a control diet over a three week period followed by a high fish oil diet where 6 g/day of fat was replaced by 6 g/day of fish oil.

The researchers found that body fat decreased by an average of 0.88 kg during the fish oil diet compared to 0.30 kg during the control diet. The resting metabolic rate of the participants increased by 3.8% on average during the fish oil diet and the rate of fat oxidization increased by more than 20%.

Fat oxidization is the process by which fat in the body is converted into usable energy. When fat oxidization rates are low, some of the dietary fat consumed cannot be converted into usable energy so the individual must consume more food to cover the energy deficit. The unoxidized fat is then stored in the body in the form of adipose tissue.

The researchers calculated that the increase in fat oxidization due to the fish oil supplementation would lead to the loss of almost half a kilogram (1.1 pounds) of adipose tissue over a three week period.

The results are backed up by similar studies in mice which have found that mice fed diets high in fish oil tend to have lower overall body fat levels than mice fed conventional diets.

The 6g/day of fish oil used in this study equates to around 100-150g of oily fish per day. Oily varieties of fish include salmon, sardines, trout, mackerel, herring, and fresh tuna. Canned tuna is not classified as oily fish because the canning process removes much of the oil.

One such study, published in the Journal of Nutrition in July 2007, looked at the levels of Vitamin C in the blood of a group of 118 middle aged individuals. The researchers found that the heavier individuals tended to have lower levels of vitamin C in the blood. The average vitamin C level of the participants with a BMI lower than 25 was 54.4umol/L compared to just 37.2umol/L for individuals with a BMI greater than 35.

The researchers also found that both body fat percentage and waist circumference were inversely correlated with plasma vitamin C levels.

Vitamin C is a key component in the production of carnitine in the body. Carnitine is involved in the fat oxidation process where fatty acids are broken down into energy usable by the body. During exercise in particular, a shortage of carnitine can reduce the bodies abilility to utilise fat for energy resulting in increased fatigue and a decrease in physical performance.

A study, published in the Journal of the American College of Nutrition in 2005, found that individuals who received an adequate intake of vitamin C oxidized 30% more fat during moderate exercise than individuals who were deficient in vitamin C.

A further study, published in the journal Nutrition & Metabolism, found that individuals with low vitamin C levels (<34umol/L) oxidized 25% less fat per kilogram of body weight during exercise compared to individuals with adequate vitamin C levels (>34umol/L). Furthermore, the individuals with low vitamin C levels reported a greater degree of fatigue during the exercise regime.

When some of the low vitamin C group were put on a vitamin C intervention of 500mg per day over four weeks, fat oxidization during exercise increased by 300% compared to the individuals who were not put on the intervention.

The authors concluded that “Low vitamin C status may partially explain the inverse relationship between vitamin C status and adiposity and why some individuals are unsuccessful in their weight loss attempts.”

Vitamin C is added to many food products and is found in a large number of fruits and vegetables. Despite this, it is believed that as many as 20% of adult Americans are deficient in the vitamin. The recommended daily intake of vitamin C lies between 70 and 90mg per day however many health organizations advocate much higher doses than this. The Linus Pauling institute, an organization dedicated to micro-nutrient research recommends 400mg of vitamin C per day. Vitamin C is well tolerated by the body and no adverse have been noted in levels up to 2g per day.

Vitamin C has a relatively short half life of 30 minutes meaning that it needs to be consumed at regular intervals for its effects to be maintained. Vitamin C should be taken around an hour before exercise in order to get the greatest benefit in terms of fat oxidization.

One study, published in the American Journal of Clinical Nutrition, looked at the effect of caffeine on total energy expenditure in a group of lean and obese women. The researchers found that 100mg of caffeine increased energy output by 3 to 4% over a 150 minute period. When the caffeine was administered at two hour intervals during a 12 hour period, total energy intake was increased by 12% in the lean group and 8% in the obese group. This equated to around 150 additional calories burned per day in the lean group and 80 calories in the obese group.

The thermogenetic effect of caffeine appears to increase linearly with the size of the caffeine dose meaning a 200mg dose of caffeine leads to twice the thermogenetic effect of a 100mg dose. Caffeine has a relatively short half life of 3-4 hours (the amount of time required for the body to metabolize half the caffeine) meaning caffeine needs to be consumed at regular intervals for the thermogenetic effects to continue throughout the day.

A pound of fat contains 3,500 calories so 200mg of caffeine consumed three to four times during the day equates to around three additional pounds of fat lost every month.

Caffeine is well tolerated by the body meaning it can be consumed in relatively high amounts with no significant long tern effects on the body. Caffeine does increase blood pressure levels and there is some concern that long term use of caffeine can increase the risk of heart disease however this is not supported by the majority of scientific studies. One of the largest studies on the effects of caffeine on heart disease involved over 40,000 men and 80,000 women from the Health Professionals Follow-Up Study (HFPS). The researchers found no association between caffeine intake and heart disease with those in the highest quartile for caffeine intake actually at a slightly lower risk of heart disease compared to those in the lowest quartile for caffeine intake.

Regular strength 100mg caffeine supplements and “extra strength” 200mg supplements are available at most health stores and pharmacies. A standard cup of coffee normally contains between 80 and 120mg of caffeine. A cup of tea contains 50mg of caffeine, a can of coca cola contains 35mg of caffeine and 100 grams of dark chocolate contains around 70mg of caffeine.

One such study, published in the journal Obesity in 2007 looked at the relationship between consumption of green tea extract and weight in a group of 240 Japanese women. The women were divided into two groups, a control group which received 96mg of green tea catechins a day, and a test group which received 583mg of green tea catechins a day.

The study was conducted over a 12 week period during which the participants made no other diet or lifestyle changes. At the conclusion of the study, the researchers found that the high catechin group had lost an average of 1.7kg of body weight compared to just 0.1kg in the control group. The high catechin group also reduced their body fat percentage by 2.5% and waist size by 2.5cm. Furthermore, the researchers noted a significant decrease in both systolic and diastolic blood pressure in the participants who had relatively high blood pressures at the beginning of the study.

A second study, published by Swish researchers in 1999 found that the equivalent of around 5 cups of green tea increased total daily energy expenditure by 4% or around 60-80 calories per day.

Green tea has a thermogenetic effect on the body meaning it increases the bodies metabolism and therefore the amount of energy burned. While much of the thermogenetic effect of green tea is due to its caffeine content it appears that catechins found in green tea may also act in conjunction with the caffeine to increase the bodies metabolism.

Catechins are powerful antioxidants that are found abundantly in most teas as well as cocoa beans and red wine. Epigallocatechin gallate (ECCG), which is a catechin found only in tea is thought to be particularly effective in increasing thermogenesis. The catechin content of green tea leaves is typically between 10 and 20% with ECCG being the most common catechin.