Glutathione

glutathione molecule

What is Glutathione? 

By Dr. Pamela Frank, BSc(Hons), ND

It is a substance produced naturally in your liver and is a powerful antioxidant, considered to be the mother of all antioxidants (antioxidants help prevent oxidation and aging).  It is made from three amino acids: glutamic acid, cysteine and glycine.  One of the primary functions of glutathione is cellular detoxification.

Why is glutathione important?

Healthy blood levels are important for protection from heart disease, dementia, Alzheimer’s and aging in general.  Reduced levels occur with aging and are associated with increased oxidative damage.

How do I get glutathione?

Foods contain it and there are glutathione supplements, however, most of what is orally ingested gets broken down in the digestive tract and so it doesn’t have an impact on increasing your blood levels.  For this reason, taking supplements is likely a waste of money, even liposomal glutathione. Consuming the above amino acids may assist your liver in producing more if you need it. Food sources of glutathione include: spinach, asparagus, avocado, squash, okra, cauliflower, broccoli, walnuts, garlic and tomatoes.

How else can I raise my blood level?

There are a number of supplements that have been shown to help raise levels in the blood, including:
  • Alpha Lipoic Acid
  • Milk Thistle
  • MSM
  • Melatonin
  • Curcumin

Your body can also recycle existing glutathione with the help of following vitamins and minerals:

  • Vitamins: C, E, B vitamins, active folic acid (L-5MTHF)
  • Minerals: selenium, zinc, vanadium, magnesium

Too Much of a Good Thing?

There is some research that has found higher levels of glutathione in cancer cells.  It may be that the cells have increased their own level as a means of protecting themselves from damage by chemotherapeutic agents or it may be that cancer cells are trying to keep themselves from undergoing normal cell demise known as apoptosis.

Should you supplement with glutathione?

I would say no, for the reasons I have already mentioned: it’s poorly absorbed and broken down in the gut and until we fully understand why glutathione is higher in cancer cells, it may be best to avoid artificially increasing it.  Use of some of the above supplements that help support healthy internal production or recycling seems like safer options.

References:

Yilin Liu, Annastasia S. Hyde, Melanie A. Simpson, and Joseph J. Barycki. Emerging regulatory paradigms in glutathione metabolism. Adv Cancer Res. 2014; 122: 69–101.

Matthew Butawan, Rodney L. Benjamin, and Richard J. Bloomer. Methylsulfonylmethane: Applications and Safety of a Novel Dietary Supplement. Nutrients. 2017 Mar; 9(3): 290.

Antonio Carrillo-Vico, Patricia J. Lardone, Nuria Álvarez-Sánchez, Ana Rodríguez-Rodríguez, and Juan M. Guerrero. Melatonin: Buffering the Immune System. Int J Mol Sci. 2013 Apr; 14(4): 8638–8683.

Jianguo Lin, Youcai Tang, Qiaohua Kang, Yunfeng Feng, and Anping Chen. Curcumin inhibits gene expression of receptor for advanced glycation end-products (RAGE) in hepatic stellate cells in vitro by elevating PPARγ activity and attenuating oxidative stress. Br J Pharmacol. 2012 Aug; 166(8): 2212–2227.

Healthy Weight Loss

woman celebrating easy weight loss

Weight Loss: 5 Healthy Ways to Lose Weight

By Dr. Pamela Frank, BSc(Hons), ND

Swimsuit season will soon be upon us!  This is the time of year where it is relatively easy to shed your winter weight. If trying on your spring and summer wardrobe has been a depressing experience, here are some weight loss tips and tricks to help you lose weight and get into summer shape more quickly and easily:

  1. Reality check – use a program like MyFitnessPal or the Fitbit app to track your caloric intake and exercise for a day or two.  You don’t need to get obsessive about it, but people tend to underestimate how many calories they consume and overestimate how much exercise they get.
  2. Reduce your stress.  Stress is bad for weight in so many ways: Increased cortisol, emotional eating, lower T3 (active thyroid hormone), higher reverse T3 (inactive thyroid hormone).
  3. Get enough sleep.  Lack of sleep lowers your willpower, promotes sugar cravings to supply energy and even one night of less than 4 hours sleep makes you more insulin resistant the next day and higher insulin means more fat gain
  4. Don’t snack.  The old 3 meals two snacks advice was bad advice.  Research has shown that people who snack between meals consume more calories in a day than those who don’t.
  5. Exercise. If you want everything you need to do to lose weight, my amazing colleague, Dr Jade Teta, has created an exercise program for weight loss, with bonus materials that include a healthy menu and recipes.  His workout will challenge what you thought you knew about exercise, it won’t take you long (only 15-20 minutes 3 times per week), there is no gym membership required, you can do it in your own living room, with or without weights or bands, it’s science based and it is cheap. The purpose of the program is to reset your metabolism to it’s prime.  I’m in my second round of the 12 week program and I can attest to the fact that it is hard, but I feel fitter than I have in years and I have lost weight and I’m exercising less. (despite my best efforts, my weight has only been doing a slow steady climb since I hit 45). Best of all, the program creator is a naturopathic doctor as well as a personal trainer and a heck of a nice guy. Check it out here. 

Meal Replacement

why boost is no better than a chocolate bar pic

Why “Meal Replacement Drinks” are no Better Than Chocolate Bars

By Dr. Pamela Frank, BSc(Hons), ND

My mom is elderly and lives on her own and doesn’t always remember to prepare herself a nutritious meal. Her doctor recommended one of these “meal replacements”, here’s why that’s a horrible idea.

These “meal replacement” drinks are mostly composed of unhealthy fats and sugar.  If all I was concerned with was giving my mom empty calories like that, I would just give her a chocolate bar.  At least it contains real food ingredients (peanuts).  The main ingredients in these drinks are: water, sugar, corn syrup, milk protein concentrate, canola/sunflower/corn oil and soy protein isolate.

What about the vitamins and minerals in meal replacement drinks?

The quality of these vitamins and minerals is the lowest of the low, the cheapest forms for the manufacturer to put in.  Not well absorbed, not the form that the body needs.  The casein is potentially detrimental to the digestive tract and absorption of nutrients.  Caseinate forms of calcium (in these drinks), contain the protein casein which is a common food allergen and may aggravate anyone with a dairy sensitivity.

How do the nutritional facts compare?

Meal Replacement Drink:

Calories: 240/8 oz bottle

Cholesterol: 10 mg

Sodium: 150 mg

Carbohydrate: 41 g

Sugar: 20 g

Snickers:

Calories: 250/bar

Cholesterol: 5 mg

Sodium: 120 mg

Carbohydrate: 33 g

Sugar: 27 g

NesQuick:

Calories: 150/8 oz

Cholesterol: 15 mg

Sodium: 180 mg

Carbohydrate: 24 g

Sugar: 22 g

If you’re noticing some similarities between the Meal Replacement drink and NesQuik, that’s because they share the same manufacturer.

What are better alternatives to meal replacement drinks?

There are many better ways to help an aging parent or someone who is very ill meet their nutritional needs.  If they would prefer something in liquid form, purchase a high quality protein shake like Vega One or Ultra Protein Plus by Douglas Labs.  They’ll supply protein, much better quality vitamins and minerals without the sugar and canola oil.  To bump up the calories, blend these with full fat coconut milk, a banana, an avocado, some coconut oil and/or almond butter.

For mineral and protein nourishment, cook up some bone broth.  It’s easy, you just throw something with a bone in it in the slow cooker with a splash of apple cider vinegar and some chopped up veggies and leave it overnight.  A cup per day of bone broth will provide lots of readily absorbed minerals, and protein in the form of gelatin which can be used by the body to make collagen for building healthy bones, hair, skin and nails.

Chestnuts Roasting in Your Oven

roasted chestnuts

Chestnuts

The tradition of roasting chestnuts dates back to sixteenth century Rome where street vendors sold them, much like we see here in Toronto.

Nutritional Content of Chestnuts

 

Chestnuts crumble in the mouth to give a sweet flavour, they are low in fat and are in fact a seed, not a nut. They are a good source of dietary fibre, and minerals like manganese, potassium, copper, phosphorous, magnesium and iron. They are also a vitamin powerhouse with Vitamin C, Vitamin B6, Thiamine, Folate, Riboflavin, Niacin and Pantothenic Acid. Zinc and calcium are also present in small amounts.  They contain lower amounts of fat and protein than most nuts and seeds and relatively higher amounts of carbohydrate and sugar.

How to Choose Good Chestnuts

These “nuts” are freshest between October to December.  Look for fresh ones that are hard, shiny, heavy for their size and do not rattle when you shake them. Because of their starch and sugar content they, they can harbour mold, discard any with visible mold.  They are highly perishable, so refrigerate them for up to one week or freeze for up to one month.

Roasting Chestnuts

You can roast them yourself by nicking the skins so that they don’t explode as they heat up. You should cut an X in the skin with a very sharp knife. Cut right through the skin, they have very tough skin so a little pressure is needed. Watch your fingers!

Heat your oven to 400 degrees Fahrenheit, spread chestnuts out on a roasting pan and pop them into the oven for approx 15-20 mins.

Take them out of the oven and let them cool a bit before eating. If you place a towel over them while they cool, the trapped steam will help loosen the skin. When you are ready to eat them, peel them by squeezing them to loosen the skin.

Here’s a link to some chestnut recipes: http://www.epicurious.com/ingredient/chestnut

Looking for more healthy food ideas or nutrition advice?  Talk to one of our licensed naturopathic doctors.  

Prevent Colds

woman trying to prevent colds and flus

How to Prevent Colds and Flus

By Dr. Pamela Frank, BSc(Hons), ND

Looking to waltz through cold and flu season unscathed and prevent colds and flus?  Here are 5 tips to stay cold and flu free this year:

  1. Up your vitamin D intake.  This time of year you want to be taking more than your usual dose of vitamin D to make up for shorter days and very little exposed skin to sunlight. A trip south may also help increase your vitamin D but also help fend of Seasonal Affective Disorder (SAD).  Personally, I can’t wait for March Break and some warmth and sunshine.
  2. Cut food sensitivities out of your diet.  How can your immune system fight off viruses if it’s busy fighting off your lunch?  If you know you have a dairy sensitivity, avoid dairy. Ok, well maybe after the holiday parties are over.
  3. Eat more ginger – Ginger helps protect against viruses by blocking viral attachment and internalization.  Ginger tea and curries are good ways to incorporate more ginger. There are lots of healthy ways to add some extra ginger to your diet on our recipe page: Gingerbread Cookie Tea, Apricot Ginger Chicken, and Ginger Pear Energy Bars are just a few.
  4. Reduce your intake of sugar sweetened beverages – pop, energy drinks, lattés, chocolate milk, fruit drinks, shakes, and anything else with added sugar. We know sugar suppresses the immune system for at least 3 hours after consumption.  Drink this Cinnamon Chai, Gingerbread Cookie Tea, or New Year’s Resolution Smoothie instead.
  5. Drink green tea. Green tea is the perfect immune system boosting drink. Green tea contains a substance called epigallocatechin gallate, or EGCG. This substance can fight bacteria and prevent viruses from multiplying. Green Tea Cheesecake anyone?

Wanting more advice about how to prevent colds or flus, what to take or do if you get a cold or flu or what to do if you are feeling the first signs of a cold or flu?  See one of our licensed naturopathic doctors today.

Want to know when to see a doctor about a cold or flu?  Want to know what should be in your cold and flu prevention tool box? Download and save Dr. Pamela’s handy infographic here:

prevent colds and flu

 

Looking for even more information about colds and flus, natural remedies for colds and flus, tips to stay healthy, herbal remedies for colds and flus, quizzes and immune system assessments?  Purchase the full Cold and Flu ebook here: https://www.smashwords.com/books/view/595481

 

Hangover Remedy

picture of grilled asparagus as a hangover remedy?

What’s the Best Hangover Remedy?

By Dr. Pamela Frank, BSc(Hons), Naturopathic Doctor

As we’re coming up to the festive season and the libations flow, I thought it an opportune time to pass on this little hangover remedy tidbit.  I know that you personally would never overindulge but perhaps you could pass this on to your husband, brother, aunt who tends to overdo it during the holidays. 😉

A study found that asparagus is a great food for helping to cope with hangovers. We don’t generally think of vegetables as herbs, but Asparagus officinalis is an herb that is used in the treatment of several conditions.  One 2016 study, found obvious anti-tumor effects of one compound in asparagus (asparinin A) and another found that components of asparagus combat liver fibrosis.

The part of the plant that we usually consume is the young shoot, but the leaves of the asparagus plant can be eaten as well and are much higher in amino acids and inorganic minerals. A 2009 study found that the two key enzymes that increase the breakdown of alcohol are more than doubled after ingestion of an extract of asparagus shoots and leaves. That means that asparagus can be a safe, effective means of treating alcohol hangover and protecting the liver from other toxins too.

Post holiday party you should also drink lots of water to rehydrate and get your blood sugar stabilized by eating lots of veggies, including asparagus, good protein and healthy fats.

Sources:

Li XM, Cai JL, Wang L, Wang WX, Ai HL, Mao ZC. Two new phenolic compounds and antitumor activities of asparinin A from Asparagus officinalis. J Asian Nat Prod Res. 2016 Sep 13:1-8.

Zhong C, Jiang C, Xia X, Mu T, Wei L, Lou Y, Zhang X, Zhao Y, Bi X. Antihepatic Fibrosis Effect of Active Components Isolated from Green Asparagus (Asparagus officinalis L.) Involves the Inactivation of Hepatic Stellate Cells. J Agric Food Chem. 2015 Jul 8;63(26):6027-34. doi: 10.1021/acs.jafc.5b01490. Epub 2015 Jun 19.

Kim BY, Cui ZG, Lee SR, Kim SJ, Kang HK, Lee YK, Park DB. Effects of Asparagus officinalis extracts on liver cell toxicity and ethanol metabolism. J Food Sci. 2009 Sep;74(7):H204-8. doi: 10.1111/j.1750-3841.2009.01263.x.

Dry Skin

woman with dry skin

Healthier Skin from the Inside Out: Natural Solutions for Dry Skin

By Dr. Pamela Frank, BSc(Hons), ND

You’ve applied a ton of moisturizer and still you have dry skin? That’s because the health of your skin is determined by internal factors as well as external factors like cold, dry winter air. Maintaining healthy, soft skin means addressing the internal and external factors that affect your skin.

Water

Water moisturizes your skin from the inside out.  Every one of your skin cells is like a little bag of water.  If you don’t take in enough water, your skin cells may look more like dry, shriveled up raisins instead of plump, moist grapes.

EFA’s

The shell of  your skin cells is an oily layer that helps hold water in and selects what is allowed to enter and exit your cells. Taking in healthy fats, called essential fatty acids, helps build a healthy shell that keeps moisture in. Eating Omega 3 fatty acids from whole foods like eggs, nuts and seeds and fish are especially important when dry winter air hits.

Vitamin A, C & E

Because of their antioxidant value, vitamins A, C and E are important for healthy skin. The antioxidant effect helps fight free radicals and oxidative stress that can damage your skin. Topical vitamin C cream has also been shown to improve skin quality and fight aging.

Make it edible

I always recommend that you only use products on your skin that you would put in your belly. At least 25% of the chemicals that you apply to your skin are absorbed into your blood stream and the liver must then detoxify them and remove them. The chemicals in moisturizers can have unwanted estrogen like effects.  I like extra virgin organic coconut oil to protect my face and lips from winter’s dryness and wind because it’s edible (hello, it’s going on my lips!) and it has anti-inflammatory effects.

Exfoliation

Scrubbing your skin can damage it because it strips off protective oils that keep moisture in, creating redness and inflammation. Exfoliation also removes superficial layers of skin before the underneath layers are ready, exposing tender new skin cells to the elements before their time.  Gently cleaning your skin with a washcloth and water and following that with an edible moisturizer is more soothing and gentle on your skin.

Boost Metabolism

woman exercising to boost metabolism

Rev Up Your Engines! 3 Effective Ways to Boost Metabolism

By Dr. Pamela Frank, BSc(Hons), ND

Your metabolism dictates how quickly and efficiently you burn calories to use for fuel and maintaining your body temperature.  The higher your metabolism, the more calories you can consume in a day without gaining weight.  Many of the patients that I see, complain of a sluggish metabolism – they don’t feel like they overeat, they consume healthy food, exercise regularly and yet they gain weight easily.  Let’s take a look at three ways you can boost metabolism to help with weight loss.

Nourish Your Thyroid

Your thyroid is a butterfly shaped gland in your neck that regulates your metabolism.  Think of it like the gas pedal for your body.  A sluggish thyroid means a slow metabolism  In order to do it’s job, your thyroid needs iodine, zinc, selenium, copper and tyrosine to build thyroid hormones.

Our main food source of iodine is iodized salt.  As a public health measure, iodine was added to table salt as a means of gently supplementing everyone with a little extra iodine, to stave off underactive thyroids.  However, excessive salt intake is considered unhealthy, so people have either stopped using salt in food preparation or shifted to using sea salt instead.  Those of you strictly avoiding salt or using sea salt may not be getting the benefit of the iodine added to regular table salt. I recommend switching to iodized sea salt, which is readily available at most health food stores.

Selenium is a trace mineral that is vital to healthy thyroid function.  Brazil nuts are a rich food source and consuming as little as 3 Brazil nuts per day supplies plenty of selenium for your thyroid.

Zinc is found naturally in beef, pumpkin seeds, egg yolks and shellfish.

Amino acids are the building blocks of protein, so protein rich foods like beans, eggs, nuts, seeds, beef, fish and chicken will supply the amino acid, tyrosine, that is also crucial to healthy thyroid function.

Restore Your Adrenals

The adrenal glands are your stress glands; they help your body cope in times of stress. They assist the thyroid hormone to function properly by enabling transfer of the hormone into the tissues where it works to increase metabolism. The adrenal glands need substantial amounts of vitamins B5, B6, C, potassium, magnesium and zinc especially while under
stress.

To nurture healthy adrenals:

  1. Eat lots of leafy green veggies like kale, spinach, Swiss chard and spring mix salad greens.  These are chock full of B vitamins, C, zinc, potassium, magnesium and zinc, everything that the adrenals need, all in one food.
  2. Get adequate rest and sleep.  Stop working by 8 p.m. and relax.  Aim to be in bed by 10 p.m..  The adrenals operate on a daily schedule that is regulated by daylight.  They peak in the morning and taper off to their lowest between 8 p.m. and 4 a.m..  Try to work with their schedule, not against it.
  3. Reduce your stress.  Easier said than done, but whatever you can do in this regard will help your adrenals and help to maintain a healthy cortisol level.  Excessive cortisol contributes to weight gain around your waist.  Stress reduction through psychotherapy, exercise, yoga, tai chi and massage therapy can all help if stress is the problem.

Exercise

Exercise can boost metabolism for up to 48 hours afterward, so get off the couch and walk briskly, dance, go on the treadmill, do something active for 30 minutes 5-7 times per week.  Exercise, but not to the point that you feel exhausted after, you should feel energized after exercise.  It’s also the best stress reliever there is.  Weight training and high intensity interval training (HIIT) are particularly good for building muscle mass.  Muscles burn more calories than non-muscle tissue, so muscle can boost metabolism even while you are sleeping.

Sugar Substitute

sugar bowl with caption what is the best sugar substitute

Sugar Substitute: What Makes the Grade?

By Dr. Pamela Frank, BSc(Hons), Naturopathic Doctor

We are all born with a sweet tooth.  Sweetness tells our body that the food is calorie dense and in times of starvation or food scarcity, consuming such foods would be a self-preservation mechanism.  For most of us, food is not that scarce, most processed foods are unnaturally sweet and taking in sweet foods adds empty calories, causes weight gain, provokes excessive insulin, promotes diabetes, hormone imbalance and inflammation. The best sweetener is no sweetener, but if you must have something sweet, here are the pros and cons of the various sugar substitute options.

Cane sugar, cane sugar juice, dehydrated cane syrup etc.

Grade = D – often found in “natural”, “healthy” treats
Pros: Derivatives of the above retain a few trace minerals that may help the body metabolize the sugar better, but at the end of the day, they’re just sugar
Cons: Still very high glycemic index, causes excessive insulin release, promotes insulin resistance and diabetes

High Fructose Corn Syrup

Grade = F – often found in pop and processed foods
Pros: None
Cons: Promotes obesity, insulin resistance, fatty liver disease and diabetes

Agave Nectar

Grade = F – often found in “natural”, “healthy” treats
Pros: None really, just benefits from good marketing
Cons: Contains more fructose than high fructose corn syrup, therefore can promote obesity, insulin resistance, fatty liver disease and diabetes

Stevia

Grade = B – natural extract from the plant Stevia rebaudiana
Pros: natural sugar substitute, intensely sweet, doesn’t elevate blood sugar, and does increase insulin sensitivity
Cons: Does cause insulin release which can disrupt hormone balance however that may be offset by the improved insulin sensitivity, slightly bitter aftertaste

Aspartame

Grade = F – artificial sweetener
Pros: None
Cons: Can cause various neurological problems in susceptible people, despite no increase in blood sugar it increases insulin levels which can aggravate hypoglycemia, lead to excessive appetite and sugar cravings, hormone imbalances, infertility, fibroids, endometriosis, PCOS

Molasses

Grade = C – is the dark liquid byproduct of the process of refining sugar cane into table sugar. It is made from the third boiling of the sugar syrup.
Pros: somewhat natural, an excellent source of manganese, copper and iron and a good source of calcium, magnesium and potassium
Cons: not good for blood sugar, insulin levels, hormone balance

Maple Syrup

Grade = C – is the amber liquid that remains after evaporating most of the water off the sap of the maple tree.  It takes 40 gallons of sap to make 1 gallon of syrup, so the natural sugar present in sap has to be concentrated 40 times to make syrup
Pros: Excellent source of manganese, good source of zinc
Cons: not good for blood sugar, insulin levels, hormone balance

Splenda/ Sucralose

Grade = F – artificially made by replacing three hydrogen-oxygen groups on the sugar molecule with three chlorine atoms
Pros: Low calorie sweetener
Cons: Side effects may include bloating, rash, decreased coordination, dulled senses, headaches, insomnia, irritability, stomach cramps, despite no increase in blood sugar it increases insulin levels which can aggravate hypoglycemia, lead to excessive appetite, hormone imbalances, infertility, fibroids, endometriosis, PCOS

Xylitol/Erythritol

Grade = B+ – naturally occurring sugar substitute, wood alcohols
Pros: has anti-bacterial (against Strep) and anti-fungal properties, lower calorie, safe, has been shown to improve bone density in rats, tastes good, measures like sugar
Cons: can have somewhat of a laxative effect and if consumed in large quantities may cause bloating and abdominal pain, TOXIC for dogs

Honey

Grade = B+ – naturally occuring sugar substitute, no need to concentrate it
Pros: has antibiotic properties, contains vitamins and minerals, not as high glycemic index as sugar
Cons: does still increase blood sugar and requires production of insulin

Coconut Sugar

Grade = C – made from concentrating the sap from the flower buds of the coconut palm
Pros: Lower glycemic index that table sugar (according to the manufacturer)
Cons: Other than the above, not much different or better than consuming table sugar

Using No Sugar or Sugar Substitutes and just enjoying the natural sweetness of whole foods – A+++

Traditional Chinese Medicine Eating Habits

how to eat according to TCM: child eating an apple

Healthy Eating Habits: How to Eat According to TCM

In Traditional Chinese Medicine (TCM), food is heavily relied upon as medicine. What, when, the temperature of the food and how you eat it affects the function of your digestive organs (your spleen and stomach in Chinese medicine) which in turn influences the qi (energy) and function of all the other organs.

The four key rules for eating habits according to Chinese medicine principles are:

  • Timing – best to eat at the same time every day.  In TCM, the spleen and stomach are the organs most involved in digestion and they work best at certain times of the day. The stomach time is from 7-9 a.m., which is the best time of day to consume a good hearty breakfast.  The spleen time follows the stomach, from 9-11 a.m., here you are digesting that hearty breakfast and turning it into energy for your body to use.   These organs are weakest 12 hours later, so you want to avoid eating from 7 p.m. to 11 p.m. to avoid damaging them.
  • Weather temperature – External cold temperatures dictate the consumption of warmer foods like soups and stews, external heat calls for
    colder foods like salads. Excessive consumption of cold, raw foods can damage the spleen, so ease up on the salads in winter, switch to lightly stir-fried or steamed foods.
  • Be mindful of what you are doing while eating – You should be focused on eating, not watching TV, talking on the phone, surfing the internet, driving, walking etc.  Being attentive to the task of eating, helps improve digestion, increases awareness of how much you are eating and helps you recognize when you are full.  The parasympathetic nervous system is responsible for digestion, being overstimulated or stressed while eating decreases parasympathetic nervous system activity and increases sympatheic nervous system which directs resources away from your digestive tract.
  • Quantity – You should eat to the point of 2/3 satiety, to allow some reserves in the digestive tract for the process of digestion