There are so many conflicting opinions about how often and how much we need to eat. Healthy Food Guide senior nutritionist Rose Carr cuts through the confusion.
Most of us will have heard advice that we ought to eat six small meals a day, or eat every two to three hours to keep our metabolism going or, even, to speed up our metabolism. Others tell us snacks add unnecessary kilojoules and we just need three good meals spaced throughout the day. Some claim eating after 6pm (or 7pm or 8pm) promotes fat storage. And you’ve probably heard the old saying, ‘Breakfast like a king, lunch like a prince and dinner like a pauper’. But what does the science say about all this?
Although there are sometimes psychological reasons for eating, such as boredom or anxiety, and sharing a meal is an important family or social occasion, our primary reason for eating is to provide our bodies with the nutrients we need. That includes the macronutrients of carbohydrate, protein and fat, as well as all the different vitamins and minerals our bodies need to function optimally.
While we talk about specific daily requirements for these nutrients, in reality we don’t need exactly the same amount every day. Our needs for specific nutrients can be evened out over a few days, or even a week. That applies to our energy intake as well. How we meet our nutrient needs, in terms of food volume and timing, naturally varies from day to day, and that’s just fine.
After we eat we are in the fed state, which lasts for around three hours, depending on the size and composition of our meal or snack. During this time, carbohydrates are broken down into sugar molecules and our blood glucose levels increase. As blood glucose levels rise, the hormone insulin is released to help use the glucose throughout the body as well as to store any excess.
As we move into the post-absorptive state, which lasts from around three to 18 hours after a meal, there is a gradual decrease in circulating glucose. Beyond a certain threshold, our brain sends messages to several organs to produce and release hormones so we start producing glucose from our stored supplies, and to stop further insulin release.
In this way, our blood glucose levels are maintained within a specific range.
Most of us will only ever experience the fed and post-absorptive states. But, if we don’t get food for longer than around 18 hours, there are further changes to how energy is sourced and supplied to our cells and after around 48 hours without food we enter a starvation state, which is different again.
It would seem not. When we eat food, we do use some energy (kilojoules) just processing it.
Work done by our cells in the process of digestion, absorption, transport and storage of nutrients uses energy. Typically, we increase our base metabolic rate by around 10 per cent when we have a meal. This is called the thermic effect of food.
It was thought that if we ate more frequently, say six times a day rather than three, we could keep our metabolism at a higher rate, thus burn more energy and it would be easier to lose weight.
But that assumes the effect of eating a small meal lasts for as long as the effect of eating a big meal. In fact, there’s not as much work to be done by our cells when we eat a small meal. The thermic effect of food is estimated at around 10 per cent of the energy value (kilojoules) of the food. So, it doesn’t really matter how frequently we eat. If our meals are smaller, the effect will be smaller at that meal.
The thermic effect does change with what we eat, though. Protein has a greater thermic effect (20-30 per cent) than carbs, and carbs have a greater thermic effect (5-10 per cent) than fat (up to five per cent). So, swapping out a high-carb, high fat muffin for a pottle of low-fat yoghurt, with its protein and carb combo, has the potential to not only reduce the kilojoule intake, but to lessen the kilojoule impact. It’s also likely to be more satisfying for longer. A win-win.
In healthy people, blood glucose is tightly controlled. While our blood glucose does increase after we eat carbohydrate foods, the so-called sugar-rush, or energy spike, is an exaggeration. Our bodies are designed to cope with blood glucose increasing after we eat. Similarly, most of us are unlikely to experience low blood glucose, simply because our bodies won’t allow it.
There may be some people who experience what’s called reactive hypoglycaemia, where eating or drinking a big load of quickly digested carbs, such as a big sugary drink or a huge slice of pavlova, increases blood glucose higher than the normal range. But reactive hypoglycaemia is not common and, even for people who do experience it, there are not usually symptoms. In the end, your body just runs better on slow-acting carbs, such as whole grains, pulses and fruit. Read more…