Eating more of the good stuff - fruit, nuts and vegetables has long been proclaimed as a good way to lose weight. Does it work? Why?
Some scientists have suggested that eating larger portions of these healthy food may work without the subject actually dieting and trying to consume less. It may be a simple matter of replacement and the low energy content of these foods and their high fibre. By eating more of these foods you may eat less of the 'bad stuff' and lose weight.
A recent comprehensive, long term Harvard School of Public Health study of 120,000 people over more than 20 years was reported in the New England Journal of Medicine.
The subjects in the study showed and average increase in weight of about 1.5kg (3.35 lb) every four years, representing a net weight gain of about 7.5 kg (16.8 lb) over the total 20 years of the study.
The researchers say they found many lifestyle factors and diet elements were associated with long-term weight gain, including the consumption of specific foods and beverages. But this did not constitute cause and effect.
They demonstrated that for many healthy people that are non-obese, the long-term increase in weight gradual and tends to accumulates over time.
These changes represent a long-term decline in health and over-weight problems.
Although the weight changes associated with any individual lifestyle or diet element factors were relatively small, all these factors taken together accounted for the large increases in weight gain.
Looking at the average correlation of various dietary and lifestyle factors with weight gain for such a large number of peoples over such a long time is useful.
The associations were both positive - i.e. linked to an increase in weight and negative linked to a net decrease in weight.
The study found that the food type correlated with the greatest weight gain was potato chips.
Eating a portion of potato chips every day was linked to an average weight gains of 1.2 kg or 3lbs in four years. Similar weight gains occurred for people who consumed extra portions of crisps, sugary drinks and meats.
Eating more fruits, nuts and vegetables, whole grains and yogurt were correlated with small weight losses over four years.
Major changes in physical activity were also associated with reducing the rate of weight increase. Increased time spent watching TV was correlated with an increase in weight.
What were the basic results?
Researchers found that for each four-year period, participants gained an average of about 1.5kg (3.35lb).
Average weight gain for each four-year period was positively associated with :
Average weight gain over the same periods was inversely associated with the intake of:
Other lifestyle factors were also independently associated with weight change.:
Why did eating more of foods such as yoghurt, fruits, vegetables, nuts and whole grains lead to lower rates of weight gain over time ( negatively correlated with weight gain)?
The researchers claimed that these foods that have lower calorie densities mean that these foods substitute for other types with high calorie density decreasing the overall amount of calories consumed.
The higher fibre content on these foods may also slow digestion and may decrease the appetite for longer.
Other researchers were surprised that potato products (including potato chips) were more strongly linked with weight gain than other foods such as cake, cookies and doughnuts, which other studies have shown contribute the highest proportion of calories to the American diet. It has been suggested that people who regularly eat potato chips and potato fries, also tend to grossly eat too much.
This highlights the general weakness of the study, which only points to associations. not cause and effect.