Why you feel hungry or full

Why do you feel hungry when you know you shouldn’t? Or why do you still feel full when you haven’t eaten for a long period of time? Your appetite...
Always hungry?

Why do you feel hungry when you know you shouldn’t? Or why do you still feel full when you haven’t eaten for a long period of time? Your appetite is largely regulated by a group of hormones, which work together to control how much you eat. However, these hormones developed (as the rest of our body did) in a different environment to the one we live in today, and a modern diet/ lifestyle can easily disrupt it. This post will explain how your appetite is naturally regulated (with anti-obesity fail safes), and explain how a modern diet can disrupt these systems – so you know what to do/ not to do to normalise your appetite!

 

Ghrelin – The Hunger Hormone

How does Ghrelin work?

Ghrelin is usually referred to as ‘the hunger hormone’ for the simple reason that stimulates hunger. It is primarily produced in cells in the stomach where they can pass easily into the blood stream, and are able to travel to the brain and stimulate the feeling of hunger. During fasting periods the levels of ghrelin in the blood are elevated to increase appetite. In over-weight and obese people, the levels of ghrelin in circulation are consistently lower than that of leaner people. This would cause a reduction in appetite, and shows its function as a natural weight control agent- reducing the appetite of people who are overweight.

How and why it gets disrupted

A number of studies have shown that the natural production of ghrelin is disrupted by irregular sleep patterns. This can mean over sleeping, under-sleeping or sleeping at different times of the day (such as shift work). This disruption can cause ghrelin to be over produced or under produced, and there is no clear pattern as to why it can cause over production in some, and under production in others. All we know is that disrupted sleep causes disrupted ghrelin production, which causes appetite fluctuations.

Until relatively recently (especially when talking in evolutionary terms) we have never experienced alarm clocks, artificial light or the need to be anywhere at a specific time – these are all part of a modern lifestyle which is unfamiliar to our body. We have evolved with the rise and fall of the sun; which was the only way our body could tell the time of day, and this has been very important for our development into modern humans. After all, it makes perfect survival sense to be tired/ sleepy at night (so we sleep) when our sight is useless, and be awake/ active in the day time, where we are able to forage and hunt effectively. As such, many biological systems have evolved to revolve around the rise and fall of the sun into what is called our circadian rhythm (or body clock). Disruption of this body clock is the cause for irregular ghrelin production (and is associated a number of other ailments too!)

 

PYY – After Meal Satisfaction

How does PYY work?

PYY (peptide tyrosine tyrosine) is a hormone which is released roughly 15 minutes after eating, and it suppresses appetite. It is produced mainly in the ileum and colon by specialized cells, but small amounts can be produced along other parts of the digestive tract. The amount of PYY produced is roughly proportional to the calorific value of the meal – the more calories in the meal generally means a larger amount of PYY is secreted, and so the more full you feel.

How and why it gets disrupted

There is evidence to show that production of PYY is significantly decreased when the bowel is inflamed, the most common causes of this being Crohn’s disease or ulcerative colitis. Both of these diseases cause serious inflammation, but I would also think that mild inflammation (which is quite typical of a typical modern diet and lifestyle) in the bowel would also have a negative impact on PYY production.

Interestingly, over-weight and obese people produce less PYY hormone than lean people, which would cause them to feel less full than lean people after a meal. Whether this is a caused by being over-weight (which it could well do, as most over-weight people will suffer from inflammation) or as a result of being obese I don’t know for certain.


Leptin – Natures Weight Control Hormone

How does Leptin work?

Leptin acts on receptors in the hypothalamus to suppress appetite, but unlike ghrelin and PYY, leptin is produced in fat cells. The amount of leptin circulating in the blood is proportional to the amount of body fat an individual has, so, as body fat increases, appetite should decrease. There is vast quantities of research on leptin, and studies in both humans and animals, has shown that as leptin levels increase, food intake decreases and energy expenditure increases. This would result in excess fat being burnt and the individual returning to a healthier weight.

As expected, the levels of letpin in obese individuals are much higher than that of lean people, but not as expected; they do not lose weight, and often actually continue to gain weight. So the big question is- with such an effective natural weight management system already in place in our body, why is obesity such a big problem?

How and why it gets disrupted

Research has shown that high fructose corn syrup (HFCS) or glucose-fructose syrup as it is known in the UK, inhibits the action of leptin. This means that although leptin levels may rise, HFCS is preventing it from from actually having an affect on appetite. HFCS is found in a vast number of foods, especially fast foods and snacks. This inhibition could explain how people can continue to gain weight despite high levels of circulating leptin. In addition to this, leptin production is disrupted by irregular sleep in the same way that ghrelin is, which can further compound the problem.

A number of studies have shown that obese individuals are actually resistant to the appetite suppression effects of leptin in a similar way that type 2 diabetes sufferers are resistant to the effects of insulin. This offers an explanation as to why over-weight people stay over-weight.

 

Summary

Naturally, we are programmed to maintain a lean body weight, and this system is probably quite effective in a natural environment, however, modern diet/ lifestyle factors disrupt this, which can cause weight gain/ loss. With this in mind, the most important factor in maintaining/ achieving a healthy weight is simply to ensure you get a good night sleep, preferably similar to that of the rise and fall of the sun. This will help to normalize both ghrelin and leptin production in the body. The next thing to do is eliminate high fructose corn syrup/ glucose-fructose syrup. Typically, you will find this in sweet things, but it would just be best to remove all highly processed foods, as this will not only eliminate HFCS from your diet, but will undoubtedly help to reduce any bowel inflammation.

 


 

References

Cummings et al. N Engl J Med 2002;346:1623-30.

Wren et al, JCEM 2004.

Broberger. J Internal Med. 2005;258:301-327.

Langhans. Proc Nutr Soc 2007; 66: 321-30.

Harris & Martin. Am J Physiol 1984; 247: R380-R386.

Batterham et al. NEJM 2003.

I hope you enjoy the site, and like what we have worked hard to create, any feedback is very much welcome, after all this site is for you! Graduate of Nutrition & Food Science (Bsc) at Reading Uni.
  • Jamie

    I feel hungry every 3-4 hours and I never connected this to my sleep patterns which are all over the place. I will sometimes sleep from 3 AM to 8 AM, then the next day sleep from 10 PM to 3 AM, and in day 3 I would sleep from 6 PM to 12 PM. I usually get 7-9 hours of sleep a day (added up) but I do not have a clear sleep schedule and my natural production of ghrelin is most likely disrupted by my irregular sleep patterns.

    Is it that important that I sleep at the same time each night? I mean, I figured that if I get 7-9 hours of sleep I would be ok no matter what my sleep schedule would look like. I sometimes have problems sleeping at a certain hour and hate to waste 1 hour or more in bed until I fall asleep. That’s why I only try to go to bed when I feel very sleepy.

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