When it comes to diets for weight loss, we have been bombarded by a whole host of conflicting messages over the years. Is fat good or bad for you? What about carbohydrates? Is fruit good or is it too high in sugar? I believe this explains the rise of single food diets – their simplicity. In most cases, the idea is that you replace some or all of your meals with a single food. You have the grapefruit diet, the cabbage soup diet, the potato diet, the rice diet, the mushroom diet, and on and on. These diets are some of the most successful for short term weight loss but they are also the least healthy.
Single food diets are a classic case of taking a tiny bit of science and blowing it all out of proportion. For example, it is true that studies have shown a compound in grapefruit juice can help with weight loss and reduce inflammation. What they don’t tell you is that most of these studies have been done in mice and rats. You are not a rat being force fed a very high fat diet! What’s more, any increase in fat burning is modest, the equivalent of an extra 50kcal a day in a human. That’s the same amount found in a medium apple.
Regardless of the superfood chosen, no one food can cause significant weight loss on it’s own. Any weight lost on these diets is due to eating fewer calories. By their nature, these diets are incredibly restrictive. Yes, if you replace a normal 500Kcal meal with 50 Kcal of grapefruit, you will lose weight! Most people on these diets end up eating less than half the calories they normally would. That is the definition of a crash diet! Even the diets which only replace one meal a day with the chosen superfood will dictate what you can eat for the rest of the day. And invariably there is no sugar, no alcohol and no junk food. In each case, it’s the cutting back on burgers that causes weight loss, not the magical fat burning properties of cabbage.
The other reason single food diets work is boredom. On the potato diet for example, you literally eat nothing but potatoes. You can have them any way you like (except as chips or crisps), but no other foods – no vegetables, no meat, no dairy, just potatoes. In theory, you can eat as much as you want. However, after a couple of days of potatoes for breakfast, lunch and dinner, you will get so tired of them you will invariably eat less. This is backed up by research, which has shown that most of us eat far more if we are presented with many different foods. In one study, when people were presented with a buffet with a wide range of foods they consumed, on average, 50% more calories than at a buffet with large portions but fewer foods.
Needless to say, these diets are very unhealthy long term. Usually the superfood chosen is a fruit, vegetable or starch (such as potatoes or rice). Such foods tend to be low in protein and fats, and offer a limited range of vitamins and minerals. The lack of protein is of particular concern. Numerous studies have shown that consuming plenty of protein helps to preserve muscle mass when dieting. In other words, more protein makes sure it’s fat you are losing, not muscle. While you may lose loads of weight on these diets, it’s likely you are losing both water and muscle mass along with the fat. This in turn will slow down your metabolism, meaning you burn fewer calories at rest. Not ideal!
Single food diets are the very definition of unsustainable, both from a physical and mental point of view. The lack of variety raises the risk of serious vitamin and mineral deficiencies. At the same time, the very restrictive nature of these diets can encourage disordered eating behaviours. So yes, while slashing the calories will cause weight loss, single food diets are quite possibly the least healthy way to do it!