Here is the assumption almost everyone brings to this question, and it is backward: that a loan secured by an investment portfolio must somehow be an investment expense. The tax law does not work that way. What secures a loan and what the loan interest is treated as are two entirely separate questions, and confusing them is the single most common mistake in how people think about securities-based lending (SBL), Lombard loans, and other securities-collateralized borrowing.
Collateral is irrelevant. Use of proceeds is everything.
Under Treasury Regulation 1.163-8T, the federal interest-tracing rules, the deductibility of loan interest is determined by tracing the actual use of the loan proceeds to a specific expenditure, not by what property secures the debt. There is one narrow, well-known exception (qualified-residence acquisition debt, the traditional home-mortgage-interest rules), but outside that specific carve-out, the collateral simply does not enter into the analysis.
That means a $2,000,000 securities-based line, secured entirely by a diversified stock portfolio, produces:
- Personal interest (generally not deductible) if the proceeds bought a vacation home for personal use, funded a wedding, or paid for a yacht.
- Investment interest (potentially deductible, subject to the limit below) if the proceeds bought additional income-producing investments.
- Business interest (subject to its own separate rules) if the proceeds capitalized a business the borrower materially participates in.
Same collateral, same lender, same loan agreement, three completely different tax outcomes, entirely dependent on where the cash actually went.
Even "deductible" has a ceiling
Even when the proceeds did go toward investment use, the interest doesn't automatically become fully deductible. Under IRC Section 163(d), a non-corporate taxpayer's investment interest expense deduction cannot exceed net investment income for the year, investment income minus investment expenses. Interest above that cap is not lost forever; it carries forward to future years, but it is not necessarily usable in the year it accrues. A borrower expecting a full, immediate deduction on a large SBL line used for further investment can be surprised by exactly this ceiling.
The commingling problem
Loan proceeds that fund more than one purpose generally have to be allocated proportionally across those purposes, and tracked that way. Draw $1,000,000 on an SBL line, spend $600,000 on an investment and $400,000 on a personal renovation, and the interest on that single loan splits, roughly 60/40, between investment interest and personal interest, with the borrower responsible for substantiating that split if it's ever questioned.
This is the practical reason advisors commonly recommend keeping loan proceeds for different purposes in separate accounts rather than one commingled pool: clean tracing is easy to defend, and commingled tracing is a real, ongoing recordkeeping burden that gets harder to reconstruct the longer it goes unaddressed.
Why this matters more for SBL and Lombard loans specifically
Because these products are explicitly marketed as flexible, "use the proceeds for anything" facilities, precisely the feature that makes them attractive versus a purpose-restricted margin loan, they are also unusually easy to spend in ways that unintentionally scatter the interest across multiple tax treatments without the borrower realizing it happened until tax time. A single large SBL draw used gradually over a year, for a mix of a new car, a partial business investment, and some additional securities purchases, is exactly the kind of pattern that turns into a real, hard-to-untangle tracing exercise after the fact. Planning the use of proceeds before drawing the loan, not reconstructing it afterward, is the difference between a clean position and an expensive one.
What to bring to your tax advisor
- A specific accounting of exactly what the loan proceeds were used for, ideally tracked from the moment of the draw, not reconstructed from memory later.
- Your net investment income for the year, since that is the actual ceiling on any investment-interest deduction, not the size of the loan or the amount of interest paid.
- Whether you have kept, or can still separate, proceeds used for different purposes into distinct, traceable expenditures.
- Whether your state's tax code conforms to the federal investment-interest limitation or treats it differently, since state treatment is a separate question this page does not address.