Why high fat diets may not be fattening

Because it’s called fat, it makes sense and is intuitive to believe that fat is fattening. It’s also rich in calories compared to carbohydrate or protein. The thing is, though, the evidence does not strongly link fat-eating with obesity, and eating low-fat diets are, on the whole, spectacularly ineffective for the purposes of weight loss. Never mind what common sense may dictate, taken as a whole the evidence suggests fat is not particularly fattening after all.

To understand how this can be it helps to understand a little about what influences the accumulation of fat in the fat cells. One key player here is the hormone insulin. This hormone predisposes the body to fatty accumulation through a variety of mechanisms, including enhanced uptake of fat into the fat cells (through activation of the enzyme lipoprotein lipase) and suppression of fat release (by inhibiting the enzyme hormone-sensitive lipase). Here’s the thing: fat does not stimulate insulin secretion directly. Is it possibly then that someone could eat a diet of nothing but fat and lose weight, even if they were not in calorie deficit?

This concept may sound far-fetched to some, but there is some evidence for it. One piece of evidence comes in the form of a study in which individuals, on separate occasions, were allowed no food or fed with fat into a vein.

In the study in question [1], individuals were completely fasted for a total of 84 hours on once occasion. During fasting, insulin levels normally fall, and levels of fatty acids in the bloodstream rise. Levels of substances called ‘ketones’ also rise. Ketones are formed in the liver from fat, and can be used for energy and, in particular, to fuel the brain. Their production is a normal, healthy mechanism that allows the body to fuel itself when food is in short supply or if carbohydrate is quite several restricted in the diet. During the fasting phase of this study, as expected, levels of glucose and insulin fell, while levels of fatty acids and ketone bodies all rose. The rate of lipolysis (fat breakdown) also increased.

The really interesting part of this study came when the same individuals were ‘fasted’ on another occasion. The difference was that on this occasion individuals were ‘fed’ with an intravenous drip containing little else but fat. The calorific value of this fat matched the calorie needs of the individuals taking part in the study.
On this occasion, glucose and insulin levels fell, while fatty acid and ketone levels rose. Lipolysis also rose. The extent of these changes was the same as during complete fasting. In other words, feeding the body pure fat induced a metabolic state in the body that was, to all intents and purposes, the same as the state induced by fasting.

While infusing significant quantities of nothing but fat into the body, the body still readily gave up its fat. And it gave it up as readily as it did when eating nothing at all. This study provides some evidence at least that fat is not inherently fattening, and reminds us of the critical role that hormones, not mere caloric balance, have a key role to play in body fat management.


1. Klein S, et al. Carbohydrate restriction regulates the adaptive response to fasting. American Journal of Physiology – 1992;262(5):E631-E636]

16 Responses to Why high fat diets may not be fattening

  1. KP 18 March 2011 at 1:39 pm #

    This is very interesting, thank you. My one problem with this is that intuitively it sounds all wrong to be “feeding” people with fat intravenously. I would love to know has anyone ever done a study where the participants actually ate (i.e. by swallowing) 2000-2500 calories a day in the form of nothing but fat for a few days? Ideally compared against the same calorific intake of nothing but carbohydrate, then pure protein, then compared against complete fasting. If anyone knows of any such studies, I’d be grateful for a link.

  2. patricia 18 March 2011 at 1:48 pm #

    having had a stroke nearly 2 yrs ago, I am on medication to lower BP and cholesterol, also aspirin and tablets to reduce the “stickiness” of plaque. What effect would a high fat diet have on these levels?

  3. Nigel Kinbrum 18 March 2011 at 7:06 pm #

    Lipolysis is the release of fatty acids from fat cells. However, if the fatty acids don’t get burned (by muscles doing work), they re-enter fat cells again. If that didn’t happen, there would be ever-increasing levels of serum fatty acids. Lipolysis therefore means very little.

  4. Richard 18 March 2011 at 9:03 pm #

    Patricia, if you think it appropriate, why don’t you try following the eating guidance that Dr Briffa describes in his books The True You Diet and Waist Disposal? There are lots of other Primal/Paleo books around too. And then see if you feel better and if your blood test results show any improvements as well? Maybe in the future your physician may suggest that you could even reduce your medication?

    I would say try spending 2 weeks to transition onto a primal diet, then if you are feeling okay follow it strictly for at least a month, and get tested while still on the diet. These diets are quite low on carbs (and this is probably the most important change away from a “standard modern diet”, excess carbs are stored as palmitic acid/fat around the body) but I think it is a good idea to have a meal or two each week with some sweet potatoes or yams and also a little fruit, like berries, every day or so. Also allow yourself to have a ‘cheat’ meal every week if you want. Also keep nicely hydrated and be active or exercise a bit if you can.

    It is only my personal view and I am not a medic, but as a cancer survivor (6 years now) I feel this has really helped me and is the healthiest way to eat. You might find that your GP etc. are not up to speed with all this, but give them ten years and I’m sure the NHS will be recommending lower carb diets.

    There is lots of info and scientific references on Dr Briffa’s site if you want to find out more here.

    Best wishes.

  5. JohnBenson Anyanwu 18 March 2011 at 9:09 pm #

    Patricia, your case is pure medical condition and as such should not take fat in large quantities without first clarifying with your doctors. The key word in the experiment is ‘MAY’, indicating that it is not conclusive yet. The only truth about this is that many renowned doctors like Atkins have been saying this many times. Healthy individuals should put their feet down on this and later tell their findings, we will be interested.

  6. Steve 19 March 2011 at 2:29 am #

    I believe fat my not be fattening in the same way as carbohydrates because fat storing mechanisms in the absence of insulin don’t have a long term chronic dis-regulation “feedback loop”.

    However I question this intravenous feeding. Does intravenous feeding produce chylomicrons that stimulate ASP? Normally these are produced in the small intestine and Acylation Stimulating Protein responds to them.

    If the ASP mechanism was suppressed by the feeding method in this study, then the results may be very different than normal eating. I don’t know the answer to this, but ASP, like insulin, also raises LPL and suppresses HSL according to information on the site adipocyte.co.uk — So similar mechanisms of storage exist outside the presence of insulin, even though ASP and insulin work best in concert and ASP doesn’t seem to have the same issues of resistance and hypersecretion etc.. that insulin does, which would limit it’s role in long term obesity and metabolic syndrome, if not short term fat storage. And while ASP may affect LPL and HSL I’m not sure of it’s relative power compared to insulin or outside the influence of insulin.

  7. kate 19 March 2011 at 7:22 am #

    Hmm. If we would eat nothing but fat, we’d lose fat and be slim and sleek? Yah. Right. Ever tried to force someone to eat just sticks of butter for days on end and tell them it’s ‘Modified Atkins’ or a similar insane unhealthy regimen. After the first 4 hours, they’re begging you to take the butter away.

    Ah, but that’s all they would eat! That’s all they would be allowed to eat, you mean. In which case, they would sit in the corner and starve, rather than eat the fat you so generously insist is their diet. And there you have it – your ‘weight loss.’ It’s low-carbing at its finest.

  8. JT 19 March 2011 at 10:09 am #

    @ Kate – you really do know very little about what you’re commenting on don’t you? I suppose you think Atkins is a high-protein diet with no veggies or fruit? Your comments are emotive and without substance.

  9. John Briffa 19 March 2011 at 10:40 am #


    You’re right, lipolysis doesn’t tell us much, but are you suggesting that NONE of increased amounts of free fatty acids (and ketones) formed through this process are burned for energy?

  10. simon 19 March 2011 at 11:58 am #

    Nigel and Dr Briffa,

    The processes at work are perhaps more complex than is our understanding – it can be tempting to rule things in – just as it can be tempting to rule things out. I think any study and discussion that questions the value in the low-fat paradigm is a good one, so thanks Dr Briffa. With all respect to Nigel, I know he is well informed on many matters, I was struck by the thought upon reading his comment that something must support basal metabolic rate while the status of the trial is maintained.

    It’s only personal anecdote but my personal experience runs counter to one of your themes, Dr Briffa. I have found that exercise does significantly help mediate weight loss, in my case – I don’t know how precisely, but since I sense I have suffered from glycosilation (spelling?) of the muscles which causes muscle lethargy I imagined muscle activity decreased levels of glycosilation and thus somehow removed ‘something’ that stood in the way of weight loss.

    I guess I side with you both while being not nearly so well informed as either of you. (;0)

  11. Nigel Kinbrum 19 March 2011 at 5:02 pm #


    Some are burned, but when a person is sedentary, they’re burned at a very low rate. See It’s all in a day’s work (as measured in Joules).

    At rest, approx 0.1g of fatty acids are burned per minute.

    Over a wide range of exercise intensities, approx 0.4 to 0.5g of fatty acids are burned per minute, 4 to 5 times faster than at rest.

  12. Yossi 22 March 2011 at 8:04 am #

    Diabetes UK spokesperson on BBC1 again this morning giving low-fat disinformation. Look at their website and it is anti fat and pro carb. No references to evidence.

  13. Robbo 22 March 2011 at 8:10 am #

    I have never tried to force anyone to eat anything. I eat butter and other fats as part of a ‘balanced diet’ – plenty of fat and protein and a little carb from green vegetables and fruit.

    My ‘balanced diet’ led to effortless weight loss over 18 months followed by effortless maintanance over 5 years and counting.

    For me, eating fat does not result in an increase in body fat. I know this because I live with the fact every day.

  14. Irene @ H.E.S.H. 14 September 2011 at 10:25 am #

    Great insights! I learned a lot reading this article . Hope to see more of this type of article in the future! It’s nice to gain some new knowledge once in awhile.


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