Can just the sight of food raise insulin?

Ask Dr. Jason Fung

Do you recommend magnesium supplements for constipation? Is a glass of wine OK on a low-carb or keto diet? And can just the sight of food raise insulin?

It’s time for this week’s Q&A about intermittent fasting and low carb with Dr. Jason Fung:

Magnesium supplements for constipation?

Hi Dr. Fung,

I’ve been following you for several years now and know that you will recommend fiber, stool softeners, or laxatives for constipation during fasting, but have never heard you recommend taking 1-2 magnesium supplements in the morning for constipation. I have been doing this and it always works. Plus you get some extra magnesium. I just think more people should know about this great option.


In our IDM program, we get many questions about supplements. Perhaps the single most common supplement we recommend is magnesium. In diabetics and in people taking diuretics (water pills that are usually prescribed for high blood pressure), low magnesium levels are especially common. The main symptoms are constipation and cramps. Milk of magnesia is a widely available, over the counter supplement for constipation and I agree it often helps.

Dr. Jason Fung

Is a glass of wine OK?

Is a glass of wine OK when on a low-carb and intermittent-fasting regimen?


During fasting, I don’t recommend it. But otherwise, yes, it can be part of a healthy low carb diet, although you need to be careful about the type of wine. Some are very sweet and contain lots of sugar and carbs. You can find a guide to low carb wines on

Read more: 7 things you need to know about alcohol and the keto diet

Dr. Jason Fung

Can just the sight of food raise insulin?

We’ve all heard of studies that show that simply tasting something sweet without ingesting it can raise insulin, but can simply seeing certain foods trigger the same process?


Yes. this is called the cephalic response. The sight or smell of food starts you salivating and insulin starts rising immediately even if you do not eat.

Dr. Jason Fung



Intermittent fasting for beginners

Earlier Q&A

Intermittent fasting Q&A

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Dr. Fung has his own blog at He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code

His new book, The Complete Guide to Fasting is also available on Amazon.



  1. Steve
    "The sight or smell of food starts you salivating and insulin starts rising immediately even if you do not eat."

    I'm having trouble coming to grips with this. So... I'm 18 hours in to a 24 hour fast, and I see/smell food and start to salivate, insulin is released. Does this put me back in energy storage mode (meaning I stop burning my body Fat?)

    Or is the amount of insulin released such a trivial amount that it doesn't affect my metabolism?

  2. Kim
    Damn, that explains blood sugar going up on a fast when when I'm not doing anything but watching tv.
    F@$K ME!
  3. Ed
    What's the reason wine isn't recommended during fasting? What if instead of eating a meal per se, you just have wine instead?
    Reply: #4
  4. Geraldine D. Kuss
    Well wine has carbs. SO you're going to be just ingesting carbs. I've been doing intermittent fasting and I'm putting on weight. Been doing 16/8 and drinking wine. I imagine it's the wine & cheese I eat. Now going to abstain for a few weeks.
  5. Robert
    Steve's question is awesome, but I do not see a response from Dr Fung.
    Reply: #6
  6. Kristin Parker Team Diet Doctor
    The problem is that there isn't going to be a set answer. For the majority of people, seeing or smelling food won't be a problem but for others it may be. The catch is that there isn't any way to test for insulin spikes at home.

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