Some dietary myths dispelled
01 December 2009
Will caffeinated drinks make you dehydrated? Should you limit egg consumption or avoid cooking vegetables? Can eating certain nutrients at certain times of the day help you lose weight? Fact is not always easy to distinguish from fiction.
Do caffeinated drinks have a dehydrating effect and increase fluid requirements?
Studies have shown that normally consumed amounts of caffeine in drinks such as tea, coffee and colas do not increase fluid loss. But caffeine has been reported to have a diuretic effect above 250 mg per day and it can therefore lead to increased water loss and possibly to body water deficit (this effect may be less pronounced in regular caffeine consumers). So, the water we get from caffeinated drinks can contribute to our total fluid when consumed in moderation. National authorities around Europe recommend a water intake from beverages of at least 1.2 litres (4-6 glasses) in adults. This is in addition to the water we get from food and our metabolism, to replace losses through urine, faeces, perspiration and the lungs.1,2
Should you eat no more than three eggs per week?
High blood cholesterol levels are a known risk factor for coronary heart disease. This has led to the idea that egg yolks, which are rich in cholesterol (about 225 mg in a medium sized egg), must be bad for your heart. However, we make over 75% of the body’s cholesterol ourselves, and the dietary cholesterol generally has very little effect on the level in the blood. While some people may respond to dietary cholesterol, saturated fat has a much greater effect on blood cholesterol, especially LDL-cholesterol, and eggs are low in saturated fat. Most health and heart advisory bodies in Europe and elsewhere no longer set a limit on the number of eggs consumed, provided they are eaten as part of an overall healthy balanced diet that is low in saturated fats.3,4
Does cooking destroy all the goodness in vegetables?
Vitamin C and folic acid are water soluble and susceptible to oxidation, so much is lost when foods containing these vitamins, such as green vegetables, are cooked in large volumes of water which is discarded. This loss and that of other vitamins and minerals can be minimised if vegetables are not cut up, plunged straight into boiling water and served immediately, or better still are steamed or cooked with very little water in a microwave oven. However, other important constituents, such as fibre, lycopene in tomatoes and other antioxidants remain in the vegetables and may become more available to the body through cooking. Additionally, proper cooking increases the microbiological safety of foods and enhances their flavour.5-7
Can “chrono-nutrition” help you lose weight?
The concept of "chrono-nutrition" was developed by a French nutritionist in 1986. The theory behind it is that there is an ideal time for digesting the macronutrients protein, carbohydrates and fat. For example, foods containing proteins, fats and slowly digested carbohydrates (such as those from wholegrain and fibre-rich sources) should be eaten at breakfast and mainly protein containing foods at lunch. Macronutrients eaten outside the periods of optimal digestion will not be utilised but stored as fat, resulting in weight gain.
It is true that we all have a circadian rhythm, a biological clock linked to sunlight and temperature, which is reflected in small fluctuations through the day and night in the levels of various substances, such as hormones, in our bodies. However, whenever we eat proteins, fats or carbohydrates our body responds by increasing the production of all substances needed to digest and utilise them. Any weight loss experienced with this diet is likely due to the reduction of calorie intake that tends to occur when individual meals are restricted to certain food items.8,9
- Scientific Opinion of the Panel on Dietetic Products, Nutrition and Allergies on a request from the EC on dietary reference values for water. The EFSA Journal (200x) xxx, 1-49.
- Maughan RJ & Griffin J (2003). Caffeine ingestion and fluid balance: a review. Journal of Human Nutrition and Dietetics 16:411-420
- Gray J & Griffin B (2009). Eggs and dietary cholesterol – dispelling the myth. British Nutrition Foundation Nutrition Bulletin 34:66-70
- European Heart Network (2002). Food, Nutrition and Cardiovascular Disease Prevention in the European Region: Challenges for the New Millennium.
- Food Standards Agency (2008). The Manual of Nutrition, 11th ed. UK
- Miglio C, et al. (2008). Effects of different cooking methods on Nutritional and physicochemical characteristics of selected vegetables. Journal of Agriculture and Food Chemistry 56:139-47
- Shi J & LeMaquer M (2000). Lycopene in tomatoes: chemical and physical properties affected by food processing. Critical Reviews in Food Science and Nutrition 40:1-42
- Gibney MJ, et al. (eds) (2009). Introduction to Human Nutrition, 2nd ed. Oxford:Wiley-Blackwell
- Waterhouse J, et al. (1997). Chronobiology and meal times: internal and external factors. British Journal of Nutrition 77:S29-38